Combined Radial Velocity Measurements Indicate Giant Exoplanets Can Rapidly Contract After Their Formation

Artist’s impression of the planetary system V1298 Tau. Credit: Gabriel Pérez Díaz, SMM (IAC).

Giant planets are defined by their massive size, thick gaseous atmospheres, and lack of solid planetary surfaces due to low amounts of solid rocky material. The current “core-formation” model for giant planet formation begins with the accumulation of solid, rocky fragments and a gaseous envelope, followed by slow contraction and cooling over hundreds of millions of years.

However, theoretical models have not been tested so far because detecting young giant planets is difficult due to the intense stellar activity of their host stars. The size of an exoplanet can be determined using the transiting method, whereby the amount of light blocked by the planet as it passes in front of its host star is measured by a detector. Alternatively, the radial velocity method relies on the fact that a star does not remain stationary as it is orbited by a planet, but in fact moves in a periodic way due to the gravitational pull of the satellite. The resulting periodic Doppler shift in the star’s light can be observed using high-resolution spectrographs and ground-based telescopes and used to calculate the mass of close-orbiting exoplanets.

The young solar-type star V1298 Tau, which has an estimated age of 20 million years, hosts four giant exoplanets, which were detected previously using the transiting method. In a recent paper, Alejandro Suárez Mascareño and colleagues from the Instituto de Astrofísica de Canarias used multiple radial velocity determinations collected over a one-year period to constrain the masses of these giant planets. Analysis of the data revealed that the two outermost planets of the V1298 Tau system are much smaller relative to their masses than predicted based on current core-formation models for giant planetary formation. Since these planets are only approximately 20 million years old, this suggests that the contraction stage of planetary formation could occur on more rapid timescales than the models predict. READ MORE