Two young planets defy the models for planets formation. The study: “Rapid contraction of giant planets orbiting the 20-million-year-old star V1298 Tau” by A. Suarez Mascareno (Instituto de Astrofísica de Canarias) recently appeared on Nature Astronomy

Planet formation occurs in disk-like structures, called “protoplanetary disks”, observed around very young stars. The dispersal timescale of these disks (being typically 3-5 million of years) sets an important limit to the planet formation timescales predicted by theoretical models. The widely accepted model that explains the formation of the gaseous giant planets, such as Jupiter, is the “core-accretion model“. This model predicts the quick formation of a large solid core by the coalescence of planetesimals, on which large amount of gas is rapidly accreted.

 

In order to test planetary formation models, it would be necessary to study a large and variegate sample of young planets. However, this a quite complicated task. Very young stars, in fact, are characterized by an intense magnetic activity. The phenomena related to this activity produce spectroscopic signals which mimic and hide those due to the presence of exoplanets.  For these reasons, it is very important to discover very young planets accounting for the magnetic activity of the host stars.

 

In the 2019, a study based on observations of the Kepler/K2 mission, which was designed to search exoplanets with the method of transits (which are the periodic dimming of stellar luminosity due to the passage of the planets in front of their stars), reported the discovery of a system with four planets around the star V1298 Tau, a 1.2 solar masses star which is about 20 million years old. Recently, the team led by the astronomer A. Suarez Mascareno (Instituto de Astrofísica de Canarias) characterized the properties of the two planets V1298 Tau b and e by analyzing spectroscopic observations obtained with the spectrographs HARPS-N mounted on the Telescopio Nazionale Galileo, CARMENES of the Centro Astronómico Hispano-Alemán, SES of the Teide Observatory, and HERMES of the Mercator Telescope, together with photometric observations obtained with the telescopes of the LCOGT consortium necessary to study the stellar magnetic activity of the host star. The two planets have a mass of 0.64 and 1.16 Jupiter masses, respectively. Considering their radii, obtained from the Kepler/K2 data, equal to 0.92 and 0.78 Jupiter radii, the two planets are more dense that what is predicted by the core-accretion model. According to this model, in fact, such densities should be reached after a few hundreds million of years. However, there is an alternative explanation: the discrepancy could be explained if the planets have a large content of heavy elements (5%-40% for planet b, 40%-60% for planet e). This may be possible if the two planets formed at a distance of few tens of Astronomical Units from the star (1 Astronomical Unit is the mean Sun-Earth distance, equal to 150 million of km). Then, the two planets should have migrated on their actual orbits, accreting a large quantity of solid bodies during their path. The study is described in the paper: “Rapid contraction of giant planets orbiting the 20-million-year-old star V1298 Tau“, recently appeared on the prestigious journal Nature Astronomy. Among the coauthors, the astronomers S. Benatti, D. Locci, G. Micela e A. Maggio of INAF – Astronomical Observatory of Palermo.

 

The figure (click here to visualize the entire image) shows the distribution of radii and masses for planets for which these quantities are determined with a precision better than 33%. Marked also the position of Jupiter, Saturn, and planets in V1298 Tau system, and the planet AU Mic b, the only known with a similar age.

 

Mario Giuseppe Guarcello  ( follow mguarce) ( youtube)

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