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).
If I understand the data correctly, ESPRESSO has an accuracy of about 10 cm/s which compares reasonably favorably with the NEID system of about 35 cm/s (a href=”https://centauri-dreams.org/2021/07/22/radial-velocity-neid-spectrograph-goes-to-work/”>Radial Velocity: NEID Spectrograph Goes to Work. Is that correct, or are the values not comparable?
Looking forward, what might be the absolute best these systems can do?
Yes, I read on the ESO site that the goal is 10 cm/s or less, which is about Earth’s RV.
We are finally getting somewhere.
I also read that the CODEX spectrograph on the E-ELT, to be inaugurated around 2026 or ’27, will have an RV accuracy of 2 cm/s, maybe even 1 cm/s.
How much is Mars’ RV? About 1 cm/s? Or even less?
With the rate of progress, all those journal papers assuming ETI has similar technology to contemporary technology will look embarrassingly quaint in a few decades – like lighting fires in a huge triangle on the Earth’s surface to signal Martians. ET thousands, possibly millions, of years ahead of us technologically will have capabilities of observing Earth and us that we cannot even dream of.
To give you just one example, the late Robert Bradbury envisioned ETI that could turn a celestial body the size of Luna into a mega space telescope that would be able to image terrestrial type worlds down to their continents and more from a considerable distance.
https://www.gwern.net/docs/ai/1999-bradbury-matrioshkabrains.pdf
I was just watching a clip of Carl Sagan looking at images of the Earth from the then available satellite imaging technology of Earth and explaining there was no sign that intelligent life existed. Cities were just not visible even on images that had higher resolution than those we hope to get from a FOCAL mission to image an exoplanet.
Of course, our images of Earth are very much better now, and Sagan might have got a better clue looking at the nightside and city lights.
Just as with our search for life, our near-term technologies look for more subtle indications using spectrographic data. The very recent proof that light reflected back from plants is rotated and is detectable at kilometer ranges suggesting that we could potentially use that approach to detect vegetation on exoplanets. Various techno-signature approaches can be brought to bear too.
I have no doubt that the future will offer more approaches as our technology improves, and with it our appreciation of the floor of what any advanced ETI could do.
I just hope that this does not include the discovery of lots of HZ planets with clear signs of rapid forced climate heating as the most obvious techno-signature. ;(
Think you have to put NIRPS on your list.
Done.
The third planet from the star may be a deep water world. What would happen with a tidally locked planet with a warm 420 kelvin sun side. Would the dark side be cool enough to cause major circulation patterns to equalize the temperatures? Has any research been done along these lines? All I have seen is atmospheric circulation, but this may be a tropical Venus!
With the capabilities of 1 cm/s we maybe able to pick up large artificial structures in close in orbits around M dwarfs. What would be practical if we built some type interstellar launch station close enough to a star to use the huge amounts of energy available to launch FTL solitons or create wormholes for large motherships?
There might be many planets with water-rich atmospheres.
https://news.uchicago.edu/story/there-might-be-many-planets-water-rich-atmospheres
“Liquid magma is actually quite runny,” Kite said, so it also turns over vigorously, just like oceans on Earth do. There’s a good chance these magma oceans are sucking hydrogen out of the atmosphere and reacting to form water. Some of that water escapes to the atmosphere, but much more gets slurped up into the magma.
Then, after the nearby star strips away the hydrogen atmosphere, the water gets pulled out into the atmosphere instead in the form of water vapor. Eventually, the planet is left with a water-dominated atmosphere.
This stage could persist on some planets for billions of years, Kite said.”
Hi
Yes this is one interesting and exciting system here of terrestrial planets.
The paper can be found on the ESO website too
https://www.eso.org/public/news/eso2112/
Cheers Edwin
“we currently know 739 multi-planetary systems.”
That is not correct.
The NASA archive website is not up to date.
There are 791 confirmed multiplanet systems, and 958 if you add candidates:
http://exoplanet.eu/catalog/
Goal: 1 telescope dedicated to one star system, starting with alphacen then continuing in order of distance. Alphacen probably deserves one per star or even one per planet, but baby steps…