What is the biggest known planet in the Universe?

What is the biggest known planet in the Universe?
VLT NACO image, taken in the Ks-band, of GQ Lupi. The feeble point of light to the right of the star is the newly found cold companion. It is 250 times fainter than the star itself and it located 0.73 arcsecond west. At the distance of GQ Lupi, this corresponds to a distance of roughly 100 astronomical units. North is up and East is to the left.   Credit:    ESO

VLT NACO image, taken in the Ks-band, of GQ Lupi. The feeble point of light to the right of the star is the newly found cold companion. It is 250 times fainter than the star itself and it located 0.73 arcsecond west. At the distance of GQ Lupi, this corresponds to a distance of roughly 100 astronomical units. North is up and East is to the left. Credit: ESO

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Our list of known planets and exoplanets unfortunately doesn’t extend much beyond our own Milky Way galaxy - to spot a planet, you need to be able to measure the light from an individual star and monitor it over time. You’re looking either for tiny flickers in the amount of light you receive, as a planet happens to pass in front of the star you’re watching, or you’re looking for there to be a little Doppler shift in the color of the star’s light, as the planets tug it slightly off center as they orbit.  Known by the names of the transit method and the Doppler shift method respectively, both of these require really careful observations over a significant amount of time, without the light from the star mixing with the light from other stars. This limits us pretty well to the stars within or surrounding our Milky Way.

Because the measurements required to spot planets must be so precise, generally the telescopes we send out to do these measurements only look at a small patch of the sky. So while I can give you our current high scoring planets, there’s no guarantee these will remain the all-time bests, if we point our telescopes in a new direction.

There is one fundamental limitation to how massive a planet can get - if you pack too much material into a planet, it will start to fuse elements in its core, and it formally becomes a star instead of a planet. This transition happens when the object is somewhere in the range of 13 to 80 times the mass of Jupiter, and is the point at which we typically start calling objects a brown dwarf star, orbiting another star, instead of a planet. The list of biggest planets can also change if we get better measurements. It's possible to learn that what we thought was a planet should really be called a brown dwarf, which then bumps that object off the list of biggest planets, and onto the list of known brown dwarfs.

This artist's conception illustrates what a "Y dwarf" might look like. Y dwarfs are the coldest star-like bodies known, with temperatures that can be even cooler than the human body. Image credit:  NASA/JPL-Caltech

This artist's conception illustrates what a "Y dwarf" might look like. Y dwarfs are the coldest star-like bodies known, with temperatures that can be even cooler than the human body. Image credit: NASA/JPL-Caltech

However, you can still have very large, fluffy planets, well before they get to this boundary of being a star. Most of the ones we know about are Jupiter like in style - massive, gaseous planets, orbiting distant stars. The easiest to find are hot Jupters - exoplanets which are not only bigger than Jupiter, they’re much closer into their star than Jupiter is to our Sun. Currently, the majority of the biggest, fluffiest planets are about twice the radius of Jupiter. Considering that you could stack 22 and a half Earths edge to edge to match the width of Jupiter, you’re looking at a planet so large, you could line up 45 Earths behind it, and not see any of them. These planets have the very pronounceable names of ROXs 42Bb, which is estimated to be about 2.5 times the size of Jupiter, or Kepler-13 Ab, which sits around 2.2 times the size of Jupiter.

There are some larger ones, but these have preliminary estimates of their size, and may yet turn out to be brown dwarfs. The current record holder is a planet orbiting a star known as GQ Lupi, and estimates place it at somewhere around 4 times larger than Jupiter. This particular object is so large that our theoretical models of how it has formed are not particularly happy, and so the estimates on its size and mass are both pretty hazy. It is likely to remain a planet, but if it turns out that its mass is on the high end of our current estimates, it could wind up on a brown dwarf list. (This object is also extremely young, and will change and compress as it evolves.)

Artist's impression of the simultaneous stellar eclipse and planetary transit events on Kepler-1647.  Credits: Lynette Cook

Artist's impression of the simultaneous stellar eclipse and planetary transit events on Kepler-1647. Credits: Lynette Cook

These big fluffy planets are orbiting your default solar system - one with a single star, around which all the planets orbit.  If you have two stars (which isn’t that uncommon), it seems to be much harder to build very large planets. The largest planet known to circle two stars at once was only confirmed in 2016, and is almost identical to Jupiter in size. At “only” 22.5 Earths in size, it orbits its parent star once every three years.


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Would A Brown Dwarf Near Us Cook The Earth?

 

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How hot are brown dwarf stars when they are burning deuterium?

A few definitions are in order!

Deuterium is hydrogen, plus a neutron. Most hydrogen in the universe, and on Earth, is one proton, one electron, and no neutrons. It’s the simplest atom out there, and hydrogen in this form is the most abundant atom in the universe. Deuterium, with its one proton, one electron, and one neutron, is much less common than its simpler counterpart - it’s only 0.016% of all the hydrogen out there. That’s approximately 160 atoms of deuterium for every million standard hydrogen atoms.

The other fun fact about deuterium is that it’s effectively not produced by any ongoing natural process. Stars don’t create it - in fact, they actively destroy deuterium. So, we think all the deuterium in the universe was created in the Big Bang, and we’ve been slowly eating away at it ever since.

Brown dwarfs, meanwhile, are little balls of gas that didn’t quite make it to being a star. They don’t have enough mass to create the temperatures and pressures in their cores needed to start burning hydrogen the way our sun does. Brown dwarfs are typically so much smaller than the rest of the stars in the universe that instead of being weighed in units of “stellar masses”, which are multiples of or fractions of the mass of our sun, we use “Jupiter masses”. Brown dwarfs tend to come in somewhere between 13 times the mass of Jupiter and about 83 times the mass of Jupiter. The upper limit here is the mass required to ignite hydrogen burning - in different units, this is 0.08 times the mass of the sun. The lower mass limit is a bit fuzzier, and that’s when you tend to run into the issue of “Is this a really large planet or is it a star”, particularly if it’s hanging around another star that is much larger than it is. You can have brown dwarfs that are smaller than 13 Jupiter masses, and planets larger than 13 Jupiter masses - it depends what’s going on inside of the object.

By definition, brown dwarfs can’t burn hydrogen, but it turns out that they can burn deuterium. Deuterium burns at lower temperatures and pressures, so if the brown dwarf is above the 13 Jupiter-mass cutoff, the internal pressure of a brown dwarf can trigger the start of deuterium burning. The temperature at which this happens is about 10^6 Kelvin - one million degrees. Keep in mind that this is the temperature at the very core of the star, not at the surface! By the time you get to the surface of the star, you’re down to somewhere between 2000 degrees Kelvin and 750 degrees Kelvin, depending on the size of your brown dwarf.

Remember that there are still only 160 atoms of deuterium per million atoms of hydrogen - these brown dwarfs are made of this same mixture. This means of course, that there is not a lot of deuterium around in the star to be burned. Most brown dwarfs race through their deuterium in about 100 million years - a flash, in cosmic time. (By comparison, our sun will be stable for about 8 billion years.) Once the star has burned all the deuterium it can reach, it’s burning days are over. The heat it built up through burning deuterium will slowly fade away, and the dwarf will truly be a “failed star”.

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What kinds of dwarf star are there?

What different types of dwarf star are there and could you mention a bit about each? e g. Some brown dwarf stars have liquid iron raining down on them.

“Dwarf” was originally a term used to distinguish between the two kinds of red stars in the universe - very massive, and very small. These were termed “red giants” and “red dwarfs”. The dwarf terminology gradually expanded to mean “not giant” stars of any colour, and the line between “giant” and “dwarf is somewhat poorly defined; the Sun is technically a "yellow dwarf” star.

What most people think of when they hear “dwarf star” are brown dwarf, red dwarf, and white dwarf stars. There are also a few theoretical kinds of dwarf stars, which is where black dwarfs fall. These stars are all classified based upon their colour, although confusingly these are not usually the colors they would appear to our eyes. (Brown dwarfs, for instance, would appear a deep pink - see above for 3 brown dwarfs as they would appear to us.)

Yellow and red dwarf stars are normal stars - they burn hydrogen in their cores and live on the main sequence of stellar lifetimes. Red dwarfs are smaller than our sun, only getting up to 50% the size of our sun. As a result, their surfaces are cooler, hence the colour shift towards the red. They don’t consume their hydrogen as quickly as our sun does, so even though they’re less massive and thus have less hydrogen, they still live for a much longer time than our sun will. Because red dwarfs require less matter to create, they are the easiest to make. Red dwarfs are therefore the most abundant type of star in the galaxy - our nearest stellar neighbor is a red dwarf.

Brown dwarfs are failed stars. They’re essentially massive Jupiters - large collections of gas that are not massive enough to create the pressures required to start burning hydrogen into helium. These dwarfs can be pretty cold; there was one found not too long ago that was only as warm as a cup of coffee. A brown dwarf can’t do anything except sit there and slowly radiate away its heat - it won’t ever become a fully fledged star. The iron rain you refer to was the conclusion of a study from 2006; evidence was found that at the temperatures of the star they were looking at, the iron they detected in its atmosphere should be forming liquid droplets and raining down towards the surface of the star. Further studies have found evidence for massive, Jupiter-style storms in the atmospheres of these stars. The behavior of the metals and other elements in a brown dwarf’s atmosphere will depend strongly on the temperature of the star in question. Since “brown dwarf” is a rather broad term, some of these stars will be too cold for iron rain, and some will be too warm. Of course, the presence or absence of a particular element will depend on the gas the dwarf formed out of, since the brown dwarf is not building any new elements itself.

White dwarfs are the most exciting to make. They are what is left over after a main sequence star (like our Sun) dies. The star will have gone through the red giant phase, and then shrugs off its less dense outer layers into a planetary nebula. At the end, all that is left is a hot, dense core of what was once the centre of the star in a volume about that of the Earth. They are so dense that the pressure provided by the electrons of the atoms within the star pushing against each other is what keeps them from becoming any smaller, and so hot they glow white just from trapped heat. This is the end-point of our Sun.

The black dwarf - still a theoretical object - is the name we would give to a white dwarf star which had managed to completely lose all of its heat, effectively going completely out. The length of time it takes for a white dwarf star to lose all of its heat is longer than the length of time the Universe has been around, so we don’t expect to see many of these around.

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