One of the ongoing puzzles in astronomy is that very few solar systems that we’ve examined to date look anything like ours. It’s common, for example, for solar systems to have so-called Hot Jupiters, planets the size of Jupiter that circle their host stars at a closer distance than Mercury circles our own Sun. Many solar systems have planets that fall in-between the size of Earth and Neptune, while we have no such worlds. In other solar systems, Super-Earths — rocky planets — are often found much closer to the Sun, while our largest and farthest planets are all gas giants.
These differences imply that our own theories of planetary formation and evolution may need tweaking to account for how our solar system appears to be odd, for lack of a better way of putting it. As if to reinforce that argument, astronomers have found a planet that appears to be much too large for the star it’s orbiting. Their work, published in Science, raises questions about how a system like this could form in the first place.
GJ 3512 b is a gas giant orbiting a tiny red dwarf, GJ 3512. The planet itself is in an eccentric, 204-day orbit around its star, but spends most of its time closer to its parent than Mercury is to Sol. GJ 3512, however, can only manage ~0.2 percent of our Sun’s solar output. But a planet the size of GJ 3512 b isn’t supposed to exist.
“The discovery was surprising because theoretical formation models suggest that low-mass stars typically host small planets, similar to Earth or small Neptunes,” said astrophysicist Juan Carlos Morales of the Institute of Space Studies of Catalonia, who led the research. “In this case, we have found a gas giant planet similar to Jupiter around a very small star,” Morales said.
To put this in perspective, GJ 3512 is only about 250 times the mass of GJ 3512 b, whereas the Sun is more than 1,000 times the mass of Jupiter.
Credit: C. Bickel/Science
According to their measurements, GJ 3512 is only about 35 percent larger than Jupiter, while the planet, GJ 3512 b, is at least 46 percent the size of Jupiter. Thus, we have a Jupiter-sized planet circling a very small star, where Jupiter-sized planets are not expected to form. Furthermore, GJ 3512 b isn’t the only planet orbiting GJ 3512; there’s a GJ 3512 c signal in the data as well. The standard model of planetary formation, in which pebbles accrete together until a core of at least 5-15 Earth masses exists followed by a relatively quick growth to gas giant size, does not fit the known circumstances in this case.
In order to form a gas giant, there has to be enough gas in a protoplanetary disk in the first place. As a solar system forms, the amount of gas within it is continually reduced by the wind of the nascent star and by other planetesimals and planets, which pull material into their own orbits. Around red dwarfs, the accretion process is typically too slow for gas giants to form this way before gas is lost to surrounding space. Giant planets only have a limited window to form around their host stars, typically estimated at 3-10 million years.
There is, however, an alternate theory of planetary formation known as gravitational disk instability. According to this theory, clumps of dust and gas may have differential temperatures, with cold areas becoming denser over time until the gas cloud collapses into a planet. Unlike theories of core or pebble accretion, this process could theoretically occur extremely rapidly, within a matter of some thousands of years as opposed to over millions of years.
Up until now, core accretion has been the favored explanation for how planets form, but GJ 3512 b is enough of an oddball that scientists don’t think our standard models can explain it. It may be an example of a planet that formed via gravitational disk instability, evidence that our existing models don’t account for all necessary variables in the process, or both. Either way, we now know there’s at least one other oddball solar system out there — even if it’s still completely different than our own.
Top image credit: Guillem Anglada-Escude—IEEC/Science-wave, using SpaceEngine.org (CC BY 4.0))
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