An Earth-sized Exoplanet Found Orbiting a Jupiter-Sized Star

Red dwarf stars, also known as M-dwarfs, dominate the Milky Way’s stellar population. They can last for 100 billion years or longer. Since these long-lived stars make up the bulk of the stars in our galaxy, it stands to reason that they host the most planets.

Astronomers examined one red dwarf star named SPECULOOS-3, a Jupiter-sized star about 55 light-years away, and found an Earth-sized exoplanet orbiting it. It’s an excellent candidate for further study with the James Webb Space Telescope.

SPECULOOS stands for the Search for habitable Planets EClipsing ULtra-cOOl Stars. It’s a European Southern Observatory effort that searches for terrestrial planets orbiting cool stars like red dwarfs. (Its odd name is an homage to a Belgian sweet biscuit.) Its goal is to find planets that are good targets for spectroscopy with the JWST and the ELT.

The new planet is named SPECULOOS-3b, and its discovery was presented in a recent paper in Nature Astronomy. The paper is titled “Detection of an Earth-sized exoplanet orbiting the nearby ultracool dwarf star SPECULOOS-3.” The lead author is Michaël Gillon from the Astrobiology Research Unit, Université de Liège, Belgium.

SPECULOOS is an automated search using four telescopes around the world: one at the Paranal Observatory in Chile, one at the Teide Observatory in Tenerife, one at the La Silla Observatory in Chile, and one at the Oukaïmden Observatory in Morocco. The project is searching 1,000 ultra-cool stars and brown dwarfs for terrestrial planets.

One of the problems in detecting planets around these stars is their low luminosity. Since they’re so dim, transiting exoplanets are difficult to detect, making their planetary populations difficult to characterize and study. So far, astronomers have found only one planetary system around one of these stars, and it’s rather well-known: the TRAPPIST-1 system. When it began, the SPECULOOS program expected to find at least one dozen systems similar to TRAPPIST-1.

“We designed SPECULOOS specifically to explore nearby ultra-cool dwarf stars in search of rocky planets,” lead author Gillon said. “With the SPECULOOS prototype and the crucial help of the NASA Spitzer Space Telescope, we discovered the famous TRAPPIST-1 system. That was an excellent start!”

The dimness of these stars can’t be understated. “Though this particular red dwarf is more than a thousand times dimmer than the Sun, its planet orbits much, much closer than the Earth, heating up the planetary surface,” said co-author Catherine Clark, a postdoctoral researcher at NASA’s JPL in Southern California.

The new planet is an Earth-sized world that orbits its star in only 17 hours. The star has a spectral type M6.5, and it delivers 16.5 more solar irradiation to its planet than the Sun does to Earth. That may sound surprising since the star is much cooler than the Sun. The Sun’s surface temperature is 5,772 K (5,500 C), while SPECULOOS-3’s temperature is only 2,900 K (2,627 C.) But SPECULOOS 3 bombards the planet with radiation due to the small distance separating them.

Since the irradiation is largely infrared and the star is only Jupiter-sized, it makes the planet an exceptional candidate for follow up observations, which is exactly what the SPECULOOS program is all about. The SPECULOOS Program 1 has found about 365 temperate, Earth-sized targets for further study with the JWST.

This chart shows the classifications by spectral type for main sequence stars according to the Harvard classification. Image Credit: By Pablo Carlos Budassi – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=92588077

The SPECULOOS-3 system is about 6.6 billion years old. Its luminosity, mass and radius are 0.084%, 10.1% and 12.3% of those of the Sun. “Just slightly larger than TRAPPIST-1, SPECULOOS-3 is the second-smallest main sequence star found to host a transiting planet,” the authors explain in their paper.

Two different telescopes observed the planetary transits around the star in 2021 and 2022 over eight nights. “Visual inspection of the 2021 and 2022 light curves showed some transit-like structures that motivated future intensive monitoring of the star,” the authors explain. The star was re-observed in 2023.

This figure from the study shows the transit of SPECULOOS-3b around its dim, cool star. Image Credit: Gillon et al. 2024.

The researchers determined that SPECULOOS-3b is about the same size as Earth, about 96% of our planet’s radius. But the planet’s density and mass are so far unconstrained. “Nevertheless,” the authors write in their paper, “several factors strongly suggest a rocky composition.”

There are two empirical reasons why the planet is likely rocky, though. The first is that its radius is on the rocky side of the radius gap. The second is that “all of the known Earth-sized planets in the NASA exoplanet archive have masses that imply rocky compositions,” Gillon and his co-authors explain.

This figure from the research compares SPECULOOS-3b to other transiting terrestrial exoplanets with less than 1.6 Earth radii. All of these planets are also cool enough to have rocky daysides rather than molten daysides. The shaded green area highlights planetary radii most similar to Earth’s (0.9–1.1R). Image Credit: Gillon et al. 2024.

But the big question concerns the planet’s potential atmosphere.

“From a theoretical point of view, the intense extreme ultraviolet emission of low-mass stars during their early lives makes it unlikely that such a small planet on such a short orbit could have maintained a substantial envelope of hydrogen.” the authors explain.

Red dwarfs are known to emit extreme radiation that strips away planetary atmospheres. However, there is some evidence that some planets can hold on to their atmospheres despite intense radiation, as with the recently discovered TIC365102760 b. Only time and more observations can tell us if the planet has an atmosphere and what type it has.

The researchers watched closely to see if there was a second planet around the star but didn’t find one. They also examined the planet spectroscopically with ground-based facilities. But we’ll have to wait for the JWST to examine the planet before we can really understand its atmosphere. The two most likely types of atmospheres for hot rocky planets are CO2-dominated and H2O-dominated.

The JWST will be able to examine SPECULOOS-3b with emission spectroscopy. This means it can examine the light the planet is emitting rather than just the light from the star as it passes through the atmosphere, which is called transmission spectroscopy. Emission spectroscopy is unaffected by irregular stellar behaviour, which red dwarfs are known to exhibit. JWST emission spectroscopy can also help determine the surface mineralogy if there’s no atmosphere. There’s a potential wealth of information waiting to be uncovered.

“We’re making great strides in our study of planets orbiting other stars,” said Steve B. Howell, one of the planet’s discoverers at NASA Ames Research Center. “We have now reached the stage where we can detect and study Earth-sized exoplanets in detail. The next step will be to determine whether any of them are habitable or even inhabited.”

Evan Gough

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