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This Earth-sized exoplanet is on a death spiral

Our circumstances here on the wondrous, life-supporting Earth can give us a false understanding of what the universe is really like. But our blue-skied, temperate planet is the extreme exception when it comes to other worlds. There's nothing remotely like Earth in our solar system, and exoplanet studies reinforce that idea. While some exoplanets have hints of habitability, most exoplanets are extremely inhospitable.

Ultra-short-period (USP) planets are one example of these hostile worlds. USP planets follow orbits shorter than one Earth day long, meaning they're very close to their stars. They're so close that their surfaces are molten, and they've likely lost whatever atmospheres they had to their star's intense output. These planets are also imperiled: they can be torn apart by their stars' massive gravitational force, or even spiral into their stars and be totally destroyed.

Astronomers working with data from TESS and other facilities have found a USP that follows an extremely short 5-hour-and-22-minute orbit around its star. They've presented their findings in new research submitted to Astronomy and Astrophysics titled "An Earth-Sized Planet in a 5.4h Orbit Around a Nearby K dwarf." The lead author is Kaya Han Tas from the Anton Pannekoek Institute for Astronomy at the University of Amsterdam in the Netherlands. The paper is on the arXiv preprint server.

The planet is named TOI-2431 b and everything about it is extreme. It completes more than four orbits in a single Earth day, making it one of the shortest period exoplanets ever discovered. There are more than 6,000 confirmed exoplanets and just over 100 of them are USP planets.

The planet is very likely tidally-locked, and its surface temperature is greater than 2,000 Kelvin (1700 C./3140 F), meaning it's probably a lava world. While Earth went through a magma ocean phase in its early years, it cooled and solidified soon after, setting the stage for life. This planet never got the chance.

The exoplanet contains 6.2 Earth masses in only 1.5 Earth radii, which reveals something about its past. It suggests that it may have been larger at one point, but has been stripped down to its rocky core by its star's intense stellar radiation over time.

TOI-2431 b is heading for a dramatic fate, according to the authors. "The short orbital period of TOI-2431 b suggests that it is subject to strong tidal interactions with its host star, potentially leading to tidal deformation and orbital decay," the researchers explain. Its star's strong gravitational pull means TOI-2431 b is deformed, and its shorter axis is about 10% shorter than its longest axis.

Beyond being an oddity, the planet can serve as a sort of observational lab for extreme planetary physics. Since its star is quite bright, the exoplanet has a high Emission Spectroscopy Metric (ESM) of 27, and according to the authors, this means the planet is "one of the best USP systems for atmospheric phase-curve analysis."

Emission spectroscopy creates a graph of the light coming from an object across multiple wavelengths, in this case, the exoplanet's atmosphere. It shows what chemical elements and molecules are present. The ESM ranks exoplanets in terms of their desirability for follow-up atmospheric studies with spectroscopy for telescopes like the JWST, which has a limited lifespan and way more targets than it can possibly get to.

But the most compelling thing about this planet is its upcoming demise.

The Roche limit or Roche radius is the distance from a more massive body that a less massive body must not breech if it expects to survive. Inside the Roche limit, a star's powerful gravity can tear the planet apart. Planets can do the same to moons, and astronomers think that Saturn's rings could be the remains of other moons that were torn apart after breaching the limit.

Tidal interactions with its host star not only deform the planet. They also dissipate orbital energy, "leading to a gradual inward spiral of a planet toward its host star," the authors write.

TOI-2431 b is approaching that limit. "We estimate that the current orbital period is only 30% larger than the Roche-limit orbital period, and that it has an expected orbital decay timescale of only ∼31 Myr," the authors explain. That's perilously close to destruction, in astronomical terms, where 31 million years is only a tiny fraction of Earth's 4.5-billion-year lifespan.

A distant planet like TOI-2431 b has very little meaning in our personal lives. But it and others like it provide context for humanity's existence. Each exoplanet, no matter how inhospitable and horrible, is a piece of the cosmic puzzle we find ourselves in. They show us how fortunate we are, how unlikely our civilization is, and how wondrous Earth and all its lifeforms are.

If we sit with that perspective for a while, we can realize why it's important to build telescopes and study the cosmos. Even a scorching hot lava world with no atmosphere, elongated and spiraling to its destruction, holds important lessons beyond its statistics and properties. Maybe these awful worlds will teach us some humility and we'll stop taking Earth's biosphere for granted.