Origin of super-Earths and sub-Neptunes: Understanding the radius valley as a by-product of planet formation under the core-powered mass-loss mechanism
A video explaining how the radius valley can be understood as a by-product of planet formation under the core-powered mass-loss mechanism. Credits: Emilie Eshbaugh (UCLA undergraduate student) and Hilke Schlichting.
The video above by Emilie Eshbaugh (UCLA) gives a great explanation and introduction to this project. If you prefer a textual summary and would like further details, please read below or refer Ginzburg et al. 2018 and Gupta & Schlichting (2019, 2020).
Till 1995, we only knew about the eight planets in our Solar system. Since, we have discovered thousands of planets in our galaxy orbiting other stars, i.e. exoplanets
(4031 as of Aug 1 2019; see NASA Exoplanet Archive). These discoveries have revolutionized the field
of exoplanetary science and offer new insights into the formation and evolution of planets.
One of the key findings from recent observations has been that the abundant planets in our galactic neighborhood, to-date, are 1 to 4 Earth radii in size, i.e. larger than Earth and smaller than Neptune.
Intriguingly, further observations have revealed that there is a lack of planets of sizes 1.5 - 2.0 Earth radii, i.e. a radius 'valley', in
the size distribution of such small, short-period (<100 days) exoplanets. Moreover, a transition in planet density has been noted around
~1.6 Earth radii, with smaller planets having higher bulk densities, consistent with rocky Earth-like compositions while
larger planets having lower bulk densities, suggesting that these planets are engulfed in H/He atmospheres.
It has thus been suggested that this valley likely marks a transition regime between smaller rocky planets, i.e.
'super-Earths' to larger planets with significant atmospheres, i.e. 'sub-Neptunes'.
Radius valley in the distribution of small, close-in planets separating populations of super-Earths and sub-Neptunes.
Plot based on data from Fulton et al. 2017.
Typically, atmospheric erosion due to high-energy radiation from the host stars, i.e. photoevaporation, is suggested as an explanation to these observations.
Recently, however, Ginzburg et al. 2018 and my advisor and I (Gupta & Schlichting 2019, 2020) have demonstrated that atmospheric loss due to a planet's own cooling luminosity, i.e. core-powered mass-loss,
can also explain the observed radius valley, even without photoevaporation.
Furthermore, we have demonstrated that planetary evolution under this mechanism can explain a multitude of trends observed in the planet size distribution
with orbital period, insolation flux and stellar mass, metallicity, age (Gupta & Schlichting, 2020).
Schematic demonstrating how the core-powered mass-loss mechanism results in super-Earths and sub-Neptunes and thus the radius valley. See Gupta & Schlichting, 2019 for details.
It is likely that the observed planet distribution was not just sculpted by core-powered mass-loss and photoevaporation, but by a multitude of processes over their lifetime such as giant impacts or different planet formation pathways (icy planets, formation in gas depleted disks or after dispersal of gas disks). Nevertheless,
our work shows that the valley in the size distribution of exoplanets is an inevitable by-product of the planetary formation process, i.e. through the core-powered mass-loss mechanism.
To know more, please refer:
⚬ A. Gupta & H.E. Schlichting, 2020. MNRAS. Accepted. [ADS] [arXiv]
⚬ A. Gupta & H.E. Schlichting, 2019. MNRAS 487, 24-33. [ADS] [arXiv]
Rings around small, non-spherical planetary bodies
Artistic rendition of the triaxial shaped dwarf planet Haumea with its surrounding ring.
Image credit: Wikipedia user 'Tomruen'.
For years, we have known of the rings around the giant planets of our Solar System.
Rings are also expected to exist around extrasolar planets but have not been detected so far.
However, what was not expected was the existence of rings around much smaller, non-spherical bodies of our Solar System.
This changed in 2014 with the discovery of rings around a small body named Chariklo, followed by another discovery in 2017, of rings around the dwarf planet, Haumea.
These discoveries suggest that the ring systems are much more common in our Solar System than previously thought,
and their existence has challenged our understanding of their evolution and formation.
In collaboration with my former group
from IIT Kanpur (Prof. Ishan Sharma, Dr. Sharvari Nadkarni-Ghosh, Shri B. Bharath and others), I have been trying to understand the dynamics
of rings around non-spherical bodies through N-body simulations.
To know more, please refer:
⚬ A. Gupta, S. Nadkarni-Ghosh & I. Sharma, 2018. Icarus 299, 97-116. [ADS][arXiv]