\n<\/p>\n
If we\u2019ll ever find life outside our solar system, it will not be an instant discovery. Save for the slim possibility of an intelligent civilization beaming a message in our direction, evidence for aliens will most likely come from scrutinizing nearby rocky worlds by using giant telescopes to study their atmospheres for gases that could hint at living, breathing somethings dwelling unseen on the planet\u2019s surface. This is no easy feat. For stars like our sun, no telescope will be capable of doing this for a generation. For smaller stars, we have only recently developed this capability thanks to the James Webb Space Telescope (JWST).<\/p>\n
That\u2019s why two papers published on September 8 in the Astrophysical Journal Letters are so exciting. Using JWST, for the first time, astronomers have managed to find tentative evidence for an atmosphere on a rocky planet in a clement orbit around another star some 40 light-years from Earth. Called TRAPPIST-1e, the planet is one of seven small worlds orbiting its host star, a red dwarf far smaller and dimmer than our sun. Efforts to find atmospheres on any of these planets have been otherwise unsuccessful; the three innermost worlds, it seems, are barren rocks stripped of any wisp of air. But now, thanks to JWST, we\u2019ve seen that might not be the case for TRAPPIST-1e, the fourth planet of this system. If there is life somewhere out there, right now this world seems to be our best bet of finding it.<\/p>\n
\u201cWe\u2019re seeing something tantalizing,\u201d says Ryan MacDonald at the University of St Andrews in Scotland, a co-author on the papers. \u201cIf it is confirmed, it is a huge deal. We would have the first atmosphere on a habitable zone rocky planet beyond our own solar system.\u201d<\/p>\n
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The planets of the TRAPPIST-1 system were discovered in 2016 and 2017 by exoplanet scientist Micha\u00ebl Gillon at the University of Li\u00e8ge in Belgium and colleagues using the Transiting Planets and Planetesimals Small Telescope\u2013South (TRAPPIST\u2013South) facility at the La Silla Observatory in Chile and NASA\u2019s Spitzer Space Telescope. Those discoveries showed this relatively nearby system was worthy of astronomers\u2019 attention. It contained seven rocky worlds each roughly the same size as Earth, several of which seemed to be in the star\u2019s habitable zone, the region where temperatures should be just right for liquid water, and perhaps life, to exist.<\/p>\n
For a red dwarf like TRAPPIST-1, this region is much smaller than our own solar system. In fact, all of the TRAPPIST-1 planets would fit comfortably inside the orbit of Mercury around our own sun. This proximity to a red dwarf is problematic, however, as these pipsqueak stars can punch well above their weight by regularly unleashing violent stellar outbursts. Stars like TRAPPIST-1 \u201cemit a lot of extreme ultraviolet radiation\u201d that can strip away the atmospheres of nearby worlds, says Yuka Fujii, an exoplanet scientist at the National Astronomical Observatory of Japan, who was not involved in the papers. \u201cWe have not confirmed planets in the habitable zone of red dwarfs can have atmospheres,\u201d she says. \u201cSo if there is an atmosphere on TRAPPIST-1e, it would be very encouraging.\u201d<\/p>\n
If even a star this volatile can support habitable worlds, \u201cit would mean a lot more stars than we think can have planets with atmospheres,\u201d says Caroline Piaulet-Ghorayeb, an astronomer at the University of Chicago who has studied the TRAPPIST-1 system.<\/p>\n
Ever since JWST launched in December 2021, astronomers have been desperate to use its unmatched infrared vision to probe systems like TRAPPIST-1. To do so requires staring at the star for long periods of time as each planet completes an orbit, up to about 19 days for the outermost planet, and crosses in front of the star relative to us. This is known as a transit, and when this happens, JWST can see the light from the star passing over the planet. If there is an atmosphere present, the signal looks noticeably different than that from a bare rock, such as Mercury, as the light passes through the atmosphere and is scattered by any gases afloat in the alien skies, a process called transmission spectroscopy.<\/p>\n
Until now, things had not looked promising. Reconnaissance of TRAPPIST-1 b, c and d has found no atmospheres, leaving some to wonder if any of the TRAPPIST-1 planets were potentially habitable. \u201cI was optimistic that we\u2019d be seeing atmospheres on most of the planets in the system,\u201d MacDonald says. \u201cWe\u2019re really hoping that TRAPPIST-1e is the one.\u201d<\/p>\n
The team\u2019s analysis involved four transits of TRAPPIST-1e observed in 2023. The data ruled out the planet having a super-thick atmosphere, like Jupiter\u2019s, full of hydrogen and helium\u2014something we wouldn\u2019t expect for an Earth-sized rocky world anyway. The signals the researchers saw from the star were instead consistent with an atmosphere containing nitrogen and methane but lacking carbon dioxide, ruling out the planet being a CO2-dominated orb like Venus or Mars. \u201cPerhaps the closest analogue in our own solar system would be [Saturn\u2019s moon] Titan,\u201d MacDonald says, although he notes that an Earth-like atmosphere is also a possibility.<\/p>\n
The team wasn\u2019t able to completely rule out that TRAPPIST-1e is a barren rock like its inner siblings, however, partly because of interference from its hyperactive star. \u201cIt\u2019s about equally likely if there is an atmosphere or not,\u201d says Ana Glidden of the Massachusetts Institute of Technology, the lead author on one of the papers, but crucially these are the best odds astronomers have found yet for any of TRAPPIST-1\u2019s worlds. If there is an atmosphere, the team suggests it would be a \u201csecondary\u201d one like Earth\u2019s, formed through volcanic eruptions and other processes after the planet lost its initial \u201cprimary\u201d envelope of gas early in its existence.<\/p>\n
The team calculates that, based on its distance from the star, TRAPPIST-1e might also be a relatively cool world, conceivably with liquid-water oceans or frozen expanses of ice on its surface. All of the TRAPPIST-1 planets are tidally locked, meaning the same face always points to the host star, but TRAPPIST-1e could have a substantial ocean on its \u201cstarward\u201d side. \u201cIt could be frozen except at the substellar point,\u201d where temperatures would be warmest, Glidden says. Such a world, she adds, could resemble a lidless, staring eyeball\u2014a dark, oceanic pupil surrounded by an iris of bright, gleaming ice.<\/p>\n
As alien and unearthly as this may be, it still would make TRAPPIST-1e by far the most promising of the TRAPPIST-1 worlds to host life. Sarah McIntyre, an exoplanet scientist and founder of the Australian astrophysics start-up Beyond Exo, has previously suggested that the planet\u2019s magnetic field might be similar to Earth\u2019s; coupled with its toasty estimated temperature, that would make TRAPPIST-1e arguably the most alluring astrobiological target in our celestial catalogs. \u201c[TRAPPIST]-1e was definitely a forerunner for me,\u201d she says. No better planetary system more amenable to JWST\u2019s scrutiny has been found in our pocket of the galaxy, Gillon says\u2014and TRAPPIST-1e may be its crown jewel. \u201cThere is nothing comparable,\u201d he says. \u201cIn the Earth-sized regime, this is the best target we have.\u201d<\/p>\n
JWST has also been observing TRAPPIST-1f, g and h in the system for signs of an atmosphere, and results from those other worlds are expected to appear within the next year. But for TRAPPIST-1e, astronomers still need to look at the planet in more detail, and they are already doing so. A follow-up campaign is underway right now to observe 15 more transits of the planet. \u201cWe\u2019re halfway through,\u201d says N\u00e9stor Espinoza at Johns Hopkins University in Maryland, lead author on the other TRAPPIST-1e paper. \u201cThe observations should be done this year, hopefully in December.\u201d The results should be published in 2026. \u201cNext year is going to be exciting for TRAPPIST-1e,\u201d Espinoza says.<\/p>\n
Those forthcoming results could be enthralling. The follow-ups use a fortuitous alignment with the innermost planet, TRAPPIST-1b, to reduce some of the noise from the star and better work out exactly what sort of atmosphere TRAPPIST-1e has. \u201cIf there is an atmosphere, we will see it,\u201d MacDonald says.<\/p>\n
And if the follow-ups clinch the case for an atmosphere on 1e\u2014or if one of the other planets turns out to have one, for that matter\u2014a new era will begin, in which astronomers lavish those targets with attention from JWST and other next-generation telescopes. \u201cIf we confirm an atmosphere and we detect different gases, it would be very easy to approve a very large program to dig in and go up to 50 or 100 transits,\u201d MacDonald says. \u201cIt\u2019s incredibly exciting to be at this point. It\u2019s the reason why I went into astronomy, to look at potentially habitable planets.\u201d<\/p>\n
Of course, there remains the possibility that all the TRAPPIST-1 planets are bare rocks. Perhaps the hints of air on TRAPPIST-1e are yet another bit of stellar mischief from the planet\u2019s problematic star. That would be disappointing, and it would bolster the case that red dwarfs are not as hospitable to life as many astronomers have hoped.<\/p>\n
Either way, the next phase of the hunt for habitability on exoplanets\u2014namely, looking for atmospheres on planets like Earth orbiting stars like our sun\u2014will not truly arrive until the 2040s, when NASA is set to launch its Habitable Worlds Observatory, a behemoth space telescope built from the ground up to probe promising worlds around other stars. It will be the first telescope capable of directly imaging Earth-like planets around sunlike stars to look for signs of life. \u201cTo push towards planets really similar to Earth, we need a next-generation space telescope,\u201d says Laura Kreidberg, an exoplanet scientist at the Max Planck Institute for Astronomy in Heidelberg, Germany.<\/p>\n
To recap: Within a year, we should know if any of the TRAPPIST-1 planets have atmospheres. By the end of the decade, we might know if they have a mixture of gases that suggest signs of life. But we will \u201chave to be patient\u201d in our search for any true-blue Earth 2.0, Gillon says. There\u2019s still no doubt that we are entering an extremely exciting time, where the possibility of finding life beyond the solar system is within our grasp; these TRAPPIST-1e observations represent one small, important step toward that future. \u201cTo use an analogy, we have the rocket to go to the moon, and with these results, we know it works,\u201d Gillon says. That pivotal \u201cgiant leap\u201d moment might not be too far away.<\/p>\n","protected":false},"excerpt":{"rendered":"
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