Mysterious new exoplanet rekindles fierce debate over planetary formation

The discovery of a fourth gas giant around the star HR 8799 doesn't seem like that big a deal, but it's become the latest flashpoint for a highly controversial and often intense debate about how gas planets are born.

There are two basic theories for where gas giants come from. In both models, a disc of gas, dust, and ice forms around young stars. Then, in the gravitational instability model, part of that disc suddenly fragments and clumps together into a single mass, which forms the gas giant all at once. Alternatively, the core accretion model calls for two steps, where the dust first forms a rocky core, and then that core captures enough of the surrounding gas to become a gas giant. Most calculations say the core only has about 10 million years to entrap the gas before it's all drawn into the star.

Now, the expanded planetary system of HR 8799 poses something of a mystery. The first three gas planets were discovered all at once a couple years ago. They are all bigger than Jupiter and located quite a bit further out from their star than the gas giants of our solar system, which the three planets each 2 to 2.5 times the distance of Saturn, Uranus, and Neptune, respectively. Indeed, both the planets' sizes and orbital distances makes the HR 8799 system resemble a scaled up version of our own solar system.

The fourth planet is also more massive than Jupiter, but it's quite a bit closer than the other three, sitting about midway between Saturn and Uranus if it was moved to our system. And that's the problem for its discoverers, who say it can't possibly have formed in the same way as the other three. If the three outer planets formed at the same orbital distance where they're now located, then must have formed by gravitational instability. At that sort of distance, there wouldn't be enough dust around for core accretion to take place.

On the other hand, core accretion seems far more likely for the newly discovered innermost planet. Indeed, the discoverers argue it could only have formed that way, as it's too close for gravitational instability to work. This, they argue, is a puzzling situation, as they argue core accretion and gravitational instability can't both occur in the same system.

This is where things get very contentious. Carnegie Institution researcher Alan Boss vehemently disagres with their findings:

"I have been publishing models for 10 years showing disk [instability] models that form gaseous protoplanets at these distances and even closer. This really is more of a religious split than a scientific one."

He even calls his opponents "disk-instability deniers", borrowing the language used to describe opponents of climate change. Other scientists aren't quite so forceful, but they stress that it is possible for both core accretion and gravitational instability to take place in the same system.

Another possibility is that the planets have migrated around, and that they formed elsewhere. Astronomer Sean Raymond points out the three inner planets - the newly discovered HR 8799e as well as planets d and c - appear to have orbital synchrony. They are in a 1:2:4 orbital resonance, where for every four orbits completed by the innermost planet e, planet d completes exactly two orbits and planet c completes exactly one. That arrangement is unlikely to occur naturally and is likely evidence that the planets have moved around since their formation.

Whatever the precise explanation, Raymond says this is a reminder that we've still got a long way to go in our understanding of how planetary systems work:

"This kind of system is excellent for [understanding] planet formation because it reminds us that we don't know as much as we would like, and it's a new challenge."

[arXiv via Science News; artist's conception of HR 8799b from NASA]