Astronomy

Are there any Stars we know don't have planets?

Are there any Stars we know don't have planets?

We have been looking for planets around other stars for a while (see related https://space.stackexchange.com/questions/1484 ) and we are finding lots of planets, some of them are Earth like According to an the answer at https://space.stackexchange.com/questions/4062 there are over 14,000 stars within 100 light years of Earth, the answer goes on to say there are "1,500 potentially habitable planets within 100 light-years" of Earth

Based on current trends it looks like there are lots of stars with lots of planets. I am beginning to assume that our solar system is not unique and that every star has several planets.

Are there any Stars that we know are without any planets?


I am beginning to assume that our solar system is not unique and that every star has several planets.

Not quite, but indeed a study published in Nature in 2012 found that, based on our observations so far, roughly 17% of stars host Jupiter-mass planets, 52% host "Cool Neptunes" and 62% host Super-Earths. (Note that these percentages do not add up to 100%, because they are not mutually exclusive possibilities). This was particularly surprising, because half of all visible stars are believe to be in binary systems, which would make planetary systems very unstable, but some binary systems have been found to have planets too.

So indeed it seems the majority of stars have planets, but it's very unlikely that all of them do.

However, the exact answer to your question "do we know of any stars with no planets" has got to be "no", because there remains a possibility that they have planets that we simply haven't been able to detect, because of limitations in our techniques to detect them.


Are there any stars or planets that rotate so fast, they're closer to discs than spheres?

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Are We Alone? Discovery of Billions of Earth-Like Planets May Hold the Answer

Earth is the only planet that’s capable of hosting life in the universe. For years, scientists have been hunting for exoplanets – planets beyond our solar system – to discover the probability of life elsewhere in the Milky Way. To be considered Earth-like, a planet must be rocky, about the same size as Earth and orbit Sun-like (G-type) stars. It also needs to orbit in the habitable zones of its star, the distance where the planet isn’t too hot or too cold for life to exist.

Anybody out there?

Over 4,100 exoplanets have been identified with the right size and right orbit of their star to support liquid water and potentially life. That seemed like a reasonable estimate – until now. According to new research published in The Astronomical Journal, estimates and calculations by astronomers at Canada’s University of British Columbia (UBC) who used data from NASA’s Kepler Mission indicate that there are even more Earth-like planets in our galaxy than previously known. Billions of Earth-like planets are orbiting G-type stars in the Milky Way, in fact. The study only considered planets orbiting these stars.

“Our Milky Way has as many as 400 billion stars, with seven percent of them being G-type,” observed study co-author and UBC astronomer Jaymie Matthews in a statement. “That means less than six billion stars may have Earth-like planets in our Galaxy.”

How many Sun-like stars can host an Earth-like planet?

The UBC research team claims there could be an Earth-like exoplanet for every five G-type stars in the Milky Way. Earlier estimates of the existence of Earth-like planets range from about 0.02 potentially habitable planets per G-type star to more than 1 per G-type star. “My calculations place an upper limit of 0.18 Earth-like planets per G-type star,” noted co-author and UBC researcher Michelle Kunimoto. She had previously discovered 17 exoplanets.

The research focused on Earth-like planets likely to be missed when searching because of their small size and great distance from their star. Kunimoto found the planets by using a new method to search for them. “I started by simulating the full population of exoplanets around the stars Kepler searched,” she reported. “I marked each planet as ‘detected’ or ‘missed’ depending on how likely it was my planet search algorithm would have found them. Then, I compared the detected planets to my actual catalogue of planets. If the simulation produced a close match, then the initial population was likely a good representation of the actual population of planets orbiting those stars.”

“Estimating how common different kinds of planets are around different stars can provide important constraints on planet formation and evolution theories, and help optimize future missions dedicated to finding exoplanets,” concludes Kunimoto.

Is there anyone out there? This seminal event in planet hunting may help to one day answer this age-old question.

Reference: “Searching the Entirety of Kepler Data. II. Occurrence Rate Estimates for FGK Stars” by Michelle Kunimoto and Jaymie M. Matthews, 4 May 2020, The Astronomical Journal.
DOI: 10.3847/1538-3881/ab88b0


Are there any Stars we know don't have planets? - Astronomy

Are there any green stars? If not why? I know that a star's color is based upon its temperature. Stars seem to exist in every other color in the visible spectrum. Why not green?

Your question is a good one! I actually asked one of my astronomy professors about that once, because it is true that the color of a star depends on its temperature, and stars with a wide range of temperatures do exist. The answer is that there are stars that are green, that is, they emit their peak radiation at a wavelength that we define as green. In fact, the sun is a yellow-green star so is close to that temperature.

However, stars emit radiation over a broad range of wavelengths, and the human eye is most sensitive to yellow and green radiation. When a star is green, it is pretty much right in the middle of the visible spectrum. It is radiating strongly at all visible wavelengths, with most of the radiation right in the middle. When we look at the star, then, all these colors are mixed and the result is the color white. So you won't ever see a green-looking star through a telescope.

There are also purple stars, which emit peak radiation in the violet part of the spectrum. But we don't see purple stars either because the human eye is more sensitive to blue light than to purple light. If a star is emitting a lot in the violet, it will also be radiating in the blue, and so these stars look blue to us. This is why the colors that we see for stars are:
red
yellow
white
blue
with red being the coolest stars and blue the hottest.


Planets

The following questions were answered by astronomer Dr. Cathy Imhoff of the Space Telescope Science Institute.

Do all the planets have seasons?
What causes seasons? Earth is tilted with respect to its orbit around the sun. So when our North Pole is tilted toward the sun, we get summer in the Northern Hemisphere (winter in the south). When the South Pole is tilted toward the sun, we get winter. So if a planet is tilted with respect to its orbit around the sun, it should have seasons. Here are the numbers that I was able to find this morning (as of September 1994) Venus &mdash 23 degrees tilt, Earth &mdash 23.5, Mars &mdash 24, Jupiter &mdash 3, Saturn &mdash 27, Uranus &mdash 98, Neptune &mdash 29.

But you can see that most of the planets have tilts like Earth, so they must have seasons. As I noted above, we definitely see seasons on Mars. In winter its ice caps grow, in summer they shrink. Jupiter has very little tilt, so it doesn't experience noticeable seasons. But Neptune is turned all the way over on its side! It must have very strange seasons!

How did the planets get their names?
Five of the planets were known to people thousands of years ago. They are bright enough to be seen with the naked eye and they move with respect to the stars. The name planet comes from the Greek word for "wanderer." I'm sure that people in different lands had various names for them, but the names we use come from the ancient Greeks and Romans. They named the planets for some of their gods. Mercury was the Roman god of commerce and cunning, and also messenger to the gods. Venus was the goddess of love. Mars was the god of war. Jupiter was the chief god. Saturn was the god of agriculture. When the next planet was found by Sir William Herschel in 1781, there was quite a debate about what to name it. Finally everyone decided to stay with the Roman names from mythology. So the new planet was finally named Uranus, for the father of the Titans. The next planet was named Neptune, for the god of the seas. And Pluto was named for the god of the underworld. Most of the moons and some asteroids are also named from Roman mythology.

What was the first planet discovered? Who discovered it? What kind of equipment did they use?
Five planets have been known since ancient times &mdash Mercury, Venus, Mars, Jupiter, and Saturn. The first new planet discovered was Uranus. It was discovered by the English astronomer Sir William Herschel in 1781. Herschel was one of the first modern astronomers. His patron was King George III of England (the same King George from the time of the American Revolution!). Herschel wanted to name the planet after King George, but nobody else liked that so they gave it the name Uranus.

Herschel and his sister Charlotte (who was an astronomer in her own right) used several reflecting telescopes, some of the first based on a design invented by Sir Isaac Newton. The largest was over 40 feet long and had a mirror 48 inches across. It was held up with a framework of wood, and they had to have helpers move it around using ropes and pulleys. It was the largest telescope in the world until over 100 years later.

Which planet was formed first and how was it formed?
We think that the planets all formed pretty much at the same time. However the sun probably formed first. The leftover gas and dust remained in a disk around the sun. In this disk, stuff began to clump and form "planetesimals" (pronounced pla-ne-TE-si-mals). These are small rocky bodies, something like asteroids. They crashed into each other and eventually formed the inner planets. At the same time, planetesimals formed the cores of the outer planets Jupiter and Saturn. Because of their strong gravity, they swept up a lot of gas. Uranus and Neptune did this too, but there was less gas around because Jupiter and Saturn got it first. The asteroid belt may be left-over planetesimals that never formed a planet because Jupiter's strong gravity nearby kept it from forming.

Are there any living things on any of the planets?
So far we know of only one planet with life &mdash Earth! In 1976, we landed probes on Mars that looked carefully for evidence of life. But they couldn't find any. The other planets are less likely to have life (at least life like that on Earth) because they are too cold, too hot, don't have water or air. So as far as we know, Earth is alone in this solar system in having life.

Why are all planets round?
Planets and stars are round because of gravity. Gravity pulls equally in all directions. Suppose you had a great big, tall mountain. As time goes by, rocks and dirt loosen up and fall down the mountain side. Eventually the mountain is worn down. Similarly a deep, deep valley will fill up. Of course a planet is not perfectly round &mdash look at the mountains and valleys on the Earth and on Mars! Also the bigger the planet, the stronger the gravity. So bigger planets will be rounder. Tiny planets may not be very round. For instance, some of the moons around Jupiter are not very big and are not round &mdash sort of oblong and irregular. Asteroids, which may be only a few miles long, are also irregular.

Why does every planet have gravity?
Every planet has gravity, because EVERYTHING has gravity! Even you do! All matter in the universe has gravity. The bigger something is, the more gravity it has. The earth has strong gravity, but the sun is much bigger and has much stronger gravity. You also have gravity much, much, much smaller than the earth, so your gravity is very small. That's good or else things would be sticking to you!

Why do some planets have more gravity than others?
The strength of gravity depends on two things, the mass of the planet and how far we are from the center of the planet. So the gravity we would experience standing on the surface of a planet depends on how massive the planet is (the heavier the planet, the more gravity) and how big the planet is (the bigger the planet, the further we are standing from the center, and thus the gravity is less). Most of the planets in our solar system are more massive than Earth, but they are also larger, so you have to do the calculations to figure out how the surface gravity compares.

How did the planets get the energy to rotate?
Actually in space it is hard to get something to NOT rotate!

The planets were formed from the same big cloud of gas and dust that formed the sun. That cloud, as it collapsed and started to form the sun, spun faster and faster as it got smaller. That is the way spin works &mdash something that scientists call "conservation of angular momentum." A familiar example is an ice skater. The ice skater starts to spin, and when she pulls her arms close around her the spin goes faster. You can do the same thing on a chair that lets you spin. Push yourself into a spin with your arms and legs stuck out. Then pull your arms and legs in to your body. Your chair will spin faster!

Well the same thing happened to the cloud that formed the sun (which is spinning) and to the portions of the cloud that formed the planets. As their smaller clouds collapsed, they spun faster and faster.

Could planets come together to form one large planet?
When our solar system was forming, many small "planets" did collide to make bigger planets. But this stopped and as a result we have our current collection of planets. Their orbits are all very stable and they can't collide now.

What are the relative distances of each of the planets from the sun? How long does it take for each of the planets to orbit the sun? Also, please provide any other data regarding the weight and such.
Let's see if I can put the information into a table to help you to better understand:

PLANET DIST. FROM SUN (A.U.) ORBIT PERIOD (EARTH-YEARS)
Mercury 0.387 0.241
Venus 0.723 0.615
Earth 1.000 1.000
Mars 1.524 1.881
Jupiter 5.203 11.86
Saturn 9.555 29.46
Uranus 19.22 84.01
Neptune 30.11 164.79
Pluto 39.44 248.5

NOTES:
DIST. FROM SUN is in Astronomical Units (A.U.), which is the distance between Earth and the sun, or 93,000,000 miles.
ORBIT PERIOD is in Earth years. This is length of time for the planet to circle the sun, so this is the planet's "year."

PLANET MASS RADIUS SURFACE GRAVITY (g)
Mercury 0.0558 0.381 0.38
Venus 0.815 0.951 0.90
Earth 1.000 1.000 1.00
Mars 0.107 0.531 0.38
Jupiter 317.89 10.85 2.64
Saturn 95.184 8.99 1.13
Uranus 14.536 3.96 0.89
Neptune 17.148 3.85 1.13
Pluto 0.0022 0.18 0.06

NOTES:
MASS, RADIUS, and SURFACE GRAVITY are all compared to Earth. Earth's radius is about 3,800 miles. If you could weigh Earth, it would weigh13,000,000,000,000,000,000. (24 zeros) pounds! Whoa! That's heavy!

Do the weights of planets stay the same?
Pretty much. Over millions of years, they pick up some space dust but that's very little compared to how much they weigh to start with.

Can you tell if a comet crash is going to hit a planet before it happens?
If we know about the comet and observe it, we can compute its orbit. Some astronomers specialize in big computer programs that can predict the orbits of the planets, comets, etc. So they can predict such a comet crash. That is what they did when Comet Shoemaker-Levy 9 was discovered &mdash they showed that it was going to hit Jupiter. Small asteroids are harder, because they aren't as bright as comets. They are just "rocks," not glowing gases like the comets. About once every 10 years we hear about some small asteroid that whizzed by the Earth and no one knew about it until just before or after it whizzed by. But the chance of one actually hitting Earth is still very very small. Thank goodness!

Why are there nine planets?
Instead of eight or ten? As far as I know, there is no special reason.

Do the planets ever get smaller by melting and erosion?
Melting and erosion must occur on other planets. For instance, Mars has ice caps, and the surface shows channels where water or something once flowed. Also we see dust storms. But do planets get smaller? I don't think so. Think about Earth &mdash ice may melt and rocks erode, but that just moves stuff (water, sand) around on the surface. To make the planet get smaller, stuff would actually have to leave the planet! Gravity makes sure that it doesn't.

Is there pollution on any of the planets?
By pollution, we usually mean something man-made that doesn't belong there. I guess you could say that we have polluted the moon, Mars, and Venus, because we sent satellites and probes to them and just left them there when we were done with them! Hopefully when the astronauts visited the moon, they cleaned up their Coke cans and orange peels before they flew back to Earth.

Have scientists found any microscopic organisms that live on any (other) planet?
The only place we have looked thoroughly is Mars, and nothing was found. Some people think that if we check closer to the ice caps, where there is more water, there might be a better chance. Hopefully some day we will do that.

Do you have any new information on the new planets?
Actually, there are now several stars that we know are planets. The first to be discovered was 51 Pegasi, but there are now five more stars similar to our sun that have at least one planet. The biggest planets are the easiest to find, so it's not surprising that all the planets found so far are around the size of Jupiter, our biggest planet. If you would like to read more details about these new planets around other stars, you can try the planet search Web page at http://www.physics.sfsu.edu/

Why do planets rotate on their axes?
Well, it turns out that there is a lot of "spin" in the universe (a scientist would call it "angular momentum"). Everywhere you look planets, stars, and galaxies are spinning. It's hard to find something that isn't spinning! The planets are rotating because when they were formed, the cloud of gas and dust surrounding the young sun was circling it in orbit. Small icy, rocky "lumps" formed in this cloud and swept up smaller particles and gas. These clumps began to form planets. As the rocks, ice, and such fell onto these new planets they helped to keep them spinning. The planets closest to the sun, however, don't rotate as fast as they did when they were formed. The sun's gravity has slowed them down. Mercury and Venus both rotate rather slowly. Earth's gravity slowed the rotation of the moon, which is why now it always keeps one side to Earth.

When do you think that humans will be starting to live on other planets?
In your lifetime, we should be able to start small colonies on the moon and on Mars. It will be hard for them to be entirely self-sufficient though. It would be very hard to live on any of the other planets in our solar system. We might put small scientific stations on one of Jupiter's moons or in orbit around Jupiter or Saturn. But good old Earth is the only place where life can prosper.

Have there been any newly discovered planets?
Recently there has been a planet discovered around a star named 51 Pegasi. This is a naked-eye star in the constellation Pegasus. We are particularly excited about this because the star is fairly similar to our sun. But the planet is nothing like the planets in our solar system. It is big &mdash about half the size of Jupiter &mdash but VERY close to the star. It must be very hot on that planet, and it must be made of rock and metal (not gases like Jupiter) in order to survive so close to the star. Some people have suggested that we name the planet Vulcan!

Why do different planets have more moons?
For planets, the bigger you are, the bigger your gravity is. If you have stronger gravity, then you can grab onto more moons. Jupiter and Saturn are the biggest planets, so they have the most moons.

How many moons do each of the planets in our solar system have?
Here are the moons that we know of. There may be more small moons around Jupiter, Saturn, Uranus, and Neptune that haven't been seen yet.
Mercury &mdash 0 moons
Venus &mdash 0
Earth &mdash 1
Mars &mdash 2
Jupiter &mdash 16
Saturn &mdash 18
Uranus &mdash 15
Neptune &mdash 8
Pluto &mdash 1

Do scientists think there might be a tenth planet in our solar system?
For some time, astronomers have tried various ways to search for a tenth planet. But we now think that there are no new planets. One big reason is that the Infrared Astronomy Satellite, known as IRAS, did a very good map of the sky in infrared light. If any new planet were out there, IRAS should have found it. But it didn't. So we think that there are no more planets in our solar system.

Are there any more planets in the galaxy?
We think so. Our sun is a pretty typical star, and we are guessing that many stars have planets. So far we have found only a handful of planets around other stars. Planets are small and dim compared to their stars, and that makes them hard to find.

Is there another planet like ours?
The planet (which we know of) that is most similar to Earth is Mars. As you probably know, Mars is smaller, colder, and has less atmosphere. But it does seem to have some water (mostly ice) and on a really hot day can get to 80° Fahrenheit.

Have there ever been any other planets that dinosaurs could have survived on?
I don't think so, at least for any of the planets in our solar system. Mars may have had some liquid water millions of years ago, and life might have started. But the atmosphere is too thin and too cold for life to have evolved very far. Venus is a horrible hothouse with acid rain. Mercury is terribly hot. Jupiter and the other planets are way too cold.

Of course there could be another planet circling another star where life exists. But we don't know enough about those other planets yet to even say how many there are, much less whether dinosaurs (or people) could live there. But the universe is a very big place. I believe that there are living creatures out there somewhere!

How do planets stay in orbit?
The sun's gravity holds all the planets in orbit. Their orbits are a balance between gravity and the motion of the planet (if the planet wasn't moving, it would fall into the sun!).

If the planets all lined up what would it be called?
We would call it a planetary alignment. When a planet lines up with the sun and Earth, and the planet is closer to the sun than Earth is, we call it a conjunction. If a planet lines up with the sun and Earth and the planet is farther from the sun than Earth, we call it opposition. There is also a cool word that isn't used very often except in crossword puzzles &mdash syzygy (siz-i-gee) &mdash which means any alignment of three bodies (sun, earth, moon, planet, whatever). How many words do you know that have three Y's in them?

Will we ever be able to travel to another planet?
Astronauts have walked on the moon. We have sent out spacecraft all over our solar system. So I think that within the next 20 or so years we will have people visit Mars. NASA has been working on how to do that for many years. But it is a big, difficult undertaking.

Why doesn't gravity pull the planets into the sun?
One approach is to go back to Sir Isaac Newton's reasoning &mdash the famous apple falling from a tree event. This story may not be true but it is probably not far from how he came up with the ideas of gravity and motion. Newton asked himself &mdash why doesn't the moon fall to Earth, the way the apple falls from the tree? He reasoned that the motion of the moon around Earth had something to do with it. Suppose you threw the apple very hard. It would fall to earth, but not at the same spot. Suppose you could throw the apple so hard that it never fell to earth. It would keep circling Earth, just as the moon does. Similarly Earth's motion in its orbit keeps it from falling into the sun. On the other hand, the moon can't escape from Earth (or Earth from the sun) because of gravity. I haven't tried it but you could try magnets. Two magnets at rest will pull together, but if you slide one past the other fast enough they won't stick. It would probably be hard to get them to "orbit" each other!

Have people found other solar systems with life on the planets?
So far no, we have not found life anywhere but on Earth. We are also having a hard time finding planets around other stars. The problems are first, distances are so great. Second, the star is very much brighter than any planet would be.

How do the gaseous planets get their gases?
Most of the "stuff" in the universe is gas. When our solar system formed, it was mostly gas with some dust. Earth was probably surrounded by gases. But Mercury, Venus, Earth, and Mars were all too close to the sun to hang onto most of their gases. The gases were heated or blown away. So those planets are rocky, with only thin gas atmospheres. The outer planets were far enough away from the sun that they held onto their gases. Even so they lost most of their lightest gases like hydrogen.

How do gas planets stay together? How can gas have gravity?
All matter, whether it is solid, liquid, or gas, has gravity. So the gas planets stay together because of the planets'gravity.

How did planets get their rings?
We think that small moons, or maybe comets, get too close to the planet. Then its gravity tears them apart. The bits of rock and ice then go into orbit around the planet and make rings. Some people think that Earth may have rings at one time!

The ringed planets are next to each other. Is there a reason for that?
A very good question! I think that the planets that are closest to the sun, Mercury and Venus, don't have rings (or moons!) in part because the sun's gravity would pull them apart. Earth may have had rings in the past, but I think our moon's gravity would pull them apart. Mars might have had rings but the big planet Jupiter would probably pull them apart. The big outer planets &mdash Jupiter, Saturn, Uranus, Neptune &mdash all are big enough to have strong gravity to hold onto their rings. Also they are further from the sun and from each other. Pluto has a big moon, Charon, that would probably pull apart rings.

What are the colors of all the planets?
This is a really good question! Most of the planets don't have bright colors, so often the pictures have been "enhanced" or shown in "false colors" using computers to bring out the details. So in some of the pictures the colors you see are not be real. Here are the planets'colors as I understand them.

Mercury is bare rock, a light gray color, like the moon. Venus'clouds are yellowish-white. (There is a famous picture of Venus that looks dark blue and white &mdash it is a "false color" picture that was actually taken in ultraviolet light to show the cloud patterns. I've also seen the same picture showing the clouds as light tan!) Earth is largely covered by water and there are lots of clouds, so it looks mostly blue and white. Mars really is red, a rusty orange-red color. Jupiter's clouds look yellowish white if you just look at it in the sky or through a telescope. True color pictures from Voyager show that some of the clouds look light brown and light orange. Many of the published pictures of Jupiter have been color enhanced to show the details in the clouds and the Red Spot (which is orangish-red). Saturn also looks yellowish-white. Its colors are similar to Jupiter's but not as strong. Uranus and Neptune look pale greenish-blue. Many of the pictures I have seen of them have the colors made stronger (bluer) than they really are. There aren't any detailed photos of Pluto yet, but it looks pretty much just white. This is reasonable since its surface is mostly ice.

If it were possible to put a colony of people on a planet with greater gravitational pull than Earth's, would there be a way to compensate for the greater force by using mechanical means?
I wonder about the physical strain of living under 2 g (two times earth gravity) for example. We know that jet pilots who undergo high gravity experience certain physiological effects, including problems with blood pooling in their feet and away from their brains, and thus sometimes falling unconscious. They wear special pressure suits to help counteract that effect. These are sort of like elastic pants that help force the blood back up into the upper body.

What planet is brightest in the skies in the morning?
Venus is the bright object closest to the horizon in the east at dawn. It is brighter than any star, so it's pretty recognizable.

Jupiter is also in the east, not quite as bright as Venus and further from the horizon. In late January early February Jupiter lies fairly close to a bright red star, Antares. Antares is the brightest star in the constellation. By the way, Saturn is low in the west in the evening sky. Mars is near opposition (i.e. opposite the sun) so it rises in the east around sunset. It is noticeably red and lies near the sickle shape part of the constellation Leo.

Are there planets around Vega?
There is a disk of dusty material around Vega. I don't think anyone has found planets there. But planets are hard to find, so there might be planets that we can't see.


We Have No Idea What Makes A Planet ‘Potentially Habitable’

The exoplanet Kepler-452b (R), as compared with Earth (L), a possible candidate for Earth 2.0. . [+] Looking at worlds that are similar to Earth is a compelling place to start, but it might not be the most likely place to actually find life in the galaxy or the Universe at large.

One of the most compelling scientific goals humanity has set for itself is to find extraterrestrial life: biological activity originating and continuing to occur on a world beyond Earth. It isn’t just our imaginations that have run wild with this possibility, it’s that we have a lot of indirect evidence identifying other potential locations where life could have arisen through similar processes to whatever occurred on Earth in our past. If we compare what’s out there with our expectations for what life requires, there’s a lot that appears to make sense.

While it might be a fun exercise to speculate about how many “potentially habitable” planets there might be out there — in our Solar System, in the Milky Way, in the Local Group, or even in the entire observable Universe — we have to be up front and honest about the assumptions that go into those estimates. These assumptions are all reflections of our ignorance, and the most uncomfortable fact of all cannot be ignored: in all the Universe, the only place we know of for certain where life has arisen is our own planet. Everything else is speculation. If we’re being completely honest with ourselves, we must admit we have no idea what makes a planet “potentially habitable.”

This illustration shows the young solar system at the end of its protoplanetary disk phase. Although . [+] we now believe we understand how the Sun and our solar system formed, this early view is an illustration only. When it comes to what we see today, all we have left are the survivors. What was around in the early stages was far more plentiful than what survives today.

JOHNS HOPKINS UNIVERSITY APPLIED PHYSICS LABORATORY/SOUTHWEST RESEARCH INSTITUTE (JHUAPL/SWRI)

If we didn’t know anything else about the Universe other than the facts that we live on planet Earth and that life exists here, we would still have every reason to speculate about what else might be out there. After all:

  • we live on a world that formed naturally,
  • made of raw ingredients — atoms, molecules, etc. — that formed naturally,
  • around a star that’s outputted energy at a relatively stable rate over billions of years,
  • and life on our planet formed, at the latest, only a few hundred million years after the Earth itself formed.

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If there was a natural explanation for how life on our world arose, and it’s eminently reasonable to assume that’s the case, then if other worlds have conditions on them that are similarly life-friendly to whatever we had on Earth in its early days, then perhaps those worlds could have had life arise on them, too. As long as the rules governing the Universe are the same everywhere, then all we need to do is discover and identify the worlds where the same processes that occurred to create life on Earth, and perhaps investigating those “potentially habitable” worlds will reveal life there as well.

This tree of life illustrates the evolution and development of the various organisms on Earth. . [+] Although we all emerged from a common ancestor more than 2 billion years ago, the diverse forms of life emerged from a chaotic process that would not be exactly repeated even if we rewound and re-ran the clock trillions of times.

Of course, that’s easier said than done. Why’s that? Because we run into our first great unknown: we don’t know how life was first created. Even if we look at the full body of scientific knowledge we have today, there’s a gap in the most important spot. We know how stars form, how solar systems form, and how planets form. We know how atomic nuclei form, how they fuse together in the interiors of stars to form heavy elements, and how those elements are recycled into the Universe to participate in complex chemistry.

And we know how chemistry works: atoms binding together to produce molecules in a wide variety of configurations, naturally. We find those complex molecules all over the Universe, from the interiors of meteorites to the ejecta of young stars to interstellar gas clouds to the protoplanetary disks in the process of creating planets.

But even with all of this, we don’t know how to go from complex, inorganic chemistry to a bona fide biological organism. Put simply, we don’t know how to create life from non-life.

Chao He explaining how the study’s PHAZER setup works, where PHAZER is the specially-designed . [+] Planetary HAZE chamber found in the Hörst lab at Johns Hopkins University. Organic molecules and O2 have been produced through inorganic processes, but no experiment has created life from non-life.

Chanapa Tantibanchachai / Johns Hopkins University

It’s not being hyperbolic, either, to say “we don’t know” in this situation. Despite:

  • searches to the limits of our ability for biological activity on other planets in our solar system,
  • spectroscopic imaging of the atmospheres of every exoplanet atmosphere we can obtain spectra from,
  • direct imaging of a variety of exoplanets involving a decomposition of their light,
  • attempts to synthesize life from non-life in laboratory settings,
  • and searches for technosignatures from potentially intelligent civilizations anywhere we’re capable of searching,

we have absolutely no evidence that favors life’s existence on any known world other than Earth. Despite all the suggestive evidence we’ve gathered that support the possibility of life arising in a myriad of different places, we’ve only ever found compelling evidence of it in two places: Earth, and on places we’ve sent Earth-based life to.

There are four known exoplanets orbiting the star HR 8799, all of which are more massive than the . [+] planet Jupiter. These planets were all detected by direct imaging taken over a period of seven years, and obey the same laws of planetary motion that the planets in our Solar System do: Kepler's Laws.

Jason Wang / Christian Marois

That isn’t to say that we know nothing about the possibility for life elsewhere. We know a lot, and we’re continuing to learn more with each new piece of information we gather. We know, for example, how to measure, count, and categorize stars in our own neighborhood, throughout the galaxy, and even throughout the Universe. We’ve learned that Sun-like stars are common, with about 15-20% of stars having comparable temperatures, luminosities, and lifetimes to our Sun.

About 75-80% of stars, interestingly enough, are red dwarfs: lower in temperature, lower in luminosity, and much longer-lived than our Sun. Although there are many important ways these systems are different from our own — planetary orbits are shorter their planets should be tidally locked they flare frequently these stars emit disproportionate amounts of ionizing radiation — we don’t have any way of assessing whether planets around these stars are similarly habitable (much less more habitable) to planets around stars like our Sun. In the absence of evidence, we can draw no robust conclusions.

An artist's rendition of a potentially habitable exoplanet orbiting a sun-like star. When it comes . [+] to life beyond Earth, we have yet to discover our first inhabited world, but TESS is bringing us the star systems which will be our most likely, early candidates for discovering it.

What about the lessons we’ve learned from our own Solar System? Earth may be unique among the worlds we have right here in our cosmic backyard as being the only planet to be obviously covered with life, but we may not be the only world that either had life in its past or that may have life persisting on it today.

Mars likely had liquid water on its surface for over a billion years before it froze could life have thrived there in our Solar System’s ancient history? And could that life survive in a sub-surface reservoir today?

Venus may have had a more temperate past with liquid water on its surface for quite some time. Could it have given rise to life, and could that life persist in the Venusian cloud-decks or cloud-tops, where conditions are far more similar to Earth?

What about sub-surface oceans with tidal heating, present on ice-covered worlds like Enceladus, Europa, Triton, or Pluto? What about life on worlds with liquid methane rather than liquid water, such as Titan? What about large worlds with potential groundwater, such as Ganymede?

Until we’ve exhaustively investigated these nearby worlds, we must admit our ignorance: we don’t even know how inhabited our Solar System is.

Deep under the sea, around hydrothermal vents, where no sunlight reaches, life still thrives on . [+] Earth. How to create life from non-life is one of the great open questions in science today, but if life can exist down here, perhaps undersea on Europa or Enceladus, there's life, too. It will be more and better data, most likely collected and analyzed by experts, that will eventually determine the scientific answer to this mystery.

What about life persisting in or even arising from interstellar space? Although this idea may sound far-fetched to many, if we trace back the history of life on Earth, it appears to be quite complex — with tens of thousands of base pairs of nucleic acids encoding information — from the moment it first arose.

Meanwhile, if we look back at the raw ingredients that we find throughout the Universe, they’re not just simple, inert molecules. We find organic molecules like sugars, amino acids, and ethyl formate: the molecule that gives raspberries their scent. We find complex carbon-based molecules, like polycyclic aromatic hydrocarbons.

We even find more amino acids that occur naturally than are involved in life processes on Earth. While we only have 20 active amino acids, all of which have the same “handedness” or chirality, the Murchison meteorite alone has some

80+ unique amino acids in it, some of which are left-handed and others of which are right-handed. Despite the success we’ve had on Earth, we simply don’t know whether other pathways are not only possible for life, but potentially even more likely.

Scores of amino acids not found in nature are found in the Murchison Meteorite, which fell to Earth . [+] in Australia in the 20th century. The fact that 80+ unique types of amino acids exist in just a plain old space rock might indicate that the ingredients for life, or even life itself, might have formed differently elsewhere in the Universe, perhaps even on a planet that didn't have a parent star at all.

Wikimedia Commons user Basilicofresco

What about our environment? Would a star system with greater percentages of heavy elements (or smaller percentages) have a greater chance at life arising and thriving than ours does? What about having a gas giant like Jupiter out near the frost line is that beneficial, benign, or actually harmful? What about our position within the galaxy is that special or mundane? Of the

400 billion stars in our galaxy, we don’t even know what criteria to look for when trying to select what targets might be good candidates for life.

And yet, you can find statements made all the time akin to the one that went viral just a few weeks ago: that there are 300 million potentially habitable planets right here in the Milky Way galaxy. They’ve been made before, and they’ll be made again plenty of times before we actually have our next meaningful data point: a world beyond Earth where we’ve found a compelling, robust biosignature (or at least a bio-hint). Until that day comes, you should treat all of those headlines with extreme skepticism, as we know too little about the habitability of planets to even discuss what it means to be “potentially habitable.”

By staring at a wide variety of stars for long periods of time, satellites like NASA's Kepler or . [+] TESS missions can search for periodic flux dips originating from those stars. Follow up observations can confirm these candidate planets, with all the data combined allowing us to reconstruct their masses, radii, and orbital parameters.

This isn’t to diminish the tremendous advances we’re actually making in the field of exoplanet sciences. Thanks to the combination of ultra-sensitive telescopes to periodic changes in a star’s brightness like NASA’s Kepler and TESS with large ground-based telescopes that can measure the periodic shifts in the star’s spectral lines, we’ve revealed thousands of confirmed planets around other stars. In particular, where the data is best, we can measure:

  • the mass, radius, and temperature of the star,
  • the mass, radius, and orbital period of the planet,

and that allows us to infer what the surface temperature of that planet should be, assuming it has an atmosphere similar to Earth’s. Now, all of that might sound reasonable, and it might sound reasonable to equate “potentially habitable” with “and it has the right temperatures so that liquid water could survive on its surface,” but that’s predicated on a lot of assumptions that are supported by only flimsy evidence. The truth is that we need superior data before we can draw any meaningful conclusions about habitability.

Today, we know of over 4,000 confirmed exoplanets, with more than 2,500 of those found in the Kepler . [+] data. These planets range in size from larger than Jupiter to smaller than Earth. Yet because of the limitations on the size of Kepler and the duration of the mission, the majority of planets are very hot and close to their star, at small angular separations. TESS has the same issue with the first planets it's discovering: they're preferentially hot and in close orbits. Only through dedicates, long-period observations (or direct imaging) will we be able to detect planets with longer period (i.e., multi-year) orbits. New and near-future observatories are on the horizon, and should reveal new worlds where right now there are only gaps.

NASA/Ames Research Center/Jessie Dotson and Wendy Stenzel missing Earth-like worlds by E. Siegel

In the quest for life beyond Earth, it’s important to remain both honest in where we are today and open-minded for what we might find in the future. We know that life arose (or arrived) on Earth very early, and survived and thrived ever since. We know that if we’re looking for planets with similar histories, properties, and conditions, we’re likely to find any nearby planets that may have had similar successes. That’s the conservative way to look, and it’s eminently sensible.

But thinking only along these lines could be existentially limiting. We don’t know whether other, very different worlds with very different histories, properties, and conditions might be as likely or even more likely to have life on them than Earth was. We don’t know how those probabilities are distributed across the myriad of planets present in our Universe. And we don’t know what the odds of developing complex, differentiated, macroscopic, or even intelligent life are if the early seeds of life do take hold. We have every reason to believe that life exists elsewhere in the Universe, and every motivation to go looking for it. But until we have an better idea of what is and isn’t inhabited, we have no business asserting how many “potentially habitable” worlds there might actually be.


The Solar System

The following questions were answered by astronomer Dr. Cathy Imhoff of the Space Telescope Science Institute.

How many miles long is the solar system from end to end?
Well, it depends a bit on how you define the "end." If you choose Pluto, the outermost planet, as the end, then the diameter of the solar system is 80 A.U. A.U. means "astronomical unit" then the distance between Earth and the sun equals 93,000,000 miles. So that would be 7,440,000,000 (over seven billion) miles. If we say that the end of the solar system is the cloud of comets that surrounds our solar system, then the diameter is roughly 60,000 A.U., which is 5,580,000,000,000 (over five trillion) miles. It's a big universe!

How long would it take for a rocket ship to visit all the planets in our solar system?
Well, it depends on how fast the rocket ship can go. The Voyager 2 spacecraft was launched in August 1977. It reached Jupiter in July 1979, almost two years later. It passed Saturn in August 1981, another two years later. Then it reached Uranus in January 1986, and Neptune in August 1989. So this spacecraft took 12 years to go most of the way across our solar system.

How were the planets formed?
The planets were formed when the sun was formed. Stars such as our sun are formed when a cloud of gas and dust collapses from its own gravity. The center tends to form first, with a disk of material around it. The material in the disk is in orbit and starts clumping up. Small clumps sweep through the disk, becoming larger and larger. The larger a clump gets, the more gravity it has, so it can sweep up more stuff to make the clump larger, which gives it more gravity, and on and on. Meanwhile the center, which has most of the stuff, forms the young sun. The clumps form the planets.

Is it possible to count the number of stars in our solar system?
When we talk about our solar system, we usually mean the sun and planets. But I bet you are thinking about the Milky Way (not the candy bar!), which is full of stars. There are so many that we can't count each one, but we can make a good guess. Here is how we can estimate how many there are: Suppose you would like to count all the trees in a forest, but you can't see the whole forest &mdash because there are too many trees! But you can count how many trees there are in the acre near you. And you can measure how big the forest is, or how many acres it is. So the number of trees in one acre multiplied by the number of acres equals the total number of trees. We do the same thing. We can count the number of stars within, say, 100 light-years of our sun. We can estimate how many light-years it is across our galaxy. That's how we estimate that there are about ten billion stars in our galaxy!

How big is the Milky Way?
Pretty big! Astronomers don't use miles to measure many distances because they are too large. We use other kinds of "yardsticks" like the light-year. A light-year is the distance that light travels in one year, which is equal to 5,676,000,000,000 miles! We think that our galaxy, the Milky Way, is about 100,000 light-years across.

Why are the orbits of celestial bodies not circular?
If gravity were a rope, connecting a planet to the sun, then the planet would have to orbit the sun in a circle. But it isn't. It is a force. It pulls harder when the planet and sun are closer together, and less when they are far apart.

I can't think of a convincing way to explain it or an analogy. But what about an experiment? Suppose you had a strong magnet and some steel balls. You can make the balls "orbit" the magnet in a way similar to planets orbiting the sun. The orbits may be pretty circular or they may be very elliptical. If you have several balls with very elliptical orbits, they will crash into each other. So only if they are in fairly circular orbits can they keep orbiting the magnet for a long time.

Similarly one reason that the planets have fairly circular orbits is so that they don't crash into each other! If they did, they would have been torn apart long ago. But the orbits don't have to be perfect circles. Of course this really goes back to the formation of the solar system, when there were millions of "planetesimals" (hunks of rock and ice) in a disk around the sun. They crashed into each other, sometimes being torn apart but sometimes sticking together to form larger and larger bodies. The large bodies had stronger gravity and so collected more and more rock and ice, growing and growing until they became planets. So the orbits of the planets come from that time when these swarms of rocks and iceballs orbited the sun. The stuff that was in elliptical orbits tended to get torn apart, while the stuff in fairly circular orbits was captured by the proto-planets.

Why is it cold in space, but if you travel too close to the sun, you'll burn and disintegrate?
On Earth, there are two things that keep us warm: one is the warm air the other is the light. The light of the sun or a fire striking our skin gives off heat. In space, there is no air, warm or cold. The only source of heat is sunlight. A satellite or the shuttle in orbit around Earth is very hot where the sun shines on it and very cold on the side in shadow. For instance, they like to turn the space shuttle around every few hours to warm the other side. In fact in the last shuttle mission, the astronauts were trying out new spacesuits that will help them stay warm when they are working in the cold, shaded side of the shuttle. They had problems staying warm in the old suits when they were in the shade too long. If you get closer to the sun, the light is brighter and the heat is greater. If a satellite (or a comet or an asteroid) gets too close, it can melt away!

Why does outer space appear all black even when the sun is out?
For you to see something, light must be reflected from that something to your eyes. If there is nothing there, the sunlight doesn't reflect and there is nothing to see.

What's it like in space?
I've never been there, but the IUE satellite that I work on is there and its sensors tell us a lot. You know, of course, that there is no air in space. In fact, it is very close to being a vacuum. Because it is a vacuum, there is nothing to protect you against either heat or cold. On our satellite, one side is always in the sun &mdash it is always very hot. The other side is always in the dark &mdash it is very cold. I don't remember the temperatures exactly, but they are something like +100° Fahrenheit on the sunlit side and 100° Fahrenheit on the dark side. This is a big temperature strain that spacecraft engineers have to take into account when building a satellite.

The sky looks very black, because there is no air to scatter sunlight about to make it look blue (or any other color). The stars are bright points of light &mdashsmaller yet brighter because there is no obscuring, blurring atmosphere. Our satellite, the IUE, is in synchronous orbit, about 24,000 miles up, so it is outside the earth's magnetic field. That means it is exposed to the radiation (electrons, protons, ions, cosmic rays) that the sun emits. Our cameras are shielded but they are still affected by some of the electrons and protons.

One of the effects that this has on our satellite is that it is pushed around a bit by the "solar wind." The solar wind is the stream of particles, mostly electrons and protons, blown out by the sun. They are not numerous &mdash it is still pretty close to being a vacuum by Earth standards &mdash but they move pretty fast. Since our satellite has big solar panels, the solar wind tends to push on it. If we didn't have a system to keep the pointing of the satellite stable, the solar wind would push it into a slow spinning motion. Also, we are protected by Earth's ozone layer from the sun's ultraviolet light. In space you would not be protected &mdash so one side of you would get quite a sunburn really fast! Of course there is virtually no gravity &mdash I won't say none because if you are in orbit around a planet then there must be a little gravity to keep you in orbit! This is why they now talk about "microgravity" experiments on the shuttle. Finally, in space there is no sound. There is no air to transmit the sound waves. So all those great sounds in the Star Wars and Star Trek movies are not realistic. In space, no one can hear you scream.

Why do some planets have more moons than others?
For planets, the bigger you are, the bigger your gravity is. If you have stronger gravity, then you can grab onto more moons. Jupiter and Saturn are the biggest planets, so they have the most moons.

Which planet has the most moons? Is it still Saturn?
Yes, it is still Saturn. The Voyager spacecrafts discovered many of the moons. These moons are smaller than the ones we could see from Earth. So we probably won't discover anymore until another spacecraft visits Saturn.

What was in space before the solar system?
Before our sun and its planets were formed, there were other stars (and probably planets) in our galaxy. We think that our galaxy has been around for roughly 15 billion years, but the sun is "only" five billion years old.

Which planet was the first to be discovered? Who discovered it? What kind of equipment did they use?
Five planets have been known since ancient times &mdash Mercury, Venus, Mars, Jupiter, and Saturn. The first new planet discovered was Uranus. It was discovered by the English astronomer Sir William Herschel in 1781. Herschel was one of the first modern astronomers. His patron was King George III of England (the same King George from the time of the American Revolution!). Herschel wanted to name the planet after King George, but nobody else liked that so they gave it the name Uranus.

Herschel and his sister Charlotte (who was an astronomer in her own right) used several reflecting telescopes, some of the first based on a design invented by Sir Isaac Newton. The largest was over 40 feet long and had a mirror 48 inches across. It was held up with a framework of wood, and they had to have helpers move it around using ropes and pulleys. It was the largest telescope in the world until over 100 years later.

Is there another planet like ours?
The planet (which we know of) that is most similar to Earth is Mars. As you probably know, Mars is smaller, colder, and has less atmosphere. But it does seem to have some water (mostly ice) and on a really hot day can get to 80° Fahrenheit.

How many known solar systems exist?
Just recently astronomers have found three stars with planets! You may heard about 51 Pegasi a few years ago. They are part of a project to carefully examine stars that are similar to our sun to see if there are any planets. We know there are planets because their gravity affects the motion of the stars. We can't see the small, dim planets directly. For each of the three stars, we have detected a big Jupiter-like planet. We are very excited about this. More stars are being examined, so hopefully more planets will be discovered soon!

Are there any solar systems with two or more stars?
It has been hard for astronomers to find other solar systems &mdash that is, planets around other stars. Only in the last two years have we been able to find some that we really believe exist. So far I think there are about two dozen stars that we believe have planets. To my knowledge none of these systems has more than one star in it, but that may be because they have not looked at double-star systems. So stay tuned &mdash we are still looking!

Have people found other solar systems with life on the planets?
So far no &mdash we have not found life anywhere but on Earth. We are also having a hard time finding planets around other stars. The problems are first, distances are so great. Second, the star is very much brighter than any planet would be.

Will the earth ever collide with another planet?
I think that it is extremely unlikely. All the planets in the solar system are in stable orbits, meaning that they shouldn't change by much. The only orbits that cross are Neptune and Pluto, and even there it has been shown that the two planets move in such a way that they will never collide. It is, of course, possible that a comet or asteroid could hit Earth &mdash it has apparently happened in the distant past &mdash but that is another story.

Describe some recent discoveries in the solar system.
One recent discovery in our solar system that I find interesting is that there are TWO places where comets seem to come from. One is the Oort cloud, a region out beyond the solar system where comets orbit the sun. They are so far away they are too dark to see, but every once in a while one of these comets falls in toward the sun. This is where many of the comets we know have come from.

But recently we have discovered that there is another place. This is known as the Kuiper belt. It is a region between Saturn and Pluto where some comets and other objects are in orbit. The brightest of these new objects is called Chiron. It seems like a comet, but if it is, then it is the biggest comet known.

Do scientists think there might be a tenth planet in our solar system?
For some time, astronomers have tried various ways to search for a tenth planet. But we now think that there are no new planets. One big reason is that the Infrared Astronomy Satellite, known as IRAS, did a very good map of the sky in infrared light. If any new planet were out there, IRAS should have found it. But it didn't. So we think that there are no more planets in our solar system.

We heard on the news the discovery of two new planets. Can you give us any details?
As you may know, astronomers have been trying to find planets around other stars for some time. Only recently has the technology gotten good enough that we seem to be able to find some for sure. Each of the two new planets orbits a star that is pretty similar to our sun. One star is 70 Virginis (in the constellation Virgo) and the other is 47 Ursae Majoris (in Ursa Major, also called the Big Dipper).

The planet circling 70 Vir is about nine times bigger than Jupiter. It orbits its star once every 116 days, which means it is closer to its star than Earth is to our sun. The discoverers calculated that the planet's temperature would be about 185° Fahrenheit &mdash kind of hot!

The planet circling 47 UMa is about three times the size of Jupiter and orbits its star once every 1,100 days. That puts it about twice as far from its star as Earth is from the sun. It would be a bit cold, about 100° Fahrenheit.

Both of these stars can be seen with the naked eye. But even the astronomers have not seen the planets! What we found was a wobble in the star's position that is due to the gravitational pull of the planet. That takes some pretty careful measurements to determine!

Can scientists see the new planets? If not, how do we know they are there?
Actually we can't see the new planets at all right now! We know they are there because of the effects that they have on their stars. Gravity holds a star and a planet together. It is like two kids holding hands and swinging each other around. One kid (the star) is really big, and the other kid (the planet) is small. So when they swing each other, the little kid swings around the most. But the big kid is swung around a little too. We discovered the planets by seeing that the stars are being swung around a little bit. We can even tell how big the planets are and how far away they are by carefully measuring the small movements of the stars. But we can't see the planets because they are too small and dim compared to their stars. Maybe some day we will have telescopes that will allow us to see the planets.

Do any of the new planets have rings?
We don't know whether those new planets have rings, we can't see the planets directly. But all of the big-gas planets in our solar system have rings. So maybe those new planets, which are also pretty big, also have rings. When I was in school, the only planet that we knew had rings was Saturn! We didn't know about the rings around Jupiter, Uranus, and Neptune until the 1980s.

What is an ice volcano?
Sounds crazy, doesn't it? But many of the moons around Jupiter, Saturn, Uranus, and Neptune are made up of icy material. If an area of ice under the crust is heated, it can push up and make a volcano, with slightly melted ice flowing down the sides.

Which characteristic of our solar system do you consider the most interesting and mysterious? Why?
Some people have found new types of objects, like big comets, in orbit beyond Saturn. Do they come from a big cloud surrounding our solar system, as many people believe? I think these small bodies and comets are interesting because stuff like this probably crashed into the young Earth and helped to make our oceans and atmosphere.


There’s a mysterious new planet hiding in our solar system. What do we know about it so far?

Scientists have narrowed down the location of the mysterious Planet Nine.

Wait, isn’t Pluto the ninth planet?

Not since 2006, when the International Astronomical Union (IAU) downgraded it to a dwarf planet. Ever since, astronomers have been lobbying the IAU to change its mind.

The latest attempt was to say that because Pluto passes the Star Trek test—that people can say what’s a planet when they see one—it should be reinstated to be the ninth planet. The IAU hasn’t budged, so we are currently at eight planets in the solar system and many dwarf planets.

But there may be another planet that could be the ninth planet?

Yes. Researchers at the California Institute of Technology (CalTech) announced in January that there may indeed be a mysterious planet which could explain some of the mysteries of the solar system that have yet gone unsolved.

A mystery could solve many other mysteries? I’m confused.

Let’s break it down. The main reason Pluto was downgraded to the status of a dwarf planet was because scientists had found many celestial bodies similar to Pluto, which were orbiting our sun. However, their orbits were skewed to one side of the sun and scientists couldn’t explain why.

Applying simple planetary physics, they figured, that there must be a planet, probably more massive than our own Earth, which would counter-balance the orbits of all these smaller celestial bodies. Current calculations show that the planet should be about 10 times the mass of the Earth, and its huge orbit should take it about 20,000 years to complete.

It’s just a hypothesis then. Why trouble us?

It sure is, but the evidence is mounting that Planet Nine might actually be real. That’s just what CalTech researchers were hoping would happen after their announcement. Astronomers around the world are pointing their telescopes and radio antennas to try to look for Planet Nine.

In February, researchers at Nice Observatory in France did something clever. They used data from the Cassini probe, which has been exploring Saturn and its moons for the last 10 years, to understand how all the large bodies of the solar system behave. They added the mysterious Planet Nine to the mix, and the computer simulations of the solar system eliminated half the orbits that CalTech researchers had suggested.

Now researchers at the Harvard-Smithsonian Center for Astrophysics have gone a step further. They used the same data from Cassini but ran a different set of calculations (looking for many more kinds of planets) and they have further narrowed down where Planet Nine might be hiding.

Planet-hunters around the world are giddy with excitement. “I’m just dropping everything to work my hardest to do this search,” David Gerdes, a cosmologist at the University of Michigan, told the New Scientist.

If it’s real, why has it taken us so long to find it?

Many reasons. First, before we found all these other small celestial bodies, the solar system’s eight planets were enough to explain everything we had observed. The new discoveries changed our understanding and created the need for another explanation. Second, its huge orbit means that it’s probably not come anywhere close to our sun during the period when humans built the technology to scour the skies. Third, it may be too cold to give out a heat signature and too dark to reflect light, both of which could have warned astronomers of something.

OK. So how are we going to find it?

We’ll need many eyes in the skies, many people on computers looking at the data we’ve already collected, and, finally, we’ll need to put together a whole slew of solid evidence that all the data irrefutably shows that Planet Nine exists.

If we do find it, will Planet Nine have anything special in store for us?

“YES!” any astronomer will shout. We’ve not found a new planet since the 1930 discovery of Pluto (which then we downgraded in 2006). There is also the tantalizing possibility that our sun stole Planet Nine from a passing star a long time ago.

So that’s it for Pluto then. Nobody is going to reinstate it to being a planet again, right?

Don’t worry. There’s a hardy group of researchers who are ready to fight for Pluto’s status.


Are there any planets that don’t orbit a star?

Yes, exoplanets that are in the milkyway but don't orbit a star are called Rogue Planets. There is even evidence for rogue planets outside the milkyway in intergalactic space.

Yes, definitely. The exact definition of an exoplanet is still in flux, but in any reasonable definition, there could be planets that don't orbit a star, such as a planet that formed around a star, but then was ejected after gravitational interactions with other planets in the system.

Do these exoplanets still move? And if they do, is if just random movement since they’re not orbiting anything?

The most recent definition of a planet was adopted by the International Astronomical Union in 2006. It says a planet must do three things:

It must orbit a star (in our cosmic neighborhood, the Sun).

It must be big enough to have enough gravity to force it into a spherical shape.

It must be big enough that its gravity cleared away any other objects of a similar size near its orbit around the Sun.


Scientists now have some ultra-prime targets to look for aliens

Don’t be too alarmed, but you can see Earth from more than 1,700 stars up to 300 light years away. You know, if alien astronomers exist.

In a new study published Wednesday in Nature, two astronomers at Cornell University and the American Museum of Natural History took a look at stars within 100 parsecs (about 326 light-years) of the Sun to see which ones might be able to see the Earth through the transit method. And the results could eventually help us find life in space.

The transit method is what Kepler and the Transiting Exoplanet Survey Satellite use to find planets that pass in front of their stars from our point of view, causing tiny — but detectable — dips in light.

Because of the vast number of planets found through transits, they inferred that the transit method is also the most likely way for astronomers orbiting distant stars to spot our little rocky blue world. And that could have implications for how astronomers on Earth search for life on distant worlds.

What’s New The team, consisting of Cornell’s Lisa Kaltenneger and AMNH’s Jackie Faherty, looked for the “Earth Transit Zone” — the area of space where observers around distant stars might have a chance to discover our planet with the transit method.

The key takeaways of the study:

  • Planets orbit their stars in a plane, called the ecliptic
  • Astronomers can only see transits if they happen to be lined up with that solar system’s ecliptic.
  • Today, 1,402 stars are in just the right position to watch our planet pass across the Sun
  • But in the last 5,000 years, at least 1,715 stars within 300 light years of Earth could have observed our transit at some point
  • An additional 319 stars will be able to see us in the next 5,000 years

Everything in the universe is moving relative to everything else, so stars change position over time. As a result, the night sky looked different thousands of years ago than it does now.

And just as our view of the stars has changed, so has their (potential) view of us. Some of the stars in the Earth Transit Zone now weren’t there a few centuries ago, and some stars that aren’t lined up right to watch our tiny transit now had a great view during the heyday of the Roman Empire.

Kaltenneger and Faherty worked that out by modeling the relative movements of 2,034 stars within 300 light years of Earth. What’s especially impressive: seven of these stars that could have peeked in on us have known planets in their habitable zone.

They discovered, for example, that Ross-128 — a red dwarf star about 11 light years away with a tidally-locked exoplanet just at the inner edge of its habitable zone — could have observed Earth’s transits from 900 to 3,000 years ago, or from roughly between the time of ancient Egypt and the Middle Ages.

"Any civilization with our level of technology could have seen us already on Ross 128b, but lost that vantage point about 900 years ago,” Kaltenneger tells Inverse. “Would anyone have concluded that there was intelligent life on Earth 900 years ago?"

One important system that could soon see us is TRAPPIST-1, another red dwarf about 39 light years away with 4 rocky exoplanets in its habitable zone. It will have the chance in another 1,642 years, if there’s anyone there to point a telescope.

But there’s more: of the stars that can see us transit, 75 stars could have received our radio signals already.

Humans started unwittingly sending radio waves out into the galaxy about 100 years ago, so the first technological signals from our planet will have reached star systems within 100 light years of us by now.

In 29 years, Teegarden’s Star, a star about 12 light years away with two Earth-mass planets in its habitable zone, will finally be in position to observe Earth passing across the Sun after receiving our leaked radio waves for more than a century.

It wouldn’t take a lot of work to figure out that those signals were coming from the small rocky world transiting its star every 365 days. One clue would be that the radio waves decrease in strength every time Earth passes around the other side of the Sun.

Here’s The Background Of the 4,422 planets outside the Solar System we know of, around 75 percent have been discovered through transit. This is because telescopes like TESS can stare at many stars at once.

Right now, we can get a few things out of a transit. For instance, we can figure out roughly the radius of the transiting planet. Follow-up methods can help us nail down a mass, which tells us if an Earth-sized planet also has a similar mass to our home planet.

But telescopes like Hubble can also watch how a star’s spectra— the wavelengths of light absorbed and emitted by particular chemicals — changes when a planet with an atmosphere transits. That offers some clues about which worlds might be habitable.

This goes for alien astronomers too — they could find us through transits, then perform follow-ups to determine our mass, distance from the Sun, and, if they have a big enough telescope, find out that our atmosphere is rich in oxygen and water vapor.

“The combination of oxygen and methane on Earth is the golden fingerprint for life in Earth’s atmosphere, and has been detectable for about 2 billion years on our planet,” Kaltenneger says. “If there are curious alien observers out there, they probably would draw those conclusions as well and could have found out there is life on Earth for a long time.”

In other words, if you were an astronomer on Wolf 359 (of Star Trek: The Next Generation infamy) looking for alien life, Earth would seem like a pretty good candidate. And if you’re an astronomer here on Earth, looking for alien life, stars that have been in a position to notice us are a good place to look for messages headed our way.

Why It Matters —If astronomers at projects like Breakthrough Listen or the SETI Institute’s COSMIC (which uses the same Very Large Array made famous in the movie Contact) want to scan the skies for alien radio signals, it makes sense to look – or listen – in a direction we’ve got some reason to expect a message from.

That means looking in the Earth Transit Zone, and especially at the 75 stars which could have seen us transit and received our radio signals.

"The concept of the Earth Transit Zone doesn't guarantee that we'll 'meet' an alien intelligence, but it's a good place to search," University of California, Berkeley astronomer Steve Croft tells Inverse.

Looking at stars that were in the Earth Transit Zone in the past helps expand the list of potential targets.

Louisiana State University astronomer Tabetha Boyajian tells Inverse that the recent study also helps SETI researchers make a connection between our history here on Earth and when exoplanets could have picked up signatures of life and technology coming from our planet.

What’s Next —TESS has recently entered its extended mission phase, and Kaltenneger and her colleagues plan to use the telescope to observe more stars in the Earth Transit Zone. The Breakthrough Listen project recently used the Green Bank Telescope to search 20 stars in the Earth Transit Zone for radio signals, with no luck, but the search continues.

"If a star closer than 50 light years to Earth detected some of our earliest radio transmissions, and responded right away with their own radio signal, we might be just about to receive that reply," Croft tells Inverse.