A minimal record and other exciting findings: Astronomers have gained new knowledge about a nearby planetary system that is similar to that of our sun. One of the celestial bodies has only half the mass of Venus – it is the lightest exoplanet that has been measured with the radial velocity method so far. Another planet in the system could be very water-rich, according to the data. The researchers also report a previously rather mysterious observation: In addition to the four planets that have now been confirmed, they found evidence of a fifth on which Earth-like conditions could prevail.
Astronomers have already discovered thousands of exoplanets around distant stars – now the task is to take a closer look at the particularly exciting systems. This is exactly what L 98-59 is about: The system, only 35 light years away from us, consists of a red dwarf star and its interesting planetary system. So far, three planets were known that were discovered in 2019 using the transit method. The astronomers used the shadowing of the star caused by the passing planets to identify them and determine their size. In order to uncover more details about these celestial bodies and possibly to identify more, an international team of astronomers has targeted the L 98-59 system with the Very Large Telescope (VLT) of the European Southern Observatory (ESO) in Chile.
Stellar wobbling reveals a lightweight
The researchers use the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO) to obtain data for the so-called radial velocity method. To put it simply, the shaking of the star is determined, which is caused by the gravitational forces of the planets orbiting it. In this way, they can be detected indirectly and conclusions can also be drawn about their masses and thus also about their compositions. “If we want to know what a planet is made of, we need at least its mass and its radius,” says first author Olivier Demangeon from the University of Porto. As he and his colleagues explain, it was only by combining the radial velocity measurements with previous data that more detailed insights into the features of the planets in the system were possible.
As they report, they achieved a technical breakthrough with the use of ESPRESSO. Its data made it possible to establish that the innermost planet of the system has only half the mass of Venus. This is the previous minimum record with this detection technique, say the astronomers. “This is a step forward in determining the masses of small planets outside the solar system,” says co-author María Rosa Zapatero Osorio of the Center for Astrobiology in Madrid.
Promising system
As the astronomers further report, their results suggest that the three previously known planets of L 98-59 may contain water in their interior or in their atmosphere. However, the two closest to the star are arguably quite dry. But this is probably not the case with number three: The density data provide indications that it consists of up to 30 percent water, the scientists write.
As part of the investigation, the team also identified a fourth previously unknown exoplanet in the system, but no further details are known about it. They also report on the promising trace of a possible fifth planet. “We have evidence of the presence of an Earth-like planet in the habitable zone of this system,” says Demangeon. Zapatero Osorio also considers this to be a particularly exciting lead: “In this area, a planet could have an atmosphere that could protect and enable life,” says the astrobiologist.
The planetary system of L 98-59 is now an attractive target for future observations of exoplanet atmospheres. Astronomers hope to be able to identify biosignatures through an atmospheric perspective. So far, however, telescopes have not been able to achieve the resolution required for small rocky planets. First of all, the team now hopes to be able to further investigate the system with NASA’s James Webb Space Telescope, which will be launched in late 2021. Later on, ESO’s Extremely Large Telescope, which is currently being built in the Chilean Atacama Desert and is scheduled to go into operation in 2027, could provide further insights. “The HIRES instrument at the ELT could possibly examine the atmospheres of some planets in the L 98-59 system,” says Osorio.
Source: ESO, technical article: Astronomy & Astrophysics, doi: 10.1051 / 0004-6361 / 202140728