ESPRESSO comes through. The spectrograph, mounted on the European Southern Observatory’s Very Large Telescope, has produced data allowing astronomers to calculate the mass of the lightest exoplanet ever measured using radial velocity techniques. The star is L 98-59, an M-dwarf about a third of the mass of the Sun some 35 light years away in the southern constellation Volans. It was already known to host three planets in tight orbits of 2.25 days, 3.7 days and 7.5 days. The innermost world, L 98-59b, has now been determined to have roughly half the mass of Venus.

What extraordinary precision from ESPRESSO (Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations). The three previously known L 98-59 planets were discovered in data from TESS, the Transiting Exoplanet Survey Satellite, which spots dips in the lightcurve from a star when a planet crosses its face.

Adding ESPRESSO’s data, and incorporating previous data from HARPS, has allowed Olivier Demangeon (Instituto de Astrofísica e Ciências do Espaço, University of Porto) and team to refine the planets’ mass. Because we already know their radii through transits, we can constrain the density of these rocky worlds. Intriguingly, 30% of L 98-59 d’s mass could be water.

What stands out here, though, is the confirmation of ESPRESSO’s capabilities as we continue to drill down into the centimeters-per-second range that will allow us to probe small rocky worlds around other stars. We’ve seen rapid growth in spectrography through ESPRESSO as well as NEID and, of course, HARPS (High Accuracy Radial Velocity Planet Searcher), which has long been in the forefront of the exoplanet hunt at ESO’s 3.6m telescope at La Silla Observatory in Chile.

ESPRESSO continues to push the boundaries of radial velocity planet detection. There is no hyperbole at all in the conclusion to the paper on this work, which notes that the refinement of mass for the planets in this system, particularly the innermost world:

…represents a new milestone which illustrates the capability of ESPRESSO to yield the mass of planets with RV signatures of the order of 10 cm s-1 in multi-planetary systems even with the presence of stellar activity.

The ESPRESSO data also flag a fourth planet around this star, along with hints of a possible fifth, the latter of which would be in the star’s liquid water habitable zone. The detected planet e has an orbital period of 12.80 days with a minimum mass of 3 Earth masses, while the candidate fifth planet has a period of 23.2 days and a minimum mass of 2.46 Earth masses. It would be in the star’s habitable zone and thus of high interest if confirmed, although there remains the possibility that the signal in the data is the result of stellar activity.

There are no signs of transits from either of these worlds. As this system is likely to become a benchmark for planetary analysis in nearby systems, we’ll keep an eye on the confirmation process for the planet candidate here.

Image: Comparison of the L 98-59 exoplanet system with the inner Solar System.

The three inner worlds at L 98-59 are candidates for atmospheric study through transmission spectrography, where astronomers examine light from the star as filtered through a planetary atmosphere during a transit. The astronomers note that in addition to potential analysis via the James Webb Space Telescope, the Extremely Large Telescope under construction in Chile’s Atacama Desert — scheduled to begin observations in 2027 — may be able to study the atmospheres of these planets from the ground.

In any event, further work with ESPRESSO, the Hubble Space Telescope, and future observatories like NIRPS (Near Infra Red Planet Searcher in Chile) and the Ariel space telescope (Atmospheric Remote-sensing Infrared Exoplanet Large-survey) should be available for atmospheric studies in this interesting system. Adds Demangeon:

“This system announces what is to come. We, as a society, have been chasing terrestrial planets since the birth of astronomy and now we are finally getting closer and closer to the detection of a terrestrial planet in the habitable zone of its star, of which we could study the atmosphere.”

An additional note relates to the tightness of planetary system configurations in multiple planet systems. This is from the paper’s conclusion:

According to exoplanet archive (Akeson et al. 2013), we currently know 739 multi-planetary systems. A large fraction of them (~ 60%) were discovered by the Kepler survey (Borucki et al. 2010; Lissauer et al. 2011). From a detailed characterization and analysis of the properties of the Kepler multiplanetary systems, Weiss et al. (2018, hereafter W18) extracted the “peas in a pod” configuration. They observed that consecutive planets in the same system tend to have similar sizes. They also appear to be preferentially regularly spaced. The authors also noted that the smaller the planets, the tighter their orbital configuration is… [W]e conclude that the L 98-59 system is closely following the “peas in a pod” configuration…

A useful fact, and one that, as the authors add, “further strengthens the universality of this configuration and the constraints that it brings on planet formation theories.”

The paper is Demangeon et al., “Warm terrestrial planet with half the mass of Venus transiting a nearby star,” accepted at Astronomy & Astrophysics (abstract).