# Which is the largest planet ever observed?

I don't know much about planets outside our solar system. Do we even know the limit of the size of how large a planet or satellite can be? The largest planet I know of is Jupiter, but could there be planets as large as our Sun or even larger? Is there a theoretical limit to the size of a planet?

Yes, there is a limit. Anything with a mass larger than about 13 times that of Jupiter would be called a brown dwarf (a failed star), though whether such an object would consist entirely of gas, or had a rocky/icy core as is probable for most giant planets, is not presently observable. Any larger than about 75 Jupiter masses and we would just call it a star. The exact definition of what a planet is (especially, the 13 Jupiter mass boundary) is still disputed.

Of the bonafide planets that have been detected and confirmed, the catalogue at exoplanets.org lists Kepler-435b as the one with the largest measured radius (although its radius error bar overlaps with that of other planets). The quoted radius is $1.99 pm 0.18$ times that of Jupiter.

Most giant planets have very similar radii for masses between about 0.5 and 10 times that of Jupiter. The reason for this is that they are largely supported by electron degeneracy pressure. The diversity in the radii (between about 0.7 and 2 times that of Jupiter) of such planets is not yet fully understood.

The plot below shows mass vs radius for "planets". The smaller (probably rocky/icy) planets do show a trend of increasing radius with mass (the solid line is where a theoretical relationship for rocky/icy planets has been used to estimate the mass from the radius). The gas giants above about 0.5 Jupiter masses show no trend and a small scatter.

## 'Super−Jupiter' sized planet found near massive star

Toronto, Nov 20 (IANS) A team of Canadian astrophysicists has discovered a 'super−Jupiter' sized planet around the massive star Kappa Andromedae, having a mass at least 13 times that of Jupiter and an orbit somewhat larger than Neptune's. The object could represent the first new observed exoplanet system in almost four years.

Toronto, Nov 20 (IANS) A team of Canadian astrophysicists has discovered a 'super−Jupiter' sized planet around the massive star Kappa Andromedae, having a mass at least 13 times that of Jupiter and an orbit somewhat larger than Neptune's.

The object could represent the first new observed exoplanet system in almost four years. The host star around which the planet orbits has a mass 2.5 times that of the Sun, making it the highest mass star to ever host a directly observed planet.

The star can be seen with the naked eye in the constellation Andromeda at a distance of about 170 light years, the Astrophysical Journal Letters reports.

"Our team identified a faint object located very close to Kappa Andromedae in January that looks much like other young, massive directly imaged planets but does not look like a star," said study co−author Thayne Currie, post−doctoral fellow in the Department of Astronomy & Astrophysics at the University of Toronto.

The researchers made the discovery based on an infrared imaging search carried out as part of the Strategic Explorations of Exoplanets and Disks with Subaru (SEEDS) programme using the Subaru telescope located in Hawaii, according to a Toronto statement.

"Kappa Andromedae moves fast across the sky so it will appear to change position relative to more distant, background objects," Currie says.

"When we re−observed it in July at multiple wavelengths, we saw the faint object again, located at about the same position as it was in January.

This indicates that it is bound to the star and not an unrelated background object. Labelled by the researchers as Kappa And b, it could be the first direct rendering of an exoplanet in two years and of a new exoplanet system in almost four years, ending a significant drought in the field.

## Scientists Find Largest Rocky Planet Ever Discovered, Half as Big as Neptune

The Kepler Spacecraft, which has the ability to observe distant solar systems, has discovered a rocky planet bigger than scientists ever thought was possible. Meet BD+20594b.

##### Rocky Competition

The former title holder of “biggest rocky planet” went to Kepler-10c, which has a radius more than double that of Earth. We thought this was as big as rocky planets could get.

Most of our understanding of planetary formation and solar system development has come from direct observation of our own Solar System. We simply couldn’t see any others, and we had no way of knowing how typical—or how strange—our own Solar System might be.

With the discovery of BD+20594b, though, we’re learning, and Kepler-10c has some intense competition. The new planet is about 16 times as massive as Earth and half the diameter of Neptune. Its density is about 8 grams per cubic centimeter (for comparison, Earth is 5.514 g/cm). This is rather dense, but it makes sense, as it’s composed entirely of rock.

The planet, whose existence was reported on January 28 at arXiv.org by astrophysicist Nestor Espinoza and his colleagues at the Pontifical Catholic University of Chile in Santiago, is over 500 light years away, in the constellation Aries. The planet was discovered last year, as it passed between Kepler and its host star. Luckily, the planet’s host star is exceptionally bright, allowing a more detailed observation than most exoplanets.

##### The Importance of BD+20594b and Kepler

The discovery and further study of BD+20594b is important for a couple of reasons. It shows that there is still so much to learn about planetary formation. Kepler’s mission, which started off just confirming the existence of exoplanets, and showing us how common they are, will play a key role in furthering that research.

In addition, there’s much more variety in planetary composition than just what we see in our own solar system. Now that we have this new planet to compare to Kepler-10c, astrophysicists can do further research on planetary formation theories.

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## 'Super-Jupiter' Discovery Dwarfs Solar System's Largest Planet

In a rare direct photo of a world beyond Earth, astronomers have spotted a planet 13 times more massive than Jupiter, the largest planet in our own solar system.

The planet orbits a star called Kappa Andromedae that is 2.5 times the mass of the sun and is located 170 light-years away from Earth. As a gas giant larger than Jupiter, it's classified as a "super-Jupiter."

Astronomers say the object's immense size places it right on the edge of the classifications for giant planets and a type of failed star known as a brown dwarf. Its official name is Kappa Andromedae b, or Kappa And b for short, and it likely has a reddish glow, researchers said.

"According to conventional models of planetary formation, Kappa And b falls just shy of being able to generate energy by fusion, at which point it would be considered a brown dwarf rather than a planet," Michael McElwain, a member of the discovery team at NASA's Goddard Space Flight Center in Greenbelt, Md., said in a Nov. 19 statement. "But this isn't definitive, and other considerations could nudge the object across the line into brown dwarf territory." [Super-Jupiter Discovery in Photos (Gallery)]

The object is an interesting test case for theories of planet formation, scientists say. Based on observations of this system, the super Jupiter appears to have formed in the same way ordinary, lower-mass exoplanets do, by coalescing from a "protoplanetary disk" of material orbiting a nascent star.

That's because its orbit, somewhat wider than the path Neptune takes around our sun, is at a comparable distance to planetary orbits in the solar system. Additionally, its star, Kappa Andromedae, is relatively young, at about 30 million years old (for comparison, the sun is roughly 5 billion years old). These clues point toward a formation story typical of smaller planets.

Previously, some scientists had doubted that such large stars could give birth to planets in protoplanetary disks. The new finding indicates that this star probably did just that.

The new photo was snapped by Japan's Subaru 8-meter telescope on the summit of Mauna Kea in Hawaii. Alien planets are extremely difficult to image directly because their stars are always so much brighter, and outshine any planets.

To capture this picture, astronomers looked in infrared light, and used a technique to hide the glare from the star in order to reveal the relatively faint dot of light from the planet. More than 800 planets have been discovered beyond the solar system, but only a handful have been imaged directly so far.

The parent star of Kappa And b, Kappa Andromedae, can be seen by stargazers from Earth without requiring a telescope, NASA officials said. The star can be easily seen in the night sky above the constellation of Pegasus from suburban areas, they added.

The discovery will be published in an upcoming issue of the Astrophysical Journal Letters.

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## StarLust

My name is Tom and I’m the creator of this website. If you ended up here while looking to learn more about space, astronomy and stargazing… welcome home.

Today, we know that our solar system is just one tiny part of a supermassive universe. A long time ago, we discovered that neither Earth nor the Sun is at the centre of the universe but our hunger for exploration is stronger than ever.

For people like you and I, it will never be possible to travel to those distant worlds and discover these uncharted territories. The only way for us to get a glimpse of the universe is through the eyepiece of a telescope.

Anyone interested in stargazing will probably face two obstacles: the amount of technical knowledge required for this hobby as well as the cost. StarLust aims to help you learn how to observe every celestial objects in the night sky without ruining your bank account.

Since the time of Copernicus, Kepler, and Galileo, we have learned a lot more about the cosmos. More than 3,600 extrasolar planets have been identified and the rate of discovery is increasing rapidly. This is an exciting time to start learning how to use a telescope.

## Bizarre Largest Rocky Planet

Over the past decade, astronomers have spotted thousands of exoplanets among the galaxy's star fields. Most of them fall into categories such as "hot Jupiters" or a big, gassy planet on tight orbits, and some are classified as "super-Earths," rocky planets bigger than Earth but smaller than Neptune.

But TOI-849b is neither of these categories.

Spotted by NASA's Transiting Exoplanet Survey Satellite (TESS), used for planet-hunting, this equipment is searching for 200,000 of the nearest and brightest stars. It discovered TOI-849b when the planet crossed its star's face and briefly blotted out a hint of starlight.

According to the National Geographic, the alien world revolves around its star every 18 hours, which means that its surface temperature could be at 2,800 degrees Fahrenheit or 1,538 degrees Fahrenheit.

Furthermore, TESS observations also showed that the planet is 3.4 times as wide as Earth and 85% as wide as Neptune. This makes its size unusual, given its proximity to its star. Astronomers have only observed hot Jupiters and super-Earths in tight orbits until now, but nothing as bizarre as this planet.

"There really are no planets of that mass there," says Jonathan Fortney, director of the Other Worlds Laboratory at the University of California Santa Cruz.

But while the planet is roughly as wide as Neptune, observation from the HARPS instrument reveals that it is at least twice as massive.

## The seven most extreme planets ever discovered

Scientists recently discovered the hottest planet ever found – with a surface temperature greater than some stars. As the hunt for planets outside our own solar system continues, we have discovered many other worlds with extreme features. And the ongoing exploration of our own solar system has revealed some pretty weird contenders, too. Here are seven of the most extreme.

The hottest

How hot a planet gets depends primarily on how close it is to its host star – and on how hot that star burns. In our own solar system, Mercury is the closest planet to the sun at a mean distance of 57,910,000km. Temperatures on its dayside reach about 430°C, while the sun itself has a surface temperature of 5,500°C.

### Recommended

But stars more massive than the sun burn hotter. The star HD 195689 – also known as KELT-9 – is 2.5 times more massive than the sun and has a surface temperature of almost 10,000°C. Its planet, KELT-9b, is much closer to its host star than Mercury is to the sun.

Though we cannot measure the exact distance from afar, it circles its host star every 1.5 days (Mercury’s orbit takes 88 days). This results in a whopping 4300°C – which is hotter than many of the stars with a lower mass than our sun. The rocky planet Mercury would be a molten droplet of lava at this temperature. KELT-9b, however, is a Jupiter-type gas giant. It is shrivelling away as the molecules in its atmosphere are breaking down to their constituent atoms – and burning off.

The coldest

At a temperature of just 50 degrees above absolute zero – -223°C – OGLE-2005-BLG-390Lb snatches the title of the coldest planet. At about 5.5 times the Earth’s mass it is likely to be a rocky planet too. Though not too distant from its host star at an orbit that would put it somewhere between Mars and Jupiter in our solar system, its host star is a low mass, cool star known as a red dwarf.

The planet is popularly referred to as Hoth in reference to an icy planet in the Star Wars franchise. Contrary to its fictional counterpart, however, it won’t be able to sustain much of an atmosphere (nor life, for that matter). This because most of its gases will be frozen solid – adding to the snow on the surface.

The biggest

If a planet can be as hot as a star, what then makes the difference between stars and planets? Stars are so much more massive than planets that they are ignited by fusion processes as a result of the huge gravitational forces in their cores. Common stars like our sun burn by fusing hydrogen into helium. But there is a form of star called a brown dwarf, which are big enough to start some fusion processes but not large enough to sustain them. Planet DENIS-P J082303.1-491201 b with the equally unpronounceable alias 2MASS J08230313-4912012 b has 28.5 times the mass of Jupiter – making it the most massive planet listed in NASA’s exoplanet archive. It is so massive that it is debated whether it still is a planet (it would be a Jupiter-class gas giant) or whether it should actually be classified as a brown dwarf star. Ironically, its host star is a confirmed brown dwarf itself.

The smallest

Just slightly larger than our moon and smaller than Mercury, Kepler-37b is the smallest exoplanet yet discovered. A rocky world, it is closer to its host star than Mercury is to the sun. That means the planet is too hot to support liquid water and hence life on its surface.

PSR B1620-26 b, at 12.7 billion years, is the oldest known planet. A gas giant 2.5 times the mass of Jupiter it has been seemingly around forever. Our universe at 13.8 billion years is only a billion years older.

PSR B1620-26 b has two host stars rotating around each other – and it has outseen the lives of both. These are a neutron star and a white dwarf, which are what is left when a star has burned all its fuel and exploded in a supernova. However, as it formed so early in the universe’s history, it probably doesn’t have enough of the heavy elements such as carbon and oxygen (which formed later) needed for life to evolve.

The youngest

### Recommended

The planetary system V830 Tauri is only 2m years old. The host star has the same mass as our sun but twice the radius, which means it has not fully contracted into its final shape yet. The planet – a gas giant with three quarters the mass of Jupiter – is likewise probably still growing. That means it is acquiring more mass by frequently colliding with other planetary bodies like asteroids in its path – making it an unsafe place to be.

The worst weather

Because exoplanets are too far away for us to be able to observe any weather patterns we have to turn our eyes back to our solar system. If you have seen the giant swirling hurricanes photographed by the Juno spacecraft flying over Jupiter’s poles, the largest planet in our solar system is certainly a good contender. However, the title goes to Venus. A planet the same size of Earth, it is shrouded in clouds of sulfuric acid.

The atmosphere moves around the planet much faster than the planet rotates, with winds reaching hurricane speeds of 360km/h. Double-eyed cyclones are sustained above each pole. Its atmosphere is almost 100 times denser than Earth’s and made up of over 95% carbon dioxide. The resulting greenhouse effect creates hellish temperatures of at least 462°C on the surface, which is actually hotter than Mercury. Though bone-dry and hostile to life, the heat may explain why Venus has fewer volcanoes than Earth.

Christian Schroeder is a lecturer in environmental science and planetary exploration at the University of Stirling. This article was originally published on The Conversation (www.theconversation.com)

## Planet With Largest Solid Core Ever Discovered

NASA researchers recently discovered the largest solid core ever found in an extrasolar planet, and their discovery confirms a planet formation theory.
"For theorists, the discovery of a planet with such a large core is as important as the discovery of the first extrasolar planet around the star 51 Pegasi in 1995," said Shigeru Ida, theorist from the Tokyo Institute of Technology, Japan.

When a consortium of American, Japanese and Chilean astronomers first looked at this planet, they expected one similar to Jupiter.

"None of our models predicted that nature could make a planet like the one we are studying," said Bun'ei Sato, consortium member and postdoctoral fellow at Okayama Astrophysical Observatory, Japan.

Scientists have rarely had opportunities like this to collect such solid evidence about planet formation. More than 150 extrasolar planets have been discovered by observing changes in the speed of a star, as it moves toward and away from Earth. The changes in speed are caused by the gravitational pull of planets.

This planet also passes in front of its star and dims the starlight.

"When that happens, we are able to calculate the physical size of the planet, whether it has a solid core, and even what its atmosphere is like," said Debra Fischer. She is consortium team leader and professor of astronomy at San Francisco State University, Calif.

The planet, orbiting the sun-like star HD 149026, is roughly equal in mass to Saturn, but it is significantly smaller in diameter.

It takes just 2.87 days to circle its star, and the upper atmosphere temperature is approximately 2,000 degrees Fahrenheit. Modeling of the planet's structure shows it has a solid core approximately 70 times Earth's mass.

This is the first observational evidence that proves the "core accretion" theory about how planets are formed. Scientists have two competing but viable theories about planet formation.

In the "gravitational instability" theory, planets form during a rapid collapse of a dense cloud.

With the "core accretion" theory, planets start as small rock-ice cores that grow as they gravitationally acquire additional mass. Scientists believe the large, rocky core of this planet could not have formed by cloud collapse. They think it must have grown a core first, and then acquired gas.

"This is a confirmation of the core accretion theory for planet formation and evidence that planets of this kind should exist in abundance," said Greg Henry, an astronomer at Tennessee State University, Nashville. He detected the dimming of the star by the planet with his robotic telescopes at Fairborn Observatory in Mount Hopkins, Arizona.

Support for this research came from NASA, the National Astronomical Observatory of Japan and the National Science Foundation.

A paper about this discovery was accepted for publication in the Astrophysical Journal.

## New Planet Is Largest Discovered That Orbits Two Suns

If you cast your eyes toward the constellation Cygnus, you’ll be looking in the direction of the largest planet yet discovered around a double-star system. It’s too faint to see with the naked eye, but a team led by astronomers from NASA’s Goddard Space Flight Center and San Diego State University used the Kepler Space Telescope to identify the new planet, Kepler-1647 b. The discovery was announced today in San Diego, at a meeting of the American Astronomical Society.

Planets that orbit two stars are called circumbinary planets, or sometimes “Tatooine” planets, after Luke Skywalker’s homeland in “Star Wars.” Using NASA’s Kepler telescope, astronomers look for slight dips in brightness that hint a planet might be transiting in front of a star, blocking some of the star’s light.

“But finding circumbinary planets is much harder than finding planets around single stars,” said SDSU astronomer William Welsh, one of the paper’s coauthors. “The transits are not regularly spaced in time and they can vary in duration and even depth.”

Once a candidate planet is found, researchers employ advanced computer programs to determine if it really is a planet. It can be a grueling process. Laurance Doyle, a coauthor on the paper and astronomer at the SETI Institute, noticed a transit back in 2011. But more data and several years of analysis were needed to confirm the transit was indeed caused by a circumbinary planet. A network of amateur astronomers in the KELT Follow-Up Network provided additional observations that helped the researchers estimate the planet’s mass. The research has been accepted for publication in the Astrophysical Journal with Veselin Kostov, a NASA Goddard postdoctoral fellow, as lead author. This work was supported in part by grants from NASA. A preprint of the paper can be found here.

Kepler-1647 b is 3,700 light-years away and approximately 4.4 billion years old, roughly the same age as the Earth. The stars are similar to the Sun, with one slightly larger than our home star and the other slightly smaller. The planet has a mass and radius nearly identical to that of Jupiter, making it the largest transiting circumbinary planet ever found.

“It’s a bit curious that this biggest planet took so long to confirm, since it is easier to find big planets than small ones,” said SDSU astronomer Jerome Orosz, another coauthor on the study. “It took so long to confirm because its orbital period is so long.”

The planet takes 1,107 days (just over 3 years) to orbit its host stars, the longest period of any confirmed transiting exoplanet found so far. The planet is also much further away from its stars than any other circumbinary planet, breaking with the tendency for circumbinary planets to have close-in orbits. Interestingly, its orbit puts the planet within the so-called habitable zone. Like Jupiter, however, Kepler-1647 b is a gas giant, making the planet unlikely to host life. Yet if the planet has large moons, they could potentially be suitable for life.

“Habitability aside, Kepler-1647 b is important because it is the tip of the iceberg of a theoretically predicted population of large, long-period circumbinary planets,” Welsh said.