What Is The Largest Planet Ever Found?

It didn’t take long from the invention of the telescope to realize that Jupiter is the largest planet in the solar system, leaving it the largest planet we knew for almost 400 years. Now however, with so many exoplanets (planets beyond the Solar System) its size record has been broken many times.

Nevertheless, we don’t have a neat answer to the question of which of these discoveries is the largest; instead, there are several contenders.

First What Is A Planet?

Most of the controversy when the International Astronomical Union (IAU) created a definition for planets in 2005 focused on those excluded at the smaller end, specifically Pluto . However, there can be questions in relation to very large objects as well.

We consider a planet to be a body that orbits a star (or has a mutual orbit with it ), but is not massive enough to be causing fusion at its core, as another star would. Additional requirements are that it has achieved hydrostatic equilibrium (what most of us call being almost spherical) and dynamically dominates its region. Debate continues about what domination really means, but that’s a problem for the smaller planetary candidates, not the big ones.

The trickiness for bigger objects is that it is easier to fuse deuterium than hydrogen. This creates the category of objects known as brown dwarfs , which have enough gravitational pressure to fuse deuterium atoms at the core, but not hydrogen. Since deuterium is rare, brown dwarfs produce a tiny fraction of the light of a proper star, leaving them quite hard to distinguish from ordinary planets.

Some astronomers would like to see brown dwarfs included as planets , and indeed some go further and argue for the category to extend to white dwarfs. If so, white dwarfs would be the smallest, but most massive, planets we have found.

However, the IAU rejects both expansions to the definition. It defines the boundary as the; “Limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity)”. As the last part of that statement indicates, the mass required to initiate fusion depends somewhat on composition. Without knowing the ratio of hydrogen to heavier elements, there’s a range where we can’t distinguish planets and brown dwarfs.

The IAU definition also excludes objects that are too large in relation to whatever they orbit. That’s relevant in the case of so-called “ rogue planets ”, those that float freely through the galaxy, without a star or brown dwarf to guide them. There is speculation there may be more “rogue planets” than ordinary ones, but they are much harder to find. Nevertheless, almost 100 were recently spotted in the Orion Nebula.

Some of the rogue planets we have found are very large – well into the size range where they might be brown dwarfs (see 2M1207-3900 for example). Rather than argue with the IAU, (at least until they change their definition ) we will exclude these from our consideration.

So Which Is The Biggest? And The Most Massive?

Even having narrowed the competition down like this, the truest answer is: we don’t know. There’s uncertainty in our estimates of exoplanet masses, and even more when it comes to size. These error bars are large enough that the top contenders overlap. Many also can’t be categorized as planets or brown dwarfs with confidence.

GQ Lupib is one contender, depending on whether it’s fusing deuterium or not. There have been at least four attempts to estimate its radius, which have produced values ranging all the way from 1.8 times Jupiter’s radius to 4.6 times . The latter estimate comes with a particularly large uncertainty, so this beast could be up to 6 Jupiter radii (more than 200 times Jupiter’s volume). The reason this planet is so hard to estimate is that it’s very young. At that age, planets still have lots of heat from their former gravitational potential energy. GQ Lupi b is somewhere around 2000° C (3,600° F), making it hard to tell if that’s all heat of formation, or if some fusion is going on as well. On top of that, it’s so far from its star (3.3 times as far as Neptune is from the Sun) that we can’t measure its mass by observing the gravitational effects. On the plus side, the combination of heat and distance from its star are the reasons we can image this enormous object directly.

There’s even more uncertainty about HD 100546b , with wildly varying estimates of size, mass and whether it’s a planet at all. Keep an eye on it, if for no other reason than it tops the size rankings in NASA’s Exoplanet Archive , but probably best not to award any trophies just yet.

There’s less disagreement about PDS 70b . One estimate of its radius puts it at 2.72 times that of Jupiter , while another just says it’s definitely more than 1.3 Jupiter radii. That puts it as a strong contender for the most voluminous planet, if the possible brown dwarfs are ruled out. Its planetary status is not in question, with a mass somewhere between 2 and 8 times that of Jupiter making it big, but well short of possible fusion.

DH Taurib has a very similar estimated size to PDS 70b, but with speculation about possible brown dwarf status hanging over it.

If you’re finding this uncertainty frustrating, you might prefer HAT-P-67b . Unlike those mentioned before, this planet transits across the face of its star, as seen from Earth. That’s given us an estimate of 2.085 times the radius of Jupiter, with unusually tight error bars. It’s certainly not a contender for the most massive planet, however. Its mass is closer to Saturn’s than Jupiter’s, indicating a very low density from being puffed up by its proximity to its star.

There are, however, half a dozen planets with estimated sizes very similar to HAT-P-67b (one actually fractionally larger), but much wider error bars, so in reality, any of these could be the largest.

If you want the most massive planet, the quest gets even more difficult – any object carrying enough mass to be a contender will automatically be under suspicion of being a brown dwarf.