Astronomers from the Search for Planets Eclipsing Ultra-cool Stars (SPECULOOS) project found an Earth-sized exoplanet orbiting an ultracool dwarf star about 55 light-years away. The team concluded that the exoplanet, called SPECULOOS-3b, likely has a rocky surface and no atmosphere.

The SPECULOOS project is a network of six robotic telescopes, four in the Southern Hemisphere and two in the Northern Hemisphere, that aims to search for exoplanets around 1,600 nearby ultracool dwarf stars.

Moreover, some of the project’s scientists are part of an international collaboration that found a giant, but ultra-light exoplanet called WASP-193b. The exoplanet is about 1.5 times the size of Jupiter but has a mass only about one-seventh that of Jupiter. The planet is so light that the teams compare its density with that of cotton candy.

An Earth-sized exoplanet around a tiny star

The SPECULOOS project first studied SPECULOOS-3b’s star in 2019 and 2021. While the observations from 2021 showed signs of a planet, the results were inconclusive as the signal was distorted by noise caused by bad weather on the day of observation. When Artemis, the SPECULOOS Northern Observatory’s telescope in Tenerife, Spain, observed the star in July 2022, the observations showed a clear signature of an exoplanet.

Artist’s impression of SPECULOOS-3b orbiting it’s flaring, ultracool brown dwarf star. (Credit: NASA/JPL-Caltech)

When an exoplanet transits its star, which means it passes in front of its star from Earth’s perspective, the star appears slightly dimmer. Astronomers study how the star’s light changes during these transits to infer the exoplanet’s properties.

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“While there were structures in the 2021 data that didn’t look convincing, the 2022 Artemis data really got our attention,” said Artem Burdanov of the Massachusetts Institute of Technology (MIT), who manages the SPECULOOS Northern Observatory. “We started to analyze one clear transit-like signal in the Artemis data, quickly decided to launch a campaign around this star, and then things just started lining up.”

With the new observations, the scientists could measure the size of SPECULOOS-3b and its star. They found that the star was similar in size to Jupiter or about one-tenth the size of the Sun. By measuring how much the star dimmed during transit, the team concluded that SPECULOOS-3b must be about the same size as the Earth.

Additionally, the team found that the exoplanet orbits its star in about 17 hours, at a very close distance. “We believe that the planet rotates synchronously, so that the same side, called the day side, always faces the star, just like the Moon does for the Earth. On the other hand, the night side hand, would be locked in endless darkness,” said study lead Michaël Gillon of the University of Liège in Belgium.

Astronomer Michaël Gillon and the #SPECULOOS team discovered a new Earth-sized exoplanet around SPECULOOS-3, an “ultracool dwarf” star as small as Jupiter, twice as cold as our Sun, and located 55 light-years from Earth. #ULiège @NatureAstronomy @NASA

— Université de Liège (@UniversiteLiege) May 15, 2024

Even though the ultracool dwarf star is 1,000 times dimmer than the sun and about twice as cold, NASA’s Transiting Exoplanet Survey Satellite (TESS) revealed it is very active. This, combined with the short distance between SPECULOOS-3b and its star, means that the exoplanet is continuously bombarded with radiation, about 16 times more than the Earth receives from the sun. The astronomers believe this radiation has likely stripped any atmosphere away, leaving a bare rocky surface.

The team believes that SPECULOOS-3b is a great target for future studies by the joint NASA, European Space Agency, and Canadian Space Agency James Webb Space Telescope (JWST). The lack of atmosphere would allow Webb to study the exoplanet’s surface and its geological history. A brighter surface would reveal recent geological activity, as older surfaces become darker from erosion by meteoroid impacts or other influences from space.

“SPECULOOS-3b is the first planet for which we can consider moving toward constraining surface properties of planets beyond the solar system,” says co-author Julien de Wit of MIT. “With this world, we could basically start doing exoplanetary geology. How cool is that?”

Gillon et al.’s results were published in the journal Nature Astronomy on May 15.

A large, but ultra-light exoplanet around a Sun-like star

WASP-193b orbits a Sun-like star about 1,200 light years from Earth, completing one orbit every 6.25 days. By analyzing how much of the star’s light was blocked during transits, the scientists concluded that the planet is substantially bigger than Jupiter, with a radius about 1.5 times larger.

Artist’s impression of puffy Jupiter WASP-193b orbiting it’s Sun-like star. (Credit: K. Ivanov)

The exoplanet was originally detected and studied by the Wide Angle Search for Planets (WASP) project. The team used additional observations, among which observations by one of SPECULOOS telescopes and TESS, to study WASP-193b in more detail.

“Typically, big planets are pretty easy to detect because they are usually massive, and lead to a big pull on their star,” said de Wit. “But what was tricky about this planet was, even though it’s big — huge — its mass and density are so low that it was actually very difficult to detect with just the radial velocity technique. It was an interesting twist.”

Using the radial velocity technique, astronomers analyze how the star’s spectrum is distorted by mass pulling on the star. In this technique, a larger mass, or planet, distorts the spectrum more. “[WASP-193b] is so very light that it took four years to gather data and show that there is a mass signal, but it’s really, really tiny,” said lead author Khalid Barkaoui of the University of Liège and MIT.

With the exoplanet’s mass determined as 14 percent that of Jupiter, the team was able to calculate WASP-193b’s density at 0.059 grams per cubic centimeter, which is a lot lower than Jupiter’s density of about 1.33 grams per cubic centimeter and Earth’s of 5.51 grams per cubic centimeter. This makes the planet an extreme example of a class of large and light exoplanets astronomers call puffy Jupiters.

Comparison of various planets, including Jupiter and WASP-193b. (Credit: Barkaoui et al.)

“WASP-193b is the second least dense planet discovered to date, after Kepler-51d, which is much smaller,” explains Barkaoui. “Its extremely low density makes it a real anomaly among the more than five thousand exoplanets discovered to date. This extremely low density cannot be reproduced by standard models of irradiated gas giants, even under the unrealistic assumption of a coreless structure.”

The scientists looked for reasons the exoplanet’s density was so low but came away empty-handed, as none of the standard models could adequately explain the observed density. Instead, the team determined how future studies could provide answers, and concluded that only a single transit observation by Webb could provide a lot more insights into the unique planet.

“The bigger a planet’s atmosphere, the more light can go through,” said de Wit. “So it’s clear that this planet is one of the best targets we have for studying atmospheric effects. It will be a Rosetta Stone to try and resolve the mystery of puffy Jupiters.”

Barkaoui et al.’s results were published in the journal Nature Astronomy on May 14.

(Lead image: artist’s impressions of SPECULOOS-3b (left) and WASP-193b. Credit: L. Garcia (left), K. Ivanov (right))

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