For the first time, Hubble sees a rocky exoplanet slip behind its star

For the first time, the Hubble Space Telescope has caught a rocky planet around another star in a secondary eclipse, the moment the planet slips behind its star. Other telescopes, including NASA's Spitzer and the James Webb Space Telescope, have studied rocky lava worlds in infrared light before, among them 55 Cancri e; the new result is Hubble's first. The planet is TOI-2431 b, and the result, from a team led by Sebastian Zieba, is published in The Astrophysical Journal Letters.

TOI-2431 b is a lava world, a rocky planet so hot that all or part of its surface is expected to be molten. It orbits a nearby K-type dwarf, a star cooler than the Sun and about two-thirds as wide, once every 5.4 hours, at a distance of 0.0063 au, less than one hundredth of Earth's distance from the Sun. Its mass has been measured at 6.2 Earth masses, give or take 1.6, and no planet with a shorter year has had both its size and its mass measured.

Reading a planet's light by watching it disappear

A planet this close to its star cannot be seen on its own; its light is mixed in with the star's. A secondary eclipse pulls the two apart. When the planet goes behind the star, its own glow drops out, and the small dip in the combined light measures how bright the planet's day side is.

Hubble watched the system in February 2022, covering about three of the planet's orbits. The dip as the planet went behind the star is tiny: the planet adds about 69 parts per million to the star's light, less than one hundredth of one percent, in Hubble's near-infrared band, and about 37 parts per million in visible light seen by NASA's TESS satellite, which also caught the eclipse.

About as hot as it could be

If the planet reflects no starlight, so that all the light it gives off is its own heat, its day side has a brightness temperature of about 2,524 K, roughly 2,250 degrees Celsius or 4,080 degrees Fahrenheit. A brightness temperature is how hot a glowing body would have to be to give off the light we measure, and K stands for kelvins, the temperature scale scientists use, which starts at absolute zero. That is close to 2,574 K, the most a planet in that orbit should reach if it absorbs all of its starlight and moves none of the heat around to its night side; the two values agree within their uncertainties.

Hubble also found no sign of light from the night side. Winds that carried heat around the planet would warm the night side and push the hottest spot away from the point directly under the star; Hubble measured no such shift, so little heat seems to travel from day to night.

Read that way, TOI-2431 b stands apart. Several other lava worlds observed so far have day sides well below their limits, the authors note, which suggests something is cooling them.

Heat or reflected light

At the wavelengths Hubble and TESS see, both the planet's own heat and starlight bouncing off it add to its light, and these data cannot tell how much comes from each. If the surface reflects a little of the starlight that hits it, the case the authors test, the day side estimate drops to about 2,407 K, and more reflective, cooler surfaces are still allowed. The data are consistent with a planet that has no atmosphere, but they do not rule one out.

What comes next

Planned observations with the James Webb Space Telescope at longer infrared wavelengths should help tell heat from reflected light, and search for an atmosphere of vaporized rock. The authors say the result makes TOI-2431 b a benchmark target for studying lava worlds.