Earth-sized exoplanet is spiraling toward destruction

Astronomers have discovered a bizarre “lava world” spiraling towards its doom. TOI-2431 b is an Earth-sized exoplanet orbiting its star in a different solar system. 

A consortium of astronomers led by Kaya Han Tas from the University of Amsterdam, Netherlands characterized the exoplanet.

They predict the planet is on a death spiral and will be torn apart by its star in just 31 million years, a blink of time on cosmic scales.

Fate of TOI-2431 b

The Earth-sized exoplanet stands out as the shortest-period rocky world with astronomers having precisely measured both its mass and radius. These are also called ultra-short-period planets.

Unlike Earth’s year-long path around the Sun, this world completes a year in less than a quarter of an Earth day, once every 5 hours, 22 minutes.

It is also “tidally locked” – the same side always faces its star. One side is in endless day, while the other remains dark. Models suggest proximity to the star locks rotational and orbital periods together.

Being so close to its star puts it at the edge of survival because its orbit is unstable and shrinking.

Researchers estimate that in only 31 million years, TOI-2431 b will cross the Roche limit. Crossing this threshold will let the star’s tidal forces overpower the planet’s self-gravity and rip it apart.

How was TOI-2431 b characterized?

Astronomers discovered TOI-2431 b with NASA’s TESS telescope. TOI refers to “TESS object of interest.” The planet is named after its host star, TOI-2431, which is about two-thirds the size of our Sun. It’s a late K star.

They measured the planet’s size indirectly. Like insects casting shadows across a light bulb, the planet creates regular dips in starlight as it passes in front. Astronomers used this ‘shadow effect’ to determine the planet’s radius.

To measure its mass, scientists turned to the NEID and Habitable-zone Planet Finder instruments. 

These detect tiny wobbles in the star’s movement, which the planet’s gravity causes. Combining mass with size showed TOI-2431 b is rocky and unusually dense.

For astronomers, the Earth-sized exoplanet is a benchmark case, revealing how extreme orbits reshape planets.

Earth-sized exoplanet shaped by tides

Unlike Earth, which is nearly spherical, TOI-2431 b has a distorted, football-like shape. Just as the Moon pulls the Earth’s oceans, this planet faces tides – but on a catastrophic scale.

It orbits so close that the star’s pull on its near side is much stronger than on its far side. The result is an elongated body. Its longest axis, pointing toward the star, is about 9% longer than its shortest axis.

Astronomers have measured such an extreme tidal deformation in a rocky exoplanet for the first time.

A “lava world” with no atmosphere

If its orbit and shape weren’t extreme enough, TOI-2431 b’s surface is even harsher. With an equilibrium temperature above 2,000 Kelvin (3,600°F), it is hot enough to melt rock. It is likely covered in oceans of lava.

Stellar radiation stripped away any hydrogen-helium atmosphere the planet once had. For millions of years, UV rays heated the gases until they escaped into space.

What remains is a dense, rocky world with a measured density of 9.4 grams per cubic centimeter, higher than Earth’s. It may represent the first well-studied case of a rocky core that atmospheric loss has exposed.

Where did it come from?

TOI-2431 b’s orbit is so close to its star that it could not have formed there. Forming solid rocks needs cooler temperatures. Astronomers believe that the planet formed farther away. It then migrated inward, being pushed and pulled by different gravitational forces along the way.

One possibility is that TOI-2431 b began as a gas giant or mini-Neptune, later losing its atmosphere. Another is that it started as a rocky super-Earth, gradually pushed inward by gravitational interactions.

Evidence suggests it may belong to a multi-planet system. Gravitational tugs from worlds arising from the same start may have helped drive its migration. A little like the three-body problem in the science fiction series.

A world in its final chapter

TOI-2431 b is unlike any world astronomers have studied before. 

For scientists, it offers a unique laboratory to study planetary migration, atmospheric loss, and tidal interactions. For the rest of us, it is a reminder of the fragile balance that allows Earth to exist in a stable orbit.

By observing TOI-2431 b’s death spiral in real time, astronomers gain insight into how planets live and die. It is also a glimpse into the extraordinary diversity of worlds across the galaxy.

The study has been submitted to Astronomy and Astrophysics and is available as a preprint on arXiv.

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