An article to be published in “Astrophysical Journal Letters” offers a solution to the mystery of the formation of the exoplanet CI Tauri b, a very young hot Jupiter that contradicts the models that predict that a gas giant is supposed to take at least 10 million years to form. A team of researchers used the IGRINS spectrograph to observe that exoplanet and the data collected suggest that it formed with the mechanism called hot start in jargon in which a gravitational instability is a key element in the quick formation of gas giant planets. Astronomers Christopher Johns-Krull of Rice University and Lisa Prato of the Lowell Observatory, among the authors of the research, presented these conclusions at the 234th meeting of the American Astronomical Society.
The discovery of the exoplanet CI Tauri b was reported in an article published in May 2016 in the journal “Astrophysical Journal” by a team that included Christopher Johns-Krull and Lisa Prato. Its star, about 450 light years from Earth, has an estimated age of just 2 million years, much less than the 10 million years that a giant gas planet would have taken to form according to models that seemed consolidated. This planet is really massive with a mass estimated at about 11.6 times Jupiter’s, and it’s also very close to its star so its year lasts about 9 Earth days. For this reason it’s considerably heated by its star and was cataloged in the hot Jupiter category.
Discovered by the radial velocity method, the exoplanet CI Tauri b has been the subject of follow-up studies to try to explain its existence, which was called a paradox. Using the Immersion Grating Infrared Spectrograph (IGRINS) instrument on the Harlan J. Smith telescope of the McDonald Observatory and the Discovery Channel Telescope of the Lowell Observatory, the researchers obtained more information on CI Tauri b thanks to the “signatures”, or spectral lines, that the various elements and compounds leave in the light that comes from the planet and its star. They can also reveal the temperature and density of a star and its speed of movement.
An important spectral line was that of carbon monoxide, which comes from the atmosphere of the exoplanet CI Tauri b and was used to distinguish the light emitted by it from that emitted by its star. This also allowed to estimate its brightness, 134 times lower than its star’s. That report also offers information on the planet’s formation process and is what led the researchers to conclude that it was a case of hot start, a model that until now has been theoretical that describes the formation of giant planets more quickly than traditional models. In the hot start mechanism, gravitational instabilities lead to a collapse of part of the protoplanetary disk with the direct and quick formation of a giant planet.
The CI Tauri system is so young that a protoplanetary disk still exists around the star. Using the ALMA radio telescope, a team of researchers that again included Christopher Johns-Krull and Lisa Prato discovered three distinct gaps in that disk that suggest the presence of three more gas giant planets, reported in an article published in “The Astrophysical Journal” in October 2018. If there are really four very young gas giants, they must all have formed with the mechanism called hot start. The image (NASA/JPL-Caltech/T. Pyle) shows an artistic concept of various gas giant planets orbiting a young star like CI Tau together with the remains of a protoplanetary disk.
CI Tau b is by far the youngest hot Jupiter discovered so far and provides an excellent test for planetary formation models. If the other planets of that system get confirmed, the situation will be even more interesting. The proximity of CI Tau b to its star is another reason of interest: according to the models, hot Jupiters are formed farther away and then migrate towards their inner system but such a young exoplanet must have formed in that area. In essence, the CI Tau system confirms that it’s very interesting to understand the processes of planetary formation but they’ll probably need to find other very young systems with gas giants to understand them better.
