Half of this Exoplanet is Covered in Lava

Astronomers working with TESS (Transiting Exoplanet Survey Satellite) have discovered a planet that’s been left out in the Sun too long. Or at least half of it has. The newly discovered planet is tidally locked to its star, and one side is completely molten.

The new planet was discovered orbiting a star named HD 63433. The star is young, only about 400 million years old, and it’s about the same mass and radius as the Sun. It’s also a G-type star like our Sun.

The planet is named HD 63433 d, and it’s the third planet found in the system, though the other two were found a couple of years ago. It’s rocky and about the same size as Earth, but that’s where the similarities end.

HD 63433 d is less than 500 million years old. That puts it in a particular category since of the thousands of confirmed exoplanets we’ve found, only 50 are estimated to be less than half a billion years old. It’s also the smallest Earth-like planet found this close to us. It orbits its star in about 4.2 days and is about eight times closer to its star than Mercury is to the Sun. The result?

The side of the planet that faces the star gets no reprieve from the star’s powerful radiation. The planet’s dayside reaches 1,257 C (2,294 F.) That means it’s blistering hot lava and will likely spend billions of years in this state. This rules out any potential habitability, and habitability is the holy grail of exoplanet research.

But HD 63433 d is more than just another lifeless exoplanet. It’s a valuable piece of the puzzle in the quest to understand how planets form and evolve. This type of planet is such an important target in science that TESS has an entire project aimed at them: THYME.

The discovery is presented in a new paper titled “TESS Hunt for Young and Maturing Exoplanets (THYME). XI. An Earth-sized Planet Orbiting a Nearby, Solar-like Host in the 400 Myr Ursa Major Moving Group.” It was published in The Astronomical Journal and presented in a Jan. 10 presentation at the 2024 American Astronomical Society Meeting. The lead author is Benjamin Capistrant, a graduate student in astronomy at the University of Florida.

“Young terrestrial worlds are critical test beds to constrain prevailing theories of planetary formation and evolution,” the authors write. The fact that HD 63433 d is half lava doesn’t change that. Studying it will help planetary scientists study atmospheric loss. Also, the light from its star is so bright that it enables accurate spectroscopy.

This figure from the research illustrates how rare exoplanets like HD 63433 d are. The x-axis is the distance from Earth, and the y-axis is the planetary radius. Each grey circle is a known exoplanet, while each blue pentagon is a known exoplanet younger than 500 million years old. The yellow star represents HD 63433 d, the nearest, young, Earth-sized exoplanet discovered to date. Image Credit: Capistrant et al. 2024

“The apparent brightness of the stellar host makes this transiting multiplanet system favourable to further investigations, including spectroscopic follow-up to probe the atmospheric loss in a young Earth-sized world,” the authors explain.

The first few hundred million years in the life of a planet is critical. Young solar systems are dynamic places. Collisions between planets and gravitational interactions can force planets to migrate or follow eccentric orbits. There are also abundant impacts by asteroids and planetesimals, which can go on for a long time. In regions of dense star formation, neighbouring stars can even affect the planets in nearby systems.

“Detailed observations of planetary systems in such environments are, therefore, crucial to understanding the general formation history of the exoplanet population,” the authors explain.

This artist’s illustration shows an exoplanet tidally locked to its star. The side facing the star is so hot it’s molten rock. Image Credit: NASA/Kepler Mission/Dana Berry

Besides its size and proximity to Earth, why is HD 63433 d important? It comes down to exoplanet atmospheres.

“Currently, one of the most important inquiries in exoplanet science is understanding in which circumstances planets keep or lose their thick primordial hydrogen/helium atmospheres and what physical processes drive this phenomenon,” the authors write.

There’s a mass gap in the radius distribution of small exoplanets that scientists refer to as the small planet radius gap. For some reason, there’s a scarcity of small planets between about 1.5 and 2 times Earth’s radius. There’s no reason to think that planets don’t form at these radii, so scientists believe planets lose mass and end up smaller.

A histogram of planets with given radii from a sample of 900 Kepler systems. The decreased occurrence rate between 1.5 and 2.0 Earth radii is apparent. [Fulton et al. 2017]

Planetary scientists aren’t sure what drives the mass loss that creates the gap, but two primary mechanisms could be responsible. One is extreme ultraviolet photoevaporation. Young stars emit powerful UV radiation that can drive the atmosphere away from a planet into space.

The other mechanism is core-powered mass loss. With this mechanism, the luminosity of the cooling planetary core provides the energy for atmospheric loss. These cores start out hot due to their assembly and formation, as the gravitational energy that binds them together is converted into heat. As the cores cool, the heat can drive away the atmosphere.

These mechanisms work on different time scales, and that’s why the youthful HD 63433 d is such a compelling subject for study. Since its radius is below the radius gap, it’s likely rocky. But if mass loss takes longer than 500 million years, it could still have a thick atmosphere. “Because Earth-sized planets orbiting young, Sun-like stars have so far been difficult to detect, HD 63433 d presents a particularly compelling case study for atmospheric investigations of close-orbiting Earth-sized planets,” write the authors.

This discovery is important because the planet is such a valuable target for future, more detailed observations of its atmosphere. “It would be valuable to interrogate the planet’s mass using precise radial velocities and determine whether the composition is indeed rocky, as expected based on observations of older planets,” the researchers explain.

The first step is confirming that HD 63433 d is, in fact, a rocky planet. The JWST has a role to play in this, as its MIRI instrument has already been used to capture the thermal emissions of rocky exoplanets. These measurements provide a benchmark astronomers can use to compare JWST observations of HD 63433 d with other rocky planets. “Moreover, the star’s unusual brightness should provide plenty of photons to make these sensitive measurements,” the authors write.

Most rocky planets, Earth included, are magma ocean planets after they initially form. Repeated impacts keep the planet’s surface molten. But some, like HD 63433 d, remain half-molten for billions of years. That may doom them to eternal lifelessness, but as this research shows, they have much to tell us.

It could be the key that unlocks the mystery of the small planet radius gap.

Evan Gough

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