New observations show rocky exoplanet is only half the mass of Venus

A team of astronomers used the Very Large Telescope at the European Southern Observatory (ESO’s VLT) in Chile to shed new light on the planets around a nearby star, L 98-59, which resemble those of the internal solar system. Among the findings are a planet with half the mass of Venus – the lightest exoplanet ever measured using the radial speed technique – an ocean world and a possible planet in the habitable zone.

“The planet in the habitable zone may have an atmosphere that could protect and support life,” says María Rosa Zapatero Osorio, astronomer at the Center for Astrobiology in Madrid, Spain, and one of the authors of the study published today ‘hui in Astronomy & Astrophysics.

The results are an important step in the quest for life on Earth-sized planets outside the solar system. Detecting biosignatures on an exoplanet depends on the ability to study its atmosphere, but current telescopes are not large enough to achieve the resolution needed to do so for small, rocky planets. The newly studied planetary system, named L 98-59 after its star, is an attractive target for future observations of exoplanet atmospheres. It orbits a star just 35 light years away and is now home to rocky planets, like Earth or Venus, which are close enough to the star to be hot.

With input from ESO’s VLT, the team was able to deduce that three of the planets may contain water inside or in their atmosphere. The two planets closest to the star in the L 98-59 system are likely dry, but could hold small amounts of water, while up to 30% of the third planet’s mass could be of water, which would make it an oceanic world.

In addition, the team found “hidden” exoplanets that had not been spotted before in this planetary system. They have discovered a fourth planet and suspect that there is a fifth, in an area the right distance from the star for liquid water to exist on its surface. “We have clues to the presence of a terrestrial planet in the habitable zone of this system”, explains Olivier Demangeon, researcher at the Instituto de Astrofísica e Ciências do Espaço, University of Porto in Portugal and lead author of the new study .

The study represents a technical breakthrough because astronomers were able to determine, using the radial velocity method, that the innermost planet in the system is only half the mass of Venus. This makes it the lightest exoplanet ever measured using this technique, which calculates the star’s oscillation caused by the tiny gravitational pull of its orbiting planets.

The team used the ESPRESSO instrument (Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations) on the ESO VLT to study L 98-59. “Without the precision and stability provided by ESPRESSO, this measurement would not have been possible,” says Zapatero Osorio. “This is a step forward in our ability to measure the masses of the smallest planets beyond the solar system.”

Astronomers first spotted three of L 98-59’s planets in 2019, using NASA’s Transiting Exoplanet Survey (TESS) satellite. This satellite uses a technique called the transit method – where the drop in light from the star caused by a planet passing in front of it is used to infer the properties of the planet – to find the planets and measure their sizes. However, it was only with the addition of radial velocity measurements made with ESPRESSO and its predecessor, the High Accuracy Radial Velocity Planet Searcher (HARPS) to ESO La Silla’s 3.6-meter telescope, that Demangeon and his team were able to find additional planets. and measure the masses and radii of the first three. “If we want to know what a planet is made of, the minimum we need is its mass and radius,” says Demangeon.

The team hopes to continue studying the system with NASA / ESA / CSA’s future James Webb Space Telescope (JWST), while ESO’s Extremely Large Telescope (ELT), under construction in the Chilean desert of Atacama and which should start its observations in 2027, also be ideal for studying these planets. “The HIRES instrument on the ELT may have the power to study the atmospheres of some of the planets in the L 98-59 system, thus supplementing the JWST from the ground,” says Zapatero Osorio.

“This system signals what’s to come,” Demangeon adds. “We as a society have been hunting terrestrial planets since the birth of astronomy and now we are finally getting closer and closer to detecting a terrestrial planet in the habitable zone of its star, which we could study. ‘atmosphere.”

This research was the subject of an article titled “Hot Earth Planet with Half the Mass of Venus Passing Through a Nearby Star” to appear in Astronomy & Astrophysics.