ESO/M. Cornmesser
As of last month, we’re pretty sure there’s an Earth-mass planet orbiting Proxima Centauri, the closest star to Earth. This raises a rather obvious question: Can it sustain life? The planet, Proxima Centauri b, orbits within its star’s habitable zone, the distance at which water could exist in liquid form.
Whether liquid is present depends in part on whether the planet supports an atmosphere, and that’s a tough question to answer. If Proxima Centauri b had formed near its current orbit, it would 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. In addition, the planet may have formed further out and later migrated inward, in which case the star’s activity doesn’t matter.
Since we cannot reason whether there is an atmosphere, the alternative is to look for one. This is not as easy as it sounds. Despite being the closest star, it’s still about 4.25 light-years away, far enough away to be an observational challenge. However, according to a manuscript posted to the arXiv, we’re ready to launch the tool we need in 2018: the James Webb Space Telescope.
Observational Challenges
Having a planet transit for its host star makes science on exoplanets easier. For Proxima b, it is not yet known whether it will go ahead, but there is a less than 1 percent chance that it will.
So this team wanted to find an alternative method of observation. One possibility is to simply image the planet directly. However, this would be extremely difficult given that the planet is so close to its star. To discern 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 these kinds of observations, but they won’t be ready until the 2020s.
Another possibility is to measure variations in the light of Proxima Centauri as it changes with the planet’s orbit. Since the planet reflects some of the star’s light back to us, there should be changes in the star’s apparent output based on the planet’s location. As Proxima b moves away, the reflected light will be shifted to the red end of the spectrum and then to blue as it approaches us. High-resolution spectroscopy combined with high-contrast imaging could reveal the planet’s albedo and tilt angle. With current telescopes, this would take about a hundred hours of total observation time. (It would only take one night on next-generation extremely large telescopes.)
However, the method these researchers are focusing on is based on the fact that Proxima b is likely tidally locked, meaning that one side of it is permanently facing the star. If the planet has an atmosphere, the winds would serve to distribute some of the heat to the cold side of the planet. So it should be possible to measure how much the planet is radiating in the infrared. 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 has no atmosphere, the amount of heat it releases towards us should change quite dramatically over the course of its orbit. If it had an atmosphere, however, the heat would be distributed more evenly, so we would observe less of a change in heat over its orbit. In other words, if we make these observations, it should become quite clear whether Proxima b has an atmosphere.
Simulations and warnings
The researchers ran simulations of what the James Webb might see. They concluded that the Webb would give us robust confirmation, either of an atmosphere or lack thereof, 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 exits the atmosphere, creating noticeable 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 can be a sign of life.
These conclusions are based on a number of assumptions. First, we won’t really know how accurate the Webb’s readings 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 using Proxima Centauri as a test target.
The researchers’ conclusions also depend on whether we can determine the inclination of the planet, 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 measurements on the ground, and the size ratio based on data from previous observations of the star. In addition, what we ultimately see depends in part on how much IR the planet’s surface rock emits. Different rocks emit more or less IR and the researchers assume high emissivities.
Either way, the sightings would be a big deal. “Either way, these observations will represent a major advance in our understanding of terrestrial worlds beyond the solar system,” the researchers write in their paper.
The verdict
In general, the reactions of other researchers have been positive.
“In principle, the observations are possible, which makes this a tempting system,” Kevin Stevenson of the Space Telescope Science Institute told Ars. (Stevenson was not involved in this research, 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. Since we will never find a potentially habitable exoplanet closer than Proxima Centauri b, the risk is well worth the reward.”
Sara Seager from MIT was a little more cautious. “It looks good,” she told Ars, though she wondered “why [the authors] does not take into account the case of an atmosphere that does not redistribute [heat around the planet]- there’s no way we’ll know for sure if they’re seeing a signal that looks like a bare rock, if it really is.
The paper’s lead author, Laura Kreidberg of the University of Chicago, didn’t think this was a problem: “I suppose it would be possible to invent a pathological scenario where very little heat is transferred (perhaps if the atmosphere is extremely would be weak). But modeling work has shown that for a wide range of atmospheric compositions, heat circulation is efficient.”
Seager also pointed out that the search for the ozone signal would require up to a hundred hours of observation time. Seager wondered “if it would really be possible to robustly aggregate data at that time scale.”
However, Kreidberg suggested it would be a valuable test of the hardware: “But Webb’s tremendous light-gathering power and the telescope’s thermal and aiming stability are exactly what we need to make these observations successful. But we will certainly test observations from Proxima would like to receive during the early commissioning phase of JWST to confirm that the detector is operating at the required level of precision.”
John Mather, the senior project scientist at 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 with regard to the planet,” he told Ars. “We definitely didn’t design the telescope with this goal in mind, as we started working 21 years ago. We only know after launch whether the telescope has the necessary stability and sensitivity. Needless to say, we would all like to find out right away, but this is one of the most difficult goals and it will take some time to learn how to use the equipment in the best possible way. I’m certainly optimistic, as we don’t know 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 excited (we’ve got more of him below), but shared his concerns with Mather about the ozone part. “I think just based on this article, the first part of the observation, people would probably want to do it. I think trying to do the ozone observation is something that should probably wait until we understand the tools better.
And even if we detected ozone, it wouldn’t be a sure sign of life. “I think if you make that observation and you got a positive result, that’s another piece of the puzzle. It’s not the “Hail Mary” kind. I think scientists generally want to see a lot more evidence than just one line. These bio signatures generally require you to see some different lines, different parts of the tape.
Still, Clampin said if we discovered ozone, it would tell us how we feel about next-generation exoplanet-observing hardware.
arXiv2016. Abstract number: arXiv:1608.07345v1 (About the arXiv).