Astronomers have discovered an unusual Earth-sized exoplanet they believe has a hemisphere of molten lava, with its other hemisphere tidally locked in perpetual darkness. Co-authors and study leaders Benjamin Capistrant (University of Florida) and Melinda Soares-Furtado (University of Wisconsin-Madison) presented the details yesterday at a meeting of the American Astronomical Society in New Orleans. An associated paper has just been published in The Astronomical Journal. Another paper published today in the journal Astronomy and Astrophysics by a different group described the discovery of a rare small, cold exoplanet with a massive outer companion 100 times the mass of Jupiter.
As previously reported, thanks to the massive trove of exoplanets discovered by the Kepler mission, we now have a good idea of what kinds of planets are out there, where they orbit, and how common the different types are. What we lack is a good sense of what that implies in terms of the conditions on the planets themselves. Kepler can tell us how big a planet is, but it doesn’t know what the planet is made of. And planets in the “habitable zone” around stars could be consistent with anything from a blazing hell to a frozen rock.
The Transiting Exoplanet Survey Satellite (TESS) was launched with the intention of helping us figure out what exoplanets are actually like. TESS is designed to identify planets orbiting bright stars relatively close to Earth, conditions that should allow follow-up observations to figure out their compositions and potentially those of their atmospheres.
Both Kepler and TESS identify planets using what’s called the transit method. This works for systems in which the planets orbit in a plane that takes them between their host star and Earth. As this occurs, the planet blocks a small fraction of the starlight that we see from Earth (or nearby orbits). If these dips in light occur with regularity, they’re diagnostic of something orbiting the star.
This tells us something about the planet. The frequency of the dips in the star’s light tells us how long an orbit takes, which tells us how far the planet is from its host star. That, combined with the host star’s brightness, tells us how much incoming light the planet receives, which will influence its temperature. (The range of distances at which temperatures are consistent with liquid water is called the habitable zone.) And we can use that, along with how much light is being blocked, to figure out how big the planet is.
But to really understand other planets and their potential to support life, we have to understand what they’re made of and what their atmosphere looks like. While TESS doesn’t answer those questions, it’s designed to find planets with other instruments that could answer them.
Some like it hot
Two exoplanets had already been found in the young system where Capistrant, Soares-Furtado et al. made their discovery, transiting a G-type star about the size of our Sun. The team re-examined the relevant TESS data after removing the signals for the previously discovered exoplanets. This revealed yet another small object, dubbed HD 63433 d. This new exoplanet is just 400 million years old (Earth is about 4.5 billion years old) and about 1.1 times the diameter of Earth. It orbits eight times closer to its star than Mercury, which means the planet is very, very hot—about 2,294° F (1,257° Celsius)—and thus likely lacks any substantial atmosphere.
That’s hot enough for an exoplanet to be a so-called “lava world,” boasting volcanic seas known as magma oceans. Jupiter’s moon, Io, is highly volcanically active, but it’s not considered a true lava world, so there are none in our Solar System. Examples of probable lava world exoplanets identified so far include CoRoT-7 b and Kepler-10 b. HD 63433 d is more of a half lava world because it is tidally locked, meaning one side is perpetually lit and the other exists in perpetual darkness as the planet makes its 4.2-day orbit around its star. Capistrant, Soares-Furtado et al. suggest that given the scorching temperatures on the lit side, that hemisphere could be lava.
As for the discovery’s significance, “HD 63433 d is the smallest known planet with a precisely measured age that is less than 500 Myr, making it an important touchstone for understanding how Earth-sized planets evolve,” the authors wrote, adding that further study could shed light on the circumstances under which planets keep or lose their thick primordial hydrogen/helium atmospheres, as well as all the underlying physical processes. “Between HD 63433 d and the two previously known larger planets, the HD 63433 system is poised to play an important role in our understanding of planetary system evolution in the first billion years after formation.”
Some like it cold
Then there’s HD88986 b, the small, cold exoplanet just discovered by another team of astronomers using a high-precision spectrograph called SOPHIE, which detects and characterizes exoplanets by measuring tiny variations in a star’s motion caused by the orbiting planets. The team determined that HD88986 b has a mass roughly 17 times that of Earth and about twice Earth’s diameter. (The latter estimate was made after consulting complementary observational data from TESS and the ESA’s CHEOPS (CHaracterizing ExOplanet Satellite). Its orbit is unusually long for an exoplanet this size: 146 days.
“Most of the planets we’ve discovered and measured for their mass and radius have short orbits, typically less than 40 days,” said co-author Neda Heidari of Institut d’astrophysique de Paris (IAP). “To provide a comparison with our solar system, even Mercury, the closest planet to the Sun, takes 88 days to complete its orbit. This lack of detection for planets with longer orbits raises challenges in understanding how planets form and evolve in other systems and even in our solar system. HD88986 b …. potentially has the longest known orbit among the population of small planets with precise measurements.”
HD 88986 b’s atmosphere is also much colder: just 190° Celsius, compared to atmospheric temperatures above 1,000° Celsius for other exoplanets. Co-author Thomas Wilson of the University of Warwick described it as “essentially a scaled-down Neptune, between the orbits of Mercury or Venus.” So like its hot half-lava counterpart (HD 63433 d), HD 88986 b is an excellent candidate for further study to learn more about exoplanet atmospheres. As for its massive outer companion exoplanet, astronomers have yet to glean many details about the companion’s exact nature or properties.
The Astronomical Journal, 2024. DOI: 10.3847/1538-3881/ad1039 (About DOIs).
Astronomy and Astrophysics, 2024. DOI: 10.1051/0004-6361/202347897 (About DOIs).