Researchers with NASA’s James Webb Space Telescope are aiming for the big chill about TRAPPIST-1e: the rocky exoplanet may have lost an initial atmosphere to its star’s volatile nature, but eventually entered a second atmosphere phase — an idea flagged with cautious excitement. The picture that emerges is nuanced — and critical when it comes to assessing whether Earth-sized planets around red dwarfs can remain habitable.
Initial Webb measurements indicate that TRAPPIST-1e doesn’t have a puffy, hydrogen-rich atmosphere, the most-hospitable type of atmosphere for telescopes to find. That result is consistent with what we might expect for planets in the intense radiation of a ultracool dwarf. But scientists say that does not preclude a “secondary” atmosphere that was generated later by volcanic outgassing, cometary bombardment or chemical reactions on and within the planet.
Why scientists suspect an atmosphere lost
The star TRAPPIST-1 is an ultra-cool red dwarf located about 40 light years from the sun, and is home to at least seven roughly Earth-sized planets. The system is a test of violent habitability theories due to the fact young worlds around stars like this are blasted with flares and high-energy radiation. Papers by teams from Cornell University and MIT in the Astrophysical Journal Letters suggest that TRAPPIST-1e almost definitely does not have an atmosphere with hydrogen-helium of primordial origin.
Webb’s transmission spectroscopy — the watching of starlight filtering through a planet’s air during transits — didn’t reveal the broad spectral imprints that would have been characteristic of a thick, thick, lightweight atmosphere. That result dovetails with a previous finding involving the sister planet TRAPPIST-1d and the NASA and the Space Telescope Science Institute models that account for the fact that many M dwarfs, such as our local sun when it was very young, unleash high-energy flares and brightness variations at radio through X-ray wavelengths can strip light gases from near-by planets in geologically short timescales.
There is a hitch: red dwarf surfaces are covered with mottling—starspots and faculae—that can imprint their own signal on the spectrum and mimic atmospheric features. Now the key point in the analysis has to deal with the separation of the star from planet.
Could TRAPPIST-1e rebuild its atmosphere?
Earth probably lost its primordial hydrogen and then accreted a denser atmosphere formed through volcanic outgassing, which was then transformed by life. TRAPPIST-1e may have taken a similar route, particularly if it held its internal heat and fed long-lasting volcanism. Secondary atmospheres around rocky planets would generally be richer in heavier molecules, like carbon dioxide, nitrogen and water vapour — more difficult to remove, more difficult to detect, but also potentially enough to mitigate against the loss of surface water.
Climate models from several groups, including NASA-Goddard and the University of Chicago, indicate that a tidally locked world such as TRAPPIST-1e could have a temperate surface if the right amount of greenhouse gases are present to mix heat around the planet. Water could remain as a global ocean in some cases, pool on the lit half of the body in others, under clouds that would act as a heat reflective, stabilizing “thermal umbrella.”
Conversely, a thin or patchy atmosphere also risks nightside collapse, in which gases freeze out and the climate system shuts down. This makes the presence — or not — of thick CO₂ or N₂ critical.
A smart Webb test: pairing up planet b and e
To tune out that stellar racket, the Webb team intends to perform a comparative experiment: observe the transit of TRAPPIST-1e within a day or two of that of TRAPPIST-1b, a hotter, probably airless world that is closer to the star. If both planets have the same spectral quirks at the same wavelengths, then those are likely the star’s own signatures. All features specific to TRAPPIST-1e are better candidates similarly for its own atmosphere.
This “control planet” approach, GB says, in combination with Webb’s NIRSpec and NIRISS, represents an evolution of the standard transit spectroscopy game, and is a response to the lessons learned from Hubble and Spitzer. It might also serve as a template for addressing multi-planet systems around active stars, where starlight contamination has long complicated interpretations.
What Webb will search for next
Eliminating hydrogen as the gas and particle, the next consideration would be to test for gasses of heavier elements. Carbon dioxide leaves its own telltale marks around 4.3 microns, within Webb’s sensitivity, while carbon monoxide signs tend to gather around 4.6 microns. Water vapor imprints in the near-infrared can also be observable when clouds are not too thick. Secondary-eclipse and phase-curve measurements, particularly with MIRI, should be able to determine heat emission and help to separate out a bare rock from a world with air.
The transit search of TRAPPIST-1e is based on dozens of potential transits and eclipses. Releasing early results — with four observations under the belt, a few more to come — means other teams can stress-test their methods, improve models of stellar activity and combine data sets. That open approach is essential, because current signals are so near the limits of precision.
Why red dwarf worlds are of interest
Red dwarfs are the most abundant type of star in the Milky Way, and are home to a significant proportion of Earth-sized exoplanets in the galaxy. If worlds like TRAPPIST-1e can be replenish their atmospheres after initial erosion, the chances of finding habitable environments skyrocket. If they don’t, the habitable zones of such stars may be even more depopulated than their surplus suggests.
Webb was constructed by agencies such as NASA, the European Space Agency and the Canadian Space Agency to address exactly these frontier questions. The findings from TRAPPIST-1 will inform future investments in space-based and ground-based telescopes, such as the proposed Habitable Worlds Observatory, which are designed to spectrally characterize rocky planets with high precision.
For the time, the case remains open — though not unoccupied. With a missing hydrogen blanket, a promising test apparatus and a path to a rebuilt atmosphere to call its own, TRAPPIST-1e has emerged as the star of the single-most important habitability experiment in progress.