The Nature and Origins of Sub-Neptune Size Planets

Area of Science:

• Exoplanetary science
• Planetary formation and evolution
• Atmospheric characterization of exoplanets

Background:

• Sub-Neptune planets are the most abundant exoplanets discovered, intermediate in size between Earth and Neptune.
• Exoplanet surveys reveal a radius gap in sub-Neptune distribution, indicating distinct formation or evolution pathways.
• These planets orbit closer to their host stars than Mercury orbits the Sun.

Purpose of the Study:

• Investigate the bimodality in sub-Neptune radius distribution.
• Identify mechanisms responsible for atmospheric loss in sub-Neptune planets.
• Constrain the formation models (migration vs. pebble drift) for sub-Neptune planets.

Main Methods:

• Analysis of exoplanet radius distribution data from missions like Kepler.
• Theoretical modeling of atmospheric mass loss mechanisms (photoevaporation, core-powered mass loss).
• Comparison of planetary formation models (migration, pebble drift).

Main Results:

• A radius gap observed between 1.5 and 2.0 Earth radii suggests two populations of sub-Neptunes.
• Planets above the gap likely retained their primary atmospheres (gas-rich super-Earths).
• Planets below the gap likely lost their atmospheres (true super-Earths).

Conclusions:

• The radius gap points to a significant atmospheric evolution process after formation.
• Atmospheric studies are crucial for distinguishing between formation models and understanding interior structures.
• Upcoming observatories are expected to provide definitive atmospheric composition data for these common exoplanets.