Study Reveals Not All Sub-Neptune Exoplanets Have Magma Ocean

When NASA launched the Kepler spacecraft in 2009, it opened a new era of exoplanet discovery, revealing thousands of planets orbiting other stars. Among these were mini-Neptunes, or sub-Neptunes, planets smaller than Neptune but larger than Earth, and now known to be the most common type of exoplanet, reported by Universe Today.

Scientists have long assumed that, like rocky planets such as Earth, sub-Neptunes begin with molten magma oceans. Their thick hydrogen-rich atmospheres were thought to trap heat efficiently, allowing the magma ocean phase to persist far longer than on Earth.

New research led by University of Chicago professor Eliza Kempton suggests that the assumption that all sub-Neptune planets have molten silicate surfaces isn't always true.

The study, titled "Not All Sub-Neptune Exoplanets Have Magma Oceans," published in "The Astrophysical Journal Letters", challenges this idea. According to the researchers, while the evolution and structure of sub-Neptunes may be influenced by interactions between an outer gaseous envelope and a potential magma ocean. Some sub-Neptunes may have solid silicate surfaces.

In this research, the team modelled the internal structure of sub-Neptunes, taking into account various planetary and atmospheric properties. All models included an iron core, a silicate mantle, and a hydrogen-helium-water mantle, with an iron-to-silicate ratio similar to Earth's.

The team varied five key factors, the planet's mass, the pressure at the radiative-convective boundary, the photospheric temperature, the mantle's mass fraction, and the mean molecular weight (MMW), which reflects the average mass of atmospheric molecules. One case study focused on GJ 1214 b, a well-known mini-Neptune whose hazy and high-MMW atmospheric structure suggests a heavier atmosphere than previously thought. Such conditions could create enough pressure to solidify magma on the planet's surface.

The study identified three key factors that determine whether a magma ocean will exist on a sub-Neptune. First, a high mantle mass fraction and a high MMW can create pressures that solidify a silicate surface. Second, cold temperatures and a thin atmospheric layer can also create a solid surface, making these planets more akin to super-Earths than traditional mini-Neptunes. Third, the pressure at the radiative-convective boundary, which reflects the planet's age, affects the persistence of a magma ocean; Magma is more likely to exist on young planets with lower pressure.

Professor Kempton said this research changes a key assumption about sub-Neptunes. He explained that planets can either have a "lava floor" or a solid surface, and atmospheric properties play a key role in determining this.

This finding is also important for ongoing observations by the James Webb Space Telescope (JWST), as a planet's atmospheric and overall properties can indicate whether a magma ocean exists on that planet.

Study co-author Matthew Nixon said that before the discovery of exoplanets, astronomers believed that other solar systems would be similar to our own. Observations of sub-Neptunes have shown the limitations of this assumption and underscored the need to revise models of planet formation. Understanding these distant planets not only highlights the diversity of planetary systems but also helps explain the process of Earth's formation and evolution.