A new view on planet formation
Many hot, rocky planets outside of our own solar system started out as large, gaseous Neptunes. This is what astronomers at Leiden University contend in a recent online publication.
This new finding means that many exoplanets (planets outside of our solar system) that astronomers once labeled 'exo-earth' may have histories and compositions entirely different from Earth's - and that our assessments of which planets could or could not support life may have been missing the mark.
Calculating planet sizes
Exoplanet astronomer Vincent van Eylen and his team calculated the sizes of 117 exoplanets, varying from the size of the Earth to four times larger. They did so with far greater precision than had been attained thusfar. As the researchers expected, the planets clustered into two clearly defined groups: one around 1.5 times the size of earth, the other around 2.5 times. 'There's two separate populations of planets, and the influence of the star is what creates the separation,' explains Van Eylen. The members of one of those groups starts out as gas giants that become rocky only later, after their parent stars blow away their thick atmospheres, leaving the bare, solid planet core.
Proximity to parents
The group also looked at the proximity of the various types of planets to their parent stars. They found that large rocky planets are usually found close to their star, whilst smaller gaseous planets are often found farther out. The paper states that 'great care must be taken when extrapolating findings of small planets at short orbital periods to more temperate Earth-sized planets.' Prior models based on our own solar system show large gas giants former father out and small terrestrial planets close to their parent star, but this new study shows that this is likely not the most common scenario.
V. Van Eylen et al. An astroseismic view of the radius valley: stripped cores, not born rocky. arXiv:1710.05398. Posted on 15 October, 2017.