As of last month, we're pretty certain there's an Earth-mass planet orbiting Proxima Centauri, the closest star to Earth. This raises a rather obvious question: can it support life? The planet, Proxima Centauri b, orbits within its star’s habitable zone, the distance at which water might exist in liquid form.

Whether there is any liquid present depends in part on whether the planet supports an atmosphere, and that is a hard question to answer. If Proxima Centauri b had formed near its present orbit, it might have seen its early atmosphere blown away during one of its host star’s more active phases. But researchers know frustratingly little about the evolution of red dwarf stars like Proxima Centauri. Furthermore, the planet might have formed farther out and migrated inward later, in which case the star's activity wouldn't matter.

Since we can't reason out whether there's an atmosphere, the alternative is to look for one. This isn’t as easy as it sounds. Despite being the closest star, it’s still about 4.25 light-years away, far enough to be an observational challenge. According to a manuscript posted to the arXiv however, we’re set to launch the tool we'd need in 2018: the James Webb Space Telescope.

Observational challenges

Having a planet transit in front of its host star makes doing science on exoplanets easier. For Proxima b, it’s not yet known whether it transits, but there’s a less than 1-percent chance that it does.

So this team wanted to find an alternate method of observation. One possibility is simply imaging the planet directly. This would be extremely difficult, however, since the planet is so close to its star. To make out this separation would require at least a 30-meter telescope. For comparison, the James Webb will have a diameter of 6.5 meters. Next-generation, “extremely large” ground-based telescopes will be able to make this kind of observation, but those won’t be ready 'til the 2020s.

Another possibility is to measure variations in Proxima Centauri's light as it changes with the planet’s orbit. Since the planet is reflecting some of the star's light toward us, there should be changes in the star’s apparent output based on the location of the planet. When Proxima b is moving away, the reflected light will be shifted toward the red end of the spectrum, then toward blue as it approaches us. High-resolution spectroscopy in conjunction with high-contrast imaging could reveal the planet’s albedo and angle of inclination. On current telescopes, this would take roughly a hundred hours of total observation time. (It would take just a single night on next-generation, extremely large telescopes.)

The method these researchers focus on, however, relies on the fact that Proxima b is likely tidally locked, which means that one side of it permanently faces its star. If the planet has an atmosphere, its winds would serve to distribute some of the heat to the cold side of the planet. Thus, it should be possible to measure how much the planet is radiating in the infra-red. As the planet goes through its orbit, we’ll see more or less of its cold side, depending on its orientation.

That way, if the planet lacks an atmosphere, the amount of heat it’s releasing in our direction should change pretty starkly during the course of its orbit. If it has an atmosphere, however, the heat would be more evenly distributed, so we’d observe less of a stark change in heat across its orbit. If we make these observations, in other words, it should become pretty obvious whether Proxima b has an atmosphere.

Simulations and caveats

The researchers ran simulations of what the James Webb might see. They concluded the Webb would give us a robust confirmation, either of an atmosphere or of a lack of one, with five-sigma confidence.

They also considered the possibility of detecting ozone in the planet’s atmosphere. If it were present, ozone would absorb some wavelengths of the IR light as it leaves the atmosphere, leaving distinctive gaps or lines in the spectrum. A hypothetical alien civilization looking at Earth would detect strong ozone lines in Earth’s spectrum, because we have an ozone layer. It’s an interesting gas to look for because oxygen could be a signature of life.

These conclusions are based on a few assumptions. First, we won't really know how precise the Webb's measurements will be until it's in space. The researchers recommend testing this as part of the James Webb Space Telescope Early Release Science program. Unsurprisingly, they suggest Proxima Centauri be used as a test target.

The researchers’ conclusions also depend on whether we can determine the planet’s inclination angle, as well as the ratio of its size to that of the star. Again, there’s good reason to think these can be measured: the inclination with ground-based measurements and the size ratio based on data from previous observations of the star. Furthermore, what we end up seeing depends in part on how much IR the planet’s surface rock is emitting. Different rocks emit more or less IR, and the researchers assume high emissivities.

Regardless, the observations would be a big deal. “Either way, these observations will provide a major advance in our understanding of terrestrial worlds beyond the Solar System,” the researchers write in their paper.

The Verdict

In general, the reaction of other researchers was positive.

“In principle, the observations are possible, which makes this a tantalizing system,” Kevin Stevenson of the Space Telescope Science Institute told Ars. (Stevenson was not involved in this study but has worked with the lead author in the past.) “However, the authors have made numerous assumptions, which they acknowledge, regarding JWST and the system. Given that we will never find a potentially habitable exoplanet closer than Proxima Centauri b, the risk is certainly worth the rewards.”

Sara Seager of MIT was a bit more cautious. “It looks fine,” she told Ars, though she went on to question “why [the authors] didn’t consider the case of an atmosphere that does not redistribute [heat around the planet]—we will have no way of knowing for sure if they see a signal that looks like a bare rock if it actually is.”

The paper’s lead author, Laura Kreidberg of the University of Chicago, didn't think this was an issue: “I suppose it would be possible to cook up a pathological scenario in which very little heat is transferred (perhaps if the atmosphere were extremely tenuous). But modeling work has shown that for a wide range of atmospheric compositions, the heat circulation is efficient.”

Seager also pointed out that searching for the ozone signal would require up to a hundred hours of observation time. Seager wondered “if it would really be possible to bin together data on that time scale robustly.”

Kreidberg, however, suggested it would be a valuable test of the hardware: “But the tremendous light-gathering power of Webb and the thermal and pointing stability of the telescope are exactly what we need to make these observations successful. But we will definitely want to get test observations of Proxima during the early commissioning phase of JWST to confirm that the detector works at the level of precision required.”

John Mather, the senior project scientist on the James Webb Space Telescope, was also optimistic. “I think the paper in question is pretty good; the authors know what they are talking about regarding the planet,” he told Ars. “We definitely did not design the telescope with this target in mind, considering that we started work 21 years ago. We won’t know whether the telescope has the needed stability and sensitivity until after launch. Needless to say we would all like to find out right away, but this is one of the most difficult targets, and it will take a while to learn how to use the equipment in the best possible way. I am certainly optimistic, since we don’t know of anything in the hardware that would prevent the observations.”

Mark Clampin, project scientist for the James Webb Space Telescope at NASA's Goddard Space Flight Center, was enthusiastic (we have more from him below) but shared concerns with Mather about the ozone part. “I think based on this paper alone, the first part of the observation, people would probably want to do. I think that trying to do the ozone observation is something that would probably have to wait until we understand the instruments better.”

And even if we detect ozone, that wouldn’t be a sure sign of life. “I think if you make that observation and you were able to get a positive result, it’s another piece of the puzzle. It’s not the sort of ‘hail Mary.’ I think scientists generally want to see a lot more evidence than just one line. These bio-signatures generally require you to see a number of different lines, different parts of the band.”

Still, Clampin went on to say that, if we did spot ozone, it would inform how we think about the next generation of exoplanet-observing hardware.

arXiv, 2016. Abstract number: arXiv:1608.07345v1 (About the arXiv).