How do rogue planets orbit around stars in other planetary systems?

How do rogue planets orbit around stars in other planetary systems?

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I got some interesting answers for What would happen if a rogue planet hit one of the planets in our Solar System?

But I have seen some documentaries that state that rogue planets from other planetary systems (which are ejected from the planetary systems in which they formed) have become part of the other planetary systems and orbit around those new stars as planets.

For example, please check this link:

The latest research suggests that sometimes, these rogue, nomadic worlds can find a new home by being captured into orbit around other stars.

How is this possible? Stars' gravity can pull the rogue planet into them. How do rogue planets escape this and orbit around the new stars. This seems to be a bit mysterious.

There are a few ways in which this can happen.

One of them is by a supernova explosion. A star explodes pushing out the planets who were once orbiting this into free space, going too fast for recapture by other stars. Some planets closer to the star would obviously be decimated but the outer ones could survive the blast(as said here by UserLTK).

Another is that a space object with enough velocity and mass to knock out a planet from its orbit into space. Such as if the originally solar system has too much planets, planet to planet interaction could send one if not both out of the system.

A rare occasion for this way of separating planets by their host star is by a black hole. There is current 16 (as far as I could remember) stars travelling at hyper speeds through the galaxy because it came too close to the black hole at the center of the milky way, then spinning outwards at incredible speeds and then the velocity finally comes over the blackhole's gravitational pull and shoots out. The speeds in which they come out is about 7-10 million miles per hour and the top speed achievable is at 30 million miles per hour. That's about crossing earth's diameter in about 10 seconds.

As LocalFluff said and UserLTK's contribution to it in the comment above, that's one of the ways too for a planet to be ejected out of the system. Link for that:

Here's some good sites for answering your question:

Capture of any object is never common. A rogue planet that passes a star will be accelerated by the star's gravity, and provided that it doesn't hit anything will pass the star in a hyperbolic path.

For capture to occur, the rogue planet has to lose momentum, and there are a few ways in which this can happen. The most general way is for the rogue planet to interact with one of the stars existing planets. If the rogue planet passes close one of the star's orbiting planets, it can interact gravitationally, and transfer momentum to the planet, and slow down in the process. This is rather like the reverse of a gravitational slingshot.

It is also possible that a double planet can be captured, if one of the bodies transfers momentum to the other. One of the double planet will escape, the other is captured by the star's gravity.

Capture of rogue planets is very rare. There is no evidence that it has occurred in our solar system (never in 4.7 billion years is a big Never) though some have speculated that Sedna might be a captured object. But we know that captures like this are possible, as Jupiter and Saturn have a collection of moons, some of which seem to have been captured from the asteroid belt.

Rogue planet discoveries with LSU Astronomy

We grew up learning about the eight planets—sorry, Pluto, I don’t make the rules—but there’s so much more to the cosmos than that. Some exciting news for the LSU community: LSU Department of Physics & Astronomy Assistant Professor Matthew Penny is part of a team looking to improve the understanding of planet demographics by searching for rogue planets.

But what are rogue planets, you might ask?

“Astronomers and planetary scientists have learned a huge amount about how the solar system and other planetary systems form, but we still lack a good understanding of all the physical processes involved and how they fit together,” explains Penny. “We think that the Earth and moon took their final form after a collision between two large protoplanets, but the same kind of encounter between planets orbiting a star can result in one of the planets being flung off into space, no longer orbiting the star they were born around. If we want to fully understand how planetary systems form, we need to be able to account for the planets that go missing—planets we call rogue planets.”

A rogue planet captured by the Nancy Grace Roman Space Telescope. Photo by NASA/JPL-Caltech/R. Hurt (Caltech-IPAC).

In the past, rogue planets have been extremely hard to capture. But with the help of NASA’s Nancy Grace Roman Space Telescope, it should be a bit easier.

“The trouble is, these rogue planets don’t emit any significant amount of light, nor do they have an effect on their parent star, so for the most part, astronomers have no way of studying them,” he explains. “The Nancy Grace Roman Telescope can search for them, though, by using the gravitational field of the rogue planet itself to bend the light of a background star and cause it to temporarily get brighter in what’s called a gravitational microlensing event. It’s an extremely unlikely, short-lived event, but by monitoring hundreds of millions of stars every 15 minutes for over a year in aggregate, our simulations have shown that the Roman telescope can expect to detect around 200 rogue planets with masses stretching from one-tenth of Earth’s mass to larger than Jupiter.”

What makes the Roman telescope different than other telescopes, though? In short, it can take high quality and widespread images.

“In the past, space telescopes have been able to do either one of two things: produce very high-resolution images by being above the atmosphere—like the Hubble telescope—or provide wide-field but low-resolution images—like the Kepler telescope,” explains Penny. “Roman is the first space telescope to be able to do both. It provides images with a similar resolution to Hubble, but are 100 times larger. It has also been built as a survey telescope, so it is extremely efficient and can complete some projects at 1,000 times the speed of Hubble. It’s these features that enable Roman to conduct its survey for regular exoplanets that still orbit a star and rogue planets.”

And there you have it, your daily dose of planetary science. Stay tuned for more on the Roman telescope, which will launch sometime in the mid-2020s.

Rogue planets piqued your interest? Hear more from Penny in the upcoming event Visitors from Outer Space: Interstellar Comets, Asteroids, and Rogue Planets on September 10, hosted by Astronomy on Tap Baton Rouge.

How Do Planets Go Rogue?

We’re accustomed to thinking about solar systems as places of order. All the planets orbit their parent star, everything is neatly arranged in ellipses and rings. Even the asteroid belt has division lines of dry and icy. Planets do what they’re told: orbit that star until the end of time. No Pluto, you may not go outside and play with the other planets. You’ll spend your lunch hour in detention with Haumea until we decide what we’re going to do with you for not cleaning up your play area.

Some planets just can’t be held down. They’re the Jimmy Deans, the greasers, the Marlon Brandos, the Cool Hand Lukes. They break all the laws and play by their own set of rules. They’re a rolling stone, baby. To ask them to settle down would just be to deny their nature. So instead of orbiting a star, they go rogue and fly off into the Milky Way, possibly seeking fame, fortune and adventure, but keeping to the beat of their own drummer.

A rogue planet is any planet that doesn’t orbit a star. Instead of being a member of a solar system, it orbits the Milky Way on its own. Or in the case of really deviant planets, it’s been ejected out of the Milky Way entirely. Make no mistake, this is not a small condition affecting a few planets. It’s estimated that there are billions of rogue planets out there in the Milky Way.

How does this happen? How can we get rogue planets? Is it the way they were raised? Something that happened in the way they were born? Some rogue planets started out as part of a solar system, and then something happened. Some event “kicked” them out into deep space. You could get a collision or near miss with another star, or even a black hole. As two stars pass one another, their gravitational interactions can cause all kinds of mayhem to a nice orderly orbital system. Planets can be kicked into higher or lower orbits, smashed into stars or flung out with an escape velocity that means they’ll never orbit their star again.

Planets can also escape when their star disappears. Sounds impossible? Sometimes stars go out for cigarettes and just never come back. When a massive star detonates as a supernova, the force of the explosion can eject planets at tremendous velocities away from the former star, flinging those billiard balls all over the hall. But the vast majority of rogue planets probably formed early on in their solar systems. Things were rough and chaotic back then, with planets smashing into each other with all kinds of near misses. These interactions could bully out smaller neighbors with not so much as a nod. Jupiter, I’m looking at you.

It’s also possible that planets could form as orphans, within a solar nebula, away from a star entirely. If a pocket of hydrogen collects together into a sphere, but it doesn’t have enough mass to actually ignite as a star, it’s another type of rogue planet. We’ll just pretend these ones were raised by Nuns.

What would it be like for these planets? Without the light from a star, these would be incredibly cold places. This isn’t just sad metaphor. The outer layers, exposed to space would be as cold as interstellar space, just a handful of degrees above absolute zero.But deep down below the surface, there would still be leftover heat from their formation, so it’s possible that life could survive down there, kept alive within a warm cocoon.

And who knows, maybe after billions of years, a rogue planet could get captured by a star again, and thawed out. It might get a second chance, or it could all end tragically, racing for pinks along the Devil’s elbow out past the Pillars of Creation. There are many ways that planets can go rogue, in fact, it’s possible that there are more starless planets in the Milky Way than there are stars.

So what do you think? Should we set sail from the Sun, and seek out adventure in the Milky Way?

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Some stars capture rogue planets

In this artist's conception, a captured world drifts at the outer edge of a distant star system, so far from its sun-like host that the star's disk is barely resolvable at upper right. New research shows that one in 20 stars within our galaxy might have captured a free-floating planet. Credit: Christine Pulliam (CfA)

( -- New research suggests that billions of stars in our galaxy have captured rogue planets that once roamed interstellar space. The nomad worlds, which were kicked out of the star systems in which they formed, occasionally find a new home with a different sun. This finding could explain the existence of some planets that orbit surprisingly far from their stars, and even the existence of a double-planet system.

"Stars trade planets just like baseball teams trade players," said Hagai Perets of the Harvard-Smithsonian Center for Astrophysics.

The study, co-authored by Perets and Thijs Kouwenhoven of Peking University, China, will appear in the April 20th issue of The Astrophysical Journal.

To reach their conclusion, Perets and Kouwenhoven simulated young star clusters containing free-floating planets. They found that if the number of rogue planets equaled the number of stars, then 3 to 6 percent of the stars would grab a planet over time. The more massive a star, the more likely it is to snag a planet drifting by.

They studied young star clusters because capture is more likely when stars and free-floating planets are crowded together in a small space. Over time, the clusters disperse due to close interactions between their stars, so any planet-star encounters have to happen early in the cluster's history.

Rogue planets are a natural consequence of star formation. Newborn star systems often contain multiple planets. If two planets interact, one can be ejected and become an interstellar traveler. If it later encounters a different star moving in the same direction at the same speed, it can hitch a ride.

A captured planet tends to end up hundreds or thousands of times farther from its star than Earth is from the Sun. It's also likely to have a orbit that's tilted relative to any native planets, and may even revolve around its star backward.

Astronomers haven't detected any clear-cut cases of captured planets yet. Imposters can be difficult to rule out. Gravitational interactions within a planetary system can throw a planet into a wide, tilted orbit that mimics the signature of a captured world.

Finding a planet in a distant orbit around a low-mass star would be a good sign of capture, because the star's disk wouldn't have had enough material to form the planet so far out.

The best evidence to date in support of planetary capture comes from the European Southern Observatory, which announced in 2006 the discovery of two planets (weighing 14 and 7 times Jupiter) orbiting each other without a star.

"The rogue double-planet system is the closest thing we have to a 'smoking gun' right now," said Perets. "To get more proof, we'll have to build up statistics by studying a lot of planetary systems."

Could our solar system harbor an alien world far beyond Pluto? Astronomers have looked, and haven't found anything yet.

"There's no evidence that the Sun captured a planet," said Perets. "We can rule out large planets. But there's a non-zero chance that a small world might lurk on the fringes of our solar system."

Are we in danger from a rogue planet?

Yesterday, I wrote about a new study that indicates that free-floating planets in the Milky Way may outnumber planets orbiting stars, and even be more numerous than stars themselves. It's an amazing result! The most likely scenario is that these planets formed in solar systems similar to ours, but got ejected due to gravitational interactions with other planets in the system. These planets get literally tossed out into space, wandering the galaxy forever * .

This made me wonder: if these numbers are correct, how likely is it that such a rogue planet might actually be close by on a cosmic scale? And given the kind of topic I like to write about, are we in any danger from a close encounter with one of these galactic nomads? These wandering planets are so dark and distant they are currently essentially impossible to detect using regular techniques, so we don't know if any are in our galactic neighborhood or not. The only way to get a grip on how close one might be is to look at it in a statistical sense: on average in the galaxy, how many of these planets are there per cubic light year of space? Then we can fiddle with the number a bit to see how far away one of these planets could be. Let me be clear up front about something. No doubt there will be people who may want to claim these rogue planets might explain Nibiru or Planet X or the Mayan apocalypse. These people are wrong (again, and as usual). As you'll see, the math absolutely does not support such a claim at all. So if you hear someone talking doomsday, send ɾm here. And I might as well address the TLDR crowd: the conclusions I draw here are that a) on average, a rogue planet may be closer than I would've initially guessed, but 2) not nearly close enough to be a concern in any way. OK then, got it? Onward to the math!

Crank up the volume Basically, all we need to do is take the number of rogue planets in the galaxy and divide it by the volume of the galaxy, and that gives us the density of these planets in space: how many there are in a cube a light year on a side. If the answer is, say, 1 then we expect to have one rogue planet inside a one-light-year-wide cube centered on the Sun. So let's see what the math tells us. First, there are a lot of rogue planets. In the study, they say there are very roughly as many of them as there are stars in the Milky Way. Let's call it 200 billion.

Second, the volume of the galaxy isn't hard to estimate. I've done it before

, and the details are there if you want them. Let me cut to the chase: the Milky Way has a volume of roughly 2 x 10 ^13 cubic light years: that's 20 trillion cubic light years! That's a lot, too. Dividing them to find the density, we get:

2 x 10 ^11 planets / 2 x 10 ^13 cubic light years = 0.01 planets per cubic light year

In other words, Weɽ expect to find one of these wandering planets in a volume of space encompassing 100 cubic light years. That's a cube about 4.6 light years on a side (or, if you prefer, a sphere about 3 light years in radius). Hey, wow, wait a second: The nearest star, the Alpha Centauri triple star, is about 4.3 light years away. That means there's a pretty good chance that, statistically speaking, there may be one of these rogue planets closer to us than the nearest star! That's actually quite shocking to me. Seriously: wow. I've often wondered if weɽ ever find a brown dwarf -- a faint, "failed" star -- closer than Alpha Cen, but it never occurred to me there might be a planet closer by! That's pretty cool.

Stranger planet danger OK then. Are we in any danger from these puppies? Could one pass close enough to us to cause earthquakes, say? No! And I mean categorically, no . 100 cubic light years is a vast, mind-numbing volume of space: about 10 ^41 cubic kilometers! That's a huge amount of real estate to tool around in. Even if a planet got as close as a light year away -- ten trillion kilometers, or 6 trillion miles -- the effect on us would be essentially nothing. Remember, we're talking about planets with about the same mass as Jupiter. Our Jupiter gets as close as about 600 million kilometers from Earth. The Moon itself has no substantial effect on earthquakes, and at most Jupiter's effect is a tiny fraction of that. A planet ten thousand times farther away than Jupiter may as well not exist as far as gravitational effects are concerned.

Oh comets, where Oort thou? So we're safe from direct problems. What about indirect ones?

Out way past Neptune is a population of icy bodies that, when they fall toward the Sun, turn into comets. There may be a trillion of these guys within a light year or so from the Sun, in a region called the Oort cloud

. Could a rogue planet dislodge a bunch of these and drop them toward us, triggering impacts and a mass extinction? In a word, no. Again, the volume of space we're talking about here is staggering. Even a trillion comets spread out over that amount of space makes things pretty thin out there on average those objects are a billion kilometers apart. The odds of a planet getting close enough to dislodge even a single Oort cloud object is pretty small. And even if it did, it's only one comet! The odds of it hitting the Earth are even teenier. We're a pretty small target in a whole lot of solar system. And let's have a sanity check: if this were a real danger, weɽ see evidence of it in the fossil record. A planet-wide bombardment of giant comets -- even from a single big comet -- in recent geological history would be hard to miss. We don't see it, therefore this isn't a danger. I'm not saying asteroid and/or comet impacts aren't a danger at all, just that ones triggered by a rogue planet whizzing past us is incredibly small. We need to take impacts seriously in general, no matter what the cause. But in this case, interstellar planets doing the deed specifically aren't a worry.

You can breathe easy These results both surprise and don't surprise me. I'm very much surprised that one of these interstellar nomads could be closer than even the nearest star that's amazing . But I'm not surprised they pose no real danger. The solar system and the Earth are terribly old, and there's been lots of time for disasters to occur. If these planets were a real and immediate threat it seems clear weɽ have known about it long before now (as we know about, say, asteroid impacts). The very fact that life has been around for billions of years, and complex life for hundreds of millions, means rogue planets don't create cosmic calamities often enough to be a worry. In a nutshell: I'm not concerned about it. That is, in an "Oh my FSM we're all gonna die!" kind of concern. As a scientist, I find these objects totally fascinating. If there is one within a couple of light years, and it's still retained enough heat from its formation to glow in the infrared ( as I discussed in the post yesterday

), it may be possible to detect one directly in the next few years. It would be too far away to send a space probe (let alone visit), but with sensitive telescopes it's not crazy to think we might actually be able to actually see one. And that would be truly cool.

^* For those prone to worry, this is not going to happen here in our solar system. The planets ejected likely suffered their indignity when their systems were very young, and super-Jupiter-sized planets still migrated in toward their parent star. Our solar system should be pretty stable over the next few billion years.

Astroquizzical: Do all planets rotate in orbit around their stars?

If so, how come? And if not, what are the exceptions?

Do all planets rotate as they go around their stars? Do they all rotate in the same direction (e.g. clockwise or anticlockwise?) Or does it just depend on what started them rotating in the first place?

Before we can expand our thinking out to “all planets”, the easiest way to start looking at planets and how they rotate is to look at our own solar system, which we can investigate in far more detail (and far more easily) than anywhere else in the Universe. What we see in our own solar system is that all of the major planets are rotating around their own internal axis.

We’re well acquainted with the rotation of the Earth, even if we haven’t thought about it in this way — the Earth’s rotation is what creates our days. As a result, we all know how long it takes for the Earth to rotate once — 24 hours. But 24 hours isn’t the rule within our solar system in fact, of all the other planets, only Mars rotates at a similar speed. Mars completes one rotation in 24 hours and 40 minutes nearly identical to our home planet.

Venus, our planetary sister gone horribly awry, rotates much slower than the earth — one rotation takes 243 days and 26 minutes this makes it the slowest rotator in our solar system. Mercury comes in second slowest, and rotates once in 58 days, 15 hours, and 30 minutes. The gas giants all seem to have similar rotation speeds, all of which are faster than any of the inner, rocky worlds. Jupiter rotates once every 9 hours and 55 minutes. Saturn recently had its rotation speed re-measured to be 10 hours and 36 minutes. Uranus rotates once every 17 hours and 14 minutes, whereas Neptune rotates once every 16 hours and 6 minutes.

Most everything in our solar system rotates in the same direction — the same direction as the Earth. If we had a bird’s eye view of our solar system, where we’d flown into space “up” via the North Pole, and looked back down, most of the planets would be rotating counterclockwise — or from the West towards the East.

All of the rotation axes of the major planets (plus Pluto). A horizontal line drawn through the centre of the image would give the orbital plane around the Sun. Image credit: Calvin J. Hamilton.

You can usually remember which way the earth is rotating by thinking about the time zones the further east you go, the earlier it is — they’re pushed towards daylight sooner than the west.

So why does almost everything rotate in the same way? A hint of the answer lies in the Sun. The sun also rotates, and it rotates in the same direction as the majority of the planets — counterclockwise from our bird’s eye view. Since the Sun is also following our consistent rotation pattern, we’re going to have to make our way back to the formation of the solar system in order to make sense of this.

Our sun formed out of a cloud of gas and dust — enormous molecular clouds are the only places where, under the persistent pull of gravity over time, material gets dense enough to begin to collapse and form a star (or ten). However, during the collapse of the cloud of dust and gas, any initial motion in the cloud becomes quite important.

It’s very unusual for objects in the universe to be completely still in relation to each other (in fact, they would have to be at absolute zero to prevent any motion), so there’s a little bit of stirring about within the cloud that is always present. As the cloud collapses, the average direction of motion of the cloud is kept — and as gravity pulls everything closer to the center, the conservation of angular momentum causes that average motion to speed up. You can do this experiment at home if you have a chair that spins. If you start yourself turning a little bit with your arms and legs extended, and then pull your legs and arms in quickly, you’ll find yourself spinning much more rapidly than you were to start out. Pulling your arms in plays the same role that gravity is playing in the collapse of the early solar system.

Because the rotation speed is so much faster than it was initially, the gas forms into a flat disk that’s all rotating in the same direction. If you’ve ever played with a ball of bread dough or silly putty, and spun it on your finger, you’ll find it flattens out into a thin disk fairly quickly. The star at the center is therefore being formed in an environment where everything is already rotating — as the star collapses even more, its rotation will increase even more. All the proto-planets are also forming out of gas which is rotating, so it makes sense that they too will absorb the rotation of the disk they formed in. So in principle, we expect most planets to form with a rotation that’s consistent with the star at the center of their solar system.

However, “most everything” isn’t everything — we’ve got two notable exceptions within our solar system to the counterclockwise rotation rule: Venus and Uranus. Uranus rotates 90 degrees off from everything else. If you consider the plane of all the planets’ orbits around the sun as a flat surface, most planets spin as though they were a coin spun on its edge, flicked counterclockwise. Uranus, on the other hand, spins like a bead rolled along the ground, instead of spinning vertically. Venus is even odder- it spins clockwise. As far as we can tell, this means Venus is somehow upside down.

What happened to these planets? The picture I painted above indicates that they should have formed with a rotation aligned with every other planet in our solar system. In fact, they probably did form with a rotation that matched. However, the very early solar system was a violent place, filled with many more proto-planets which were inclined to collide with other objects. Some of these collisions were likely with the early versions of the planets which exist today. A particularly bad collision or series of collisions could cause such an energetic punch to the other planet that the planet was effectively tipped over onto its side (in the case of Uranus) or upside down completely (in the case of Venus.)

To expand to the Universe outside of our own solar system physics is the same everywhere in the Universe, so solar systems should form in the same way everywhere. However, we wouldn’t expect every planet to rotate in the same direction as its parent star, since — like our own solar system — the planets may have had a very collision-intensive early life.

If you have your own questions you’d like Astroquizzical to cover, you can submit them at Astroquizzical’s ask page!

Rogue planets wander the galaxy all alone

A newfound planet (seen here in an illustration) is about the mass of Earth and has no sun. Instead, it floats through the galaxy alone.

NASA/JPL-Caltech/R. Hurt (Caltech-IPAC)

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December 23, 2020 at 6:30 am

Not all planets orbit stars. Some zip through our galaxy all on their own. And now astronomers have found the smallest of these rogue planets yet.

The newly discovered wandering world has roughly the mass of Earth. With no sun in its sky, it’s always nighttime on this lonely planet. And that sky is a lot darker and filled with more stars than can be seen from any place on Earth.

“The sky must be marvelous,” says Przemek Mróz. He is an astronomer at Caltech in Pasadena, Calif. He led the team that discovered the planet. But the lack of a sun does come at a cost, he says. “It must be freezing cold, too.”

This drifter joins a small club. Over the last 20 years, astronomers have found fewer than two dozen planets without stars in our galaxy. Most are big balls of gas that are more like Jupiter than Earth. But scientists think these worlds are the tip of an enormous iceberg. In our galaxy alone, there might be billions out there awaiting discovery.

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A valuable planet

Finding this tiny planet is “very valuable,” says Diana Dragomir. She’s an astronomer at the University of New Mexico in Albuquerque. She looks for planets around stars other than our sun. This work is helping her figure out how many worlds other than Earth may be home to some type of life. There’s probably nothing alive on this dark, frigid orphan planet. But its discovery gives Dragomir and other scientists information about worlds that are difficult to find.

“The fact that we found it means a lot, even if it’s just floating, because it means it formed in the first place,” she says.

Astronomers think that orphaned planets formed in solar systems like our own. But something kicked the planet out. Maybe the gravity of a larger world gave a planet the boot. Or perhaps a passing star got too close, and its gravity snagged a planet or two.

Dragomir says this newfound world probably formed pretty far from its home star. If it were too close, then the star’s gravity would have kept it from escaping.

Planets, especially small ones, that are far from their stars are often tricky to find. “Even though we think that there are distant planets around many, many stars, we cannot know for sure,” says Dragomir. “Finding even just one like this one, using another technique, is really helpful because it’s adding to a pretty small sample.” She adds that the discovery of this wandering planet is “telling us these small planets at a reasonable distance from their star do form.”

When is a planet not a planet?

Most people think of planets as objects that orbit stars. In fact, the official definition says a planet must orbit a star. Specifically, our sun. Rogue planets don’t meet this definition, which was decided on by the International Astronomical Union, or IAU, notes Jessie Christiansen. She’s an astronomer at Caltech. Like Dragomir, she also is tallying all the types of planets that are out there. The IAU is the group that decides on official definitions and names for things in space.

Explainer: What is a planet?

But many now argue a planet should be defined only by how it formed. The IAU goes on to say that a planet is anything that’s big enough so that its gravity molds itself into a ball. Otherwise it would be a lumpy asteroid or comet. But the object can’t be so massive that it crushes together atoms and starts to glow. Then it would be a star.

Based on their mass, rogue planets pass inspection. But “our institutions have not caught up to the fact that these planets exist yet,” says Christiansen.

“NASA is literally rewriting the definition right now,” she says. NASA keeps track of planets found outside our solar system in a computer database. It’s called the NASA Exoplanet Archive. (An exoplanet is any planet that doesn’t orbit our sun.) But Dragomir says this database does not yet include orphan worlds. “We’re in the process of redesigning our archives so that we can host them,” she says.

Really low odds

Astronomers find most planets by detecting how they influence the stars they orbit. That, of course, won’t work for the orphans. They also don’t emit light, so astronomers can’t see them directly.

However, orphan planets can alter the light from stars that are much farther away. The process is known as gravitational lensing.

If something in space passes between Earth and a star, the object’s gravity focuses light from that star onto Earth. “It’s like a magnifying lens,” Mróz says. To someone on Earth, the star brightens as the object passes by. And that’s how researchers discovered this tiny rogue planet.

In June 2016, a faint star in the constellation Sagittarius brightened a bit. It then faded back to normal. Mróz and his team measured how long it took the star to brighten and dim. The change took about five hours. That told them the approximate mass of the passing object. They estimate that its mass could be as little as one-third the mass of Earth or as much as twice as massive as our planet. They shared their discovery November 1 in Astrophysical Journal Letters.

Mróz and his team noticed the planet with a telescope called OGLE. That stands for Optical Gravitational Lensing Experiment. The telescope sits in the Atacama Desert of Chile. It stares toward parts of our Milky Way that have lots of stars, such as the center of the galaxy. It then looks for changes in starlight caused by dark objects floating by.

The odds of finding just one object are, well, astronomical. The alignment between Earth, some object and a background star has to be almost perfect. “If you observed only one star, you would need to wait on average a million years” before anything passed by, says Mróz.

No one wants to wait that long. So to increase their chances, instead of watching one star, scientists watch millions. The OGLE telescope monitors the same 200 million stars every clear night, notes Mróz. That lets them find a couple thousand floaters every year, though most are just dim stars.

Researchers found the new rogue planet using the Optical Gravitational Lensing Experiment (OGLE) telescope, here, in Chile. Krzysztof Ulaczyk/Wikimedia Commons (CC BY-SA 2.5)

What’s next

This teeny planet pushes the limit of what telescopes like OGLE can do, says Mróz. To find lots more, astronomers need a telescope in space that’s up to the challenge.

That’s where the Nancy Grace Roman Space Telescope comes in. It’s due to launch around 2025. It will be as large as the Hubble Space Telescope, but it will see 100 times more of the sky at once. The new telescope is named after Nancy Grace Roman, NASA’s first chief astronomer. In 1959, she wrote that putting a telescope in space would let astronomers find planets around other stars. (Her namesake telescope won’t be the first such planet-finding telescope in space. The Kepler space telescope, for example, found more than 2,700 exoplanets before running out of gas in 2018.)

The Roman telescope will orbit far above Earth’s shaky atmosphere. From there, it will be able to find many roaming planets (and do lots of other science, too).

The Nancy Grace Roman space telescope will search for exoplanets, as well as do lots of other science. NASA

“Right now, we know very little about free-floating planets,” says Samson Johnson. He is an astronomer at The Ohio State University in Columbus. Recently, he and other scientists calculated how many floating planets the Roman telescope might find. They estimate it could find at least 250, some as tiny as Mars. They reported these results in the September Astronomical Journal.

Such discoveries could tell astronomers a lot about how planets form. Some of the solar systems in our galaxy show hints of past messiness. They are home to planets with orbits that are tilted and spaced out in strange ways. But other solar systems are neat and orderly.

“One of the questions going forward is, which is more common?” Christiansen says. If the Roman telescope turns up lots of floating planets, she says, then it may indicate planets get kicked out of their homes often. And that may mean that many young planetary systems are messy.

Even our own solar system was once chaotic. But for hundreds of years, astronomers assumed our solar system has always looked the way it does now: nice and organized. They also thought other planetary systems would be similar to ours. But the variety of worlds we’ve discovered, including orphan planets, shows this isn’t the case. And some scientists now think that that our solar system lost a planet long ago.

“One of the nice, most amazing, and exciting things that came out of exoplanets,” says Christiansen, “is discovering that there are so many different types of planetary systems out there.”

Power Words

archive: (adj. archival) To collect and store materials, including sounds, videos, data and objects, so that they can be found and used when they are needed. The term is also for the process of collecting and storing such things. People who perform this task are known as archivists.

astronomer: A scientist who works in the field of research that deals with celestial objects, space and the physical universe.

atmosphere: The envelope of gases surrounding Earth or another planet.

atom: The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

average: (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.

comet: A celestial object consisting of a nucleus of ice and dust. When a comet passes near the sun, gas and dust vaporize off the comet’s surface, creating its trailing “tail.”

constellation: Patterns formed by prominent stars that appear to lie close to each other in the night sky. Modern astronomers divide the sky into 88 constellations, 12 of which (known as the zodiac) lie along the sun’s path through the sky over the course of a year. Cancri, the original Greek name for the constellation Cancer, is one of those 12 zodiac constellations.

database: An organized collection of related data.

exoplanet: Short for extrasolar planet, it’s a planet that orbits a star outside our solar system.

focus: (in physics) The point at which rays (of light or heat for example) converge sometimes with the aid of a lens.

galaxy: A group of stars — and usually dark matter — all held together by gravity. Giant galaxies, such as the Milky Way, often have more than 100 billion stars. The dimmest galaxies may have just a few thousand. Some galaxies also have gas and dust from which they make new stars.

gravitational lensing: The distortion of light by an intense gravitational force, such as what can be exerted by clusters of galaxies — the most massive things in the universe. The gravity can bend or focus light, making it appear brighter and in one or more different places in the sky.

gravity: The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.

journal: (in science) A publication in which scientists share their research findings with experts (and sometimes even the public). Some journals publish papers from all fields of science, technology, engineering and math, while others are specific to a single subject. The best journals are peer-reviewed: They send all submitted articles to outside experts to be read and critiqued. The goal, here, is to prevent the publication of mistakes, fraud or sloppy work.

Jupiter: (in astronomy) The solar system’s largest planet, it has the shortest day length (10 hours). A gas giant, its low density indicates that this planet is composed of light elements, such as hydrogen and helium. This planet also releases more heat than it receives from the sun as gravity compresses its mass (and slowly shrinks the planet).

literally: A term that the phrase that it modifies is precisely true. For instance, to say: "It's so cold that I'm literally dying," means that this person actually expects to soon be dead, the result of getting too cold.

Mars: The fourth planet from the sun, just one planet out from Earth. Like Earth, it has seasons and moisture. But its diameter is only about half as big as Earth’s.

mass: A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.

Milky Way: The galaxy in which Earth’s solar system resides.

monitor: To test, sample or watch something, especially on a regular or ongoing basis.

NASA: Short for the National Aeronautics and Space Administration. Created in 1958, this U.S. agency has become a leader in space research and in stimulating public interest in space exploration. It was through NASA that the United States sent people into orbit and ultimately to the moon. It also has sent research craft to study planets and other celestial objects in our solar system.

optical: An adjective that refers to light or vision.

orbit: The curved path of a celestial object or spacecraft around a galaxy, star, planet or moon. One complete circuit around a celestial body.

planet: A large celestial object that orbits a star but unlike a star does not generate any visible light.

solar system: The eight major planets and their moons in orbit around our sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.

star: The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become hot enough, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.

sun: The star at the center of Earth’s solar system. It is about 27,000 light-years from the center of the Milky Way galaxy. Also a term for any sunlike star.

telescope: Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.


Journal:​ P. Mróz et al. A terrestrial-mass rogue planet candidate detected in the shortest-timescale microlensing event. The Astrophysical Journal Letters. Vol. 903, November 1, 2020. doi: 10.3847/2041-8213/abbfad.

Journal: N.G. Roman. Planets of other suns. The Astronomical Journal. Vol. 64, October 1959. doi: 10.1086/108038.

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How Many Planets in the Milky Way Can Support Life?

Scientists have estimated that 1 in 5 stars like our Sun has at least one Earth-like planet orbiting around them, which may support life. Based upon the mapping of our Milky Way, and through simulations, there are an estimated 40 billion planets that might support life in our Milky Way galaxy.

This, however, is an average number, as there might be many more. There are many different factors when considering this. Still, one aspect that could put our calculus on a different perspective is this: We only know how life adapts and evolves based on the species that live and lived on our planet.

There is no telling on what life is limited to in Outer Space, and thus, many planets that we might consider inhospitable to us might actually be hospitable to other unknown species. This might sound like science fiction, but there is a truth in this which cannot be denied we don’t know how life can evolve and adapt we don’t know its limits, shapes, and forms. What we do know is that life exists, adapts, and evolves.

The Rogue Planets That Wander the Galaxy Alone

Astronomers are searching for mysterious, free-floating worlds across the Milky Way.

The Milky Way is home to hundreds of billions of stars, and many more planets. Some come in sets, as in our own solar system. But not every planet orbits a star.

Some planets actually wander the galaxy alone, untethered. They have no days or nights, and they exist in perpetual darkness. In a kitschy NASA collection of travel posters for destinations beyond Earth, one of these cold worlds is advertised with the motto: “Visit the planet with no star, where the nightlife never ends.”

Astronomers call these worlds free-floating, or rogue, planets. They are mysterious objects, and a small group of researchers around the world is dedicated to studying them. Of the thousands of planets that scientists have detected beyond our solar system so far, only about a dozen are sunless and coasting on their own, somewhere between us and the center of the Milky Way. At least, astronomers think they are. “We are sure that these objects are planets,” Przemek Mroz, an astronomer at Caltech, told me. “We are not fully sure whether these objects are free-floating or not.”

Mroz has spent perhaps as much time thinking about these strange objects as anyone on Earth. He and his team just announced another finding—the smallest known rogue planet—today. The object is between the masses of Earth and Mars, a blip in interstellar space so relatively tiny that it might seem insignificant. But according to scientists’ best theories about the way planetary systems arise all across the universe, rogue worlds should exist.

The term rogue planet suggests that these objects desert their stars on purpose, striking out on their own to carve a new path through the Milky Way. In reality, rogue planets are usually kicked out of their star system, banished to a solitary existence circling the center of the galaxy.

The beginnings of a planetary system, including our own, are thought to be quite messy. As planets swirl into shape out of the cosmic fog surrounding a newborn star, they jostle one another around. The gravitational game of pool can shove planets toward the edges of a system, and even eject them altogether. Nearby stars can scramble planets too. Most stars are not born alone, but in clusters of dozens to thousands, and in such a crowded environment, a passing star with its own entourage of planets could whisk away a planet from another, keeping it for itself or casting it out into space.

Some solitary planets might form another way, without the help of a parent star. These worlds emerge from collapsed clouds of gas and dust, as stars do, but they don’t have enough mass to spark the nuclear reactions that make stars shine. These objects, known as “failed stars”—wow, astronomers—resemble planets from afar.

Rogue planets are extremely difficult to detect astronomers can’t search for them like they do exoplanets, which reveal their presence by gently tugging at their parent stars or briefly blocking out their light as they go around. On the loose and nearly invisible, rogue planets evade detection in much the same way that black holes do.

So astronomers rely on a cosmic quirk of gravity. Imagine a line of sight from Earth’s telescopes to a distant star. When an object crosses that line, its presence can bend and magnify the star’s light, making the star appear more luminous than usual to us. The duration of the brightening signals the nature of the object responsible—a brightening that lasts several days indicates a star, a day means a Jupiter-mass object, and hours suggest something equaling the mass of Earth. The rogue planet recently discovered by Mroz’s team signaled its existence for just a few hours.

The tricky part is figuring out whether rogue planets are, in fact, rogue. The stars whose light they bend can’t be their parent stars because they’re simply too far away. And even if a parent star were closer by, it would be impossible to see through the luminous star’s glare. Astronomers must wait years, usually a decade, for the luminous star to move before they can check for a parent star. If no such star appears, the planet is probably going solo. The process takes long enough that scientists haven’t reached this milestone for any of the dozen rogue-planet candidates, including the latest, tiniest addition.

Mroz and other astronomers studying rogue planets don’t know how many of these worlds might be coasting through the Milky Way, nor do they know much about the ones they’ve found so far. They can discern the mass of an object through their observation and compare it with worlds in our own solar system—objects with masses similar to those of Earth and Mars, for example, are probably rocky, while objects as massive as Neptune and Uranus are icy. But those analogies cannot fill in the details of rogue planets’ unknown surfaces, or the atmospheres that separate them from space.

There’s no doubt about one thing: Without a star to warm themselves by, rogue planets must be frozen—if not to their core, certainly at their outermost layer. They might not be so alone, either planets could take their moons with them when they’re hurled out of their cosmic homes.

As they roam through the galaxy, what can happen to rogue planets? Could a free-floating world find a home out there with a different star? Michael Liu, an astronomer at the University of Hawaii, thinks it’s unlikely. Interstellar space is quite, well, spacious, and it’s difficult for even a hefty star to slow down and lasso a fast-moving planet. In 2017, an interstellar asteroid the size of a skyscraper barreled right through our solar system and just kept going. “Normally, things just whiz by each other,” Liu says.

Could something bigger—an entire rogue planet—catch us by surprise as that asteroid did? The answer to this unnerving question depends on how common rogue planets are. “Do I worry about a free-floating planet hitting the solar system? No, but maybe I should?” says Jennifer Yee, an astrophysicist at Harvard and Smithsonian’s Center for Astrophysics who uses the same line-of-sight technique to find exoplanets. “It really depends on how many there are. If there are one per star, it isn’t very likely that we would run into one.”

A surprise visit from a rogue planet would present astronomers with a great research opportunity. It would also likely terrify the rest of us. “Probably we would be fine because the solar system itself is pretty empty,” Yee says. “On the other hand, depending on how massive the planet is, it might perturb the orbits of the existing planets, which could be bad.”

The orbits of our planets will someday become perturbed anyway. About 5 billion years from now, our sun—that glowing, life-giving, seemingly immutable orb—will start to die. The star will lose mass until it can no longer hold onto its outermost planets. Neptune and Uranus—and Pluto too—will probably become rogue planets. They will drift away, taking their icy atmospheres with them. Unbothered by the cold of interstellar space, the planets will remain mostly unchanged, relics of a solar system that once huddled close around a warm sun.

Earth will meet a different fate. Dying stars lose mass because they eject gas and dust in all directions, leaving exposed their spent cores. Our planet is expected to become enveloped in this hot mist and vaporized.

For now, Earth remains safely tucked into the solar system, on a cozy orbit from which we can look out at other, lonelier worlds. Astronomers are eager for the launch of a new telescope scheduled for the mid-2020s. The Nancy Grace Roman Space Telescope—named for NASA’s first female executive, recognized for helping make the agency’s best-known space telescope, the Hubble, a reality—will have an exquisite view of the night sky. Free from the atmosphere that often stymies ground telescopes, Roman, as the telescope is called, will peer toward the heart of the Milky Way, crowded with stars. A recent study predicted that Roman could detect hundreds of rogue planets, and would provide the best estimate for these worlds yet. Right now, estimates range from tens of billions to trillions.

Roman might find fewer true rogue planets than astronomers expect, or perhaps none at all. Their obsession could turn out to be a very minor footnote in the galaxy’s story. Or it could change our understanding of the place we live. Sam Johnson, a graduate astronomy student at the Ohio State University and the lead author of the Roman study, likes to imagine himself on one of these worlds, blanketed in pure darkness, the rest of the Milky Way stretching out in front of him. “They can feel pretty lonely, I would imagine,” Johnson says.

Rogue Planets Can Find Homes Around Other Stars

As crazy as it sounds, free-floating rogue planets have been predicted to exist for quite some time and just last year, in May 2011, several orphan worlds were finally detected. Then, earlier this year, astronomers estimated that there could be 100,000 times more rogue planets in the Milky Way than stars. Now, the latest research suggests that sometimes, these rogue, nomadic worlds can find a new home by being captured into orbit around other stars. Scientists say this finding could explain the existence of some planets that orbit surprisingly far from their stars, and even the existence of a double-planet system.

“Stars trade planets just like baseball teams trade players,” said Hagai Perets of the Harvard-Smithsonian Center for Astrophysics.

Astronomers now understand that rogue planets are a natural consequence of both star and planetary formation. Newborn star systems often contain multiple planets, and if two planets interact, one can be ejected in a form of planetary billiards, kicked out of the star system to become an interstellar traveler.

But later, if a rogue planet encounters a different star moving in the same direction at the same speed, be captured into orbit around that star, say Perets and Thijs Kouwenhoven of Peking University, China, the authors of a new paper in The Astrophysical Journal.

A captured planet tends to end up hundreds or thousands of times farther from its star than Earth is from the Sun. It’s also likely to have a, orbit that’s tilted relative to any native planets, and may even revolve around its star backward.

Perets and Kouwenhoven simulated young star clusters containing free-floating planets. They found that if the number of rogue planets equaled the number of stars, then 3 to 6 percent of the stars would grab a planet over time. The more massive a star, the more likely it is to snag a planet drifting by.

While there haven’t actually been planets found yet that are definitely a ‘captured’ world, the best bet would perhaps be a planet in a distant orbit around a low-mass star. The star’s disk wouldn’t contain enough material to form a planet that distant, Perets and Kouwenhoven said.

The best evidence of a captured planet comes from the European Southern Observatory, which announced in 2006 the discovery of two planets (weighing 14 and 7 times Jupiter) orbiting each other without a star.

“The rogue double-planet system is the closest thing we have to a ‘smoking gun’ right now,” said Perets. “To get more proof, we’ll have to build up statistics by studying a lot of planetary systems.”

As for our own solar system, there’s no evidence at this time that our Sun could have captured an alien world, which would lie far beyond Pluto.

“There’s no evidence that the Sun captured a planet,” said Perets. “We can rule out large planets. But there’s a non-zero chance that a small world might lurk on the fringes of our solar system.”

Watch the video: Η περιστροφή της Γης. Ημέρα και Νύκτα. (June 2022).