Scientists have observed, for the first time, a young planet (named AB Aurigae b) still immersed in the disk of gas and dust around its host star. We know that planets are born at the same time as their host stars. Around the new star, a disk of material can be used to generate the planetary system around it. According to the most used models, this material generates small grains that gradually grow, becoming larger stones, larger bodies and, finally, planets.
The unprecedented observation, however, challenges traditional models. As the planet is massive — 10 times more massive than Jupiter, the largest in our Solar System — it would have already grown a lot, consuming the material around it. However, the images show a giant planet present in the disk still intact. Thus, it was necessary to find an alternative to explain how it came about.
Vladimir lyra, a Brazilian astronomer at the State University of New Mexico, in the United States, was primarily responsible for the theoretical aspect of the work. In addition to being a talented scientist, he is a long-time friend, and I had the opportunity to talk a little with him about the result.
Thiago Gonçalves: What did you discover? Why is discovery so important?
Wladimir Lyra: We have observed for the first time a planet still immersed in the protoplanetary disk, shrouded in nebulosity. The only other planet seen with the disk still present has already excavated a gap in the disk in its orbit. This shows that AB Aurigae b is an even more recent object, at an even more embryonic evolutionary stage.
TG: How can you guarantee that you were actually observing a planet, and not just a brighter spot in the dust disk?
WL: First, the font size is consistent with what a giant planet would look like at this distance from the star. Second, Dr. Thayne Currie (Nasa-Ames Research Center, research project leader) looked for the planet in archival data, and found it in 2007 Hubble images. So it’s not something like a dust cloud, which in that time span would have already been dispersed.
We’ve been discussing the data for five years. Currie is a careful and meticulous observer.
TG: If it really is a planet, how could it have formed under such unexpected conditions?
WL: There are two main ideas for the formation of giant planets: core accretion and gravitational instability.
Core accretion is a “bottom-up” model: the core of the giant planet, up to 30 times the mass of Earth, forms first, and this core captures gas from the nebula. Gravitational instability, on the other hand, is “top-down”: the gas breaks up into larger pieces, like what happens during star formation in giant clouds.
Up to the mass of Saturn all planets appear to be cases of core accretion. Uranus and Neptune are the cores of giant planets that failed to capture gas. Saturn has a core of 30 Earth masses.
Alternatively, gravitational instability produces huge planets. Jupiter is inconclusive until we have better data on the interior, but the data indicate that it must be one as well. There was no system or planet that could categorically say that gravitational instability is the most likely mechanism.
AB Aurigae b appears to be the first case. The disk density and the found value of the planet’s mass are in agreement with the models that predict the formation of planets in this way.
Interestingly, there’s a lot of evidence coming from independent directions, converging. If it’s not gravitational instability, it would be very coincidental, very unlikely.
TG: Does that mean then that there is no longer any doubt as to how this planet could have formed?
WL: I will emphasize that we are not saying that nucleus accretion is impossible for this object. While the traditional model of core accretion does not produce these objects at such great distances from the star, other phenomena can produce high-mass planetary embryos even at great distances from the star. Thus, core accretion is not completely ruled out.
It is difficult to give a final word on the formation mechanism. After all, we are still debating Jupiter. This observation, while not 100% conclusive, is strong evidence of gravitational instability.
In the coming years the community will analyze this object, with even more detailed observations and even more sophisticated models, and we will certainly learn a lot about planet formation.