How One Planet Jumped the JWST Queue
The Ground-Based Proof
Collin Cherubim, a Harvard PhD candidate, modeled the signature before he ever pointed a telescope at it. His prediction: a rocky planet in a red dwarf's habitable zone, if it still had a real atmosphere after billions of years of stellar radiation, should be shedding a thin stream of helium off its upper layers, a gas light enough to escape gravity and heavy enough to leave a fingerprint in starlight. He tested it on LHS 1140 b, a super-Earth 48 light-years out in the constellation Cetus, 5.6 times Earth's mass, using the WINERED spectrograph on the Magellan Clay telescope in Chile as the planet crossed its star. The helium showed up exactly where the model said it would. A second planet in the same system, LHS 1140 c, showed nothing, the negative result that argues against an instrument artifact rather than for one. Published in Science on July 16, the result took zero hours of James Webb Space Telescope time. David Charbonneau, who chairs Harvard's astronomy department, said his first reaction was that 'there's nothing like that in the solar system.' Then the data came back clean, twice.
The Queue It Jumps
James Webb time for exoplanet atmospheres is among the most oversubscribed categories the telescope runs, and LHS 1140 b's confirmed atmosphere gives its proposal something Webb allocation committees weigh heavily: evidence an atmosphere is there to characterize, rather than a bet on whether one exists at all. This desk argued last week that the discovery matters less as a planet than as proof a ground-based telescope can do Webb's expensive first pass, freeing the rarest instrument in astronomy for targets that have already cleared that bar; the follow-on question, now forming an answer, is which system claims the freed time. TRAPPIST-1, a red dwarf 40 light-years out with seven roughly Earth-sized planets, has held some of Webb's highest-profile slots, for hosting more habitable-zone rocky worlds than any other system charted. Its observations kept coming back close to empty: the innermost planets show no confirmed atmosphere, their star's radiation apparently having stripped them clean over billions of years. LHS 1140 b now makes the stronger case for time to search for carbon dioxide on a rocky, habitable-zone world with a demonstrated atmosphere, rather than to find out whether there is air at all. A carbon-dioxide search is planned. The comparison to TRAPPIST-1 cuts directly against systems whose case for atmosphere had rested on hope rather than a confirmed detection.
LHS 1140 b's helium proves a rocky world can keep an atmosphere through billions of years of stellar battering; TRAPPIST-1's silence proves most, on the evidence so far, do not. A carbon-dioxide search would settle nothing about which planet previews the galaxy's actual odds. The real test is whether the WINERED spectrograph turns up helium on the next red-dwarf rocky planet in line, and whether that system's proposal gets Webb time in the coming allocation round.
Sources
- Harvard FAS (current.fas.harvard.edu)
- ScienceDaily (sciencedaily.com)
- Astronomy.com (astronomy.com)
- Scientific American (scientificamerican.com)