The L 98-59 system has just become a particularly desirable destination for planetary system astronomers. Human visits aren't happening soon, despite recent progress, but space telescopes have revealed an intriguing set of planets – including one of lightest ever found, one that's probably 30 percent water, and signs of one perfectly placed for hosting life.
Until 2019, L 98-59 was just another dwarf star among many – although at 35 light-years away, it's close by galactic standards. Then, the Transiting Exoplanet Survey Satellite (TESS) discovered L 96-59b, the smallest planet it had found – 20 percent smaller than Earth. Finding planets this small around distant stars is an astonishing technical challenge. TESS also found two planets 40 and 60 percent larger than our home.
However, L 98-59b can't match Kepler-37b, the smallest exoplanet ever found, closer to the Moon in size than Mercury.
Now, a large team of researchers led by Dr Olivier Demangeon of the University of Porto have used the Very Large Telescope (VLT) to explore the system in more depth. In Astronomy and Astrophysics they've added a lot of information about the worlds TESS found – and at least one it missed – making the system rival Trappist-1 for the most interesting ever found.
Just as it is hard to find small planets using the transit method, radial velocity detection (also called Doppler wobble) struggles to detect low-mass worlds. However, the VLT's observations of L 96-59's movements revealed L 96-59b has less than half the mass of Venus – the lightest object ever detected this way. By comparing TESS's size measurements and the mass obtained by the VLT Demangeon determined L 96-59b's density is an intriguingly light 3.6 g/cm3, two-thirds of Mercury or Earth. L 98-59c has a density not far below the Sun's inner planets.
More intriguing is L 98-59d's density of just 2.95 g/cm3, suggesting it could be 30 percent water, despite above-boiling surface temperatures.
TESS's discoveries all orbit so close to L 98-59 that, even allowing for it being much fainter than our Sun, they get far too much heat to be habitable. However, the VLT also detected a definite fourth planet, L 98-59e, and a possible fifth (called 05 until confirmed), suspected of orbiting at a suitable distance for surface water.
“We have hints of the presence of a terrestrial planet in the habitable zone of this system,” Demangeon said in a statement.
L 98-59e is three times Earth's mass, while 05 is suspected to be 2.5 Earth masses. That might increase its chances of holding onto an atmosphere against the outbursts typical of red dwarf stars like this one, but would also make getting into space harder.
“This system announces what is to come," Demangeon added. “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.”
If the JWST finally makes it into space later this year, the planets of L 98-59 represent some of its most suitable candidates for the study of exoplanet atmospheres.