There are two primary ways to detect life on other planets: Direct observation of planetary surfaces and detecting signs of its influence. Unlike planets within our own solar system, exoplanets present a major challenge for optical telescopes, however the James Webb Space Telescope's infrared and near-infrared sensors allow for observing very distant objects. Published in The Astronomical Journal, Dr. Ben Lew at NASA Ames and collaborators focused JWST on a promising Jupiter-like planet orbiting a dim white dwarf star ~63 light years away, successfully measuring the composition of its atmosphere.
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Astro Brief is a podcast collaboration between KSMU, the Missouri Space Grant, and MSU's Department of Physics, Astronomy and Materials Science. Hosted by Dr. Mike Reed, Astro Brief focuses on astronomical events, the field of astronomy, and astronomy-related guests. It airs Thursdays at 9:45 am on KSMU.
Transcript
We have spent a lot of episodes talking about both exoplanets and possible prebiotic material in our own solar system, and a lot of that is to understand our place in the universe. We don't have aliens dropping out of the sky in UFOs or saying “howdy” in radio waves to the SETI group, so it seems up to us to determine if there is indeed life out there somewhere. To that end we search for the chemistry of life where we can. So far that has been a lot on Mars, on meteorites that come to us naturally, and a few return samples from comets and asteroids. Those are very local, and because we have reasonably high‑resolution imaging of most major bodies in our solar system, we know there’s nothing here bigger than microbes—except what we have here on Earth. I should add, at least on the surfaces. There are still the under‑ice oceans of many outer‑solar‑system moons like Europa and Titan, where there could be larger life forms, but obviously not without sufficient intelligence or technology to burrow through the ice to the surface.
Those results naturally have led us to look outside our own solar system. At this point we know that there are many, many planets out there in other solar systems orbiting other stars. What we would really like to know is whether there are habitable planets, and if so, whether they are inhabited. How do we get to that point? There are roughly only two ways: directly see the surface of Earth‑like planets—that’s certainly what we would love to do—or detect the influence of life on their environment, which means their atmosphere.
At this point we do not have the technological means to accomplish either of those, but the way to get there—and the way science accomplishes things—is to begin with what we can do and then learn how to do it better. For over a decade astronomers have been working on two approaches toward that end. The first is a method for observing exoplanet atmospheres. When a planet transits its star, not only does it block some of the starlight, but some starlight passes through the planet’s atmosphere. By isolating that portion of the light we can see what the atmosphere is made of.
However, atmospheres of Earth‑like planets are much, much smaller than those of hot, gassy planets. So the first measurements were of hot Jupiters. The resolution is still very poor, so only a couple of elements can be measured, but it was a start. Improvements have continued, and JWST has done that transit photometry on smaller, Neptune‑sized objects. But so far there are no Earth‑like exoplanets. That capability still seems beyond us, but we’re getting closer.
The second method is, of course, what we’d all love to have: direct imaging of the planet. The first successes were of big Jupiters 50 or more AU from their stars—about 50 Earth–Sun distances—or 50 AU from their stars. Improvements in light cancellation have allowed astronomers to find large planets slightly closer to their stars, now down to about 5 AU, or Jupiter’s orbital distance in our own solar system. But again those planets are really just reflecting starlight, and so we haven’t been able to see much of the planet itself—much less measure its contents—and only for really large Jupiters.
The next advance is a paper by Ben Liu of NASA Ames and collaborators published in an astronomical journal. They found a planet—a large Jupiter‑like planet—with a huge orbit around a white dwarf star. The star is called WD 0806–661. Obviously the “WD” stands for white dwarf, and the number represents its coordinates in the sky. It’s in the southern hemisphere in the constellation of Volans.
The planet—simply called lowercase b—is only 8 % larger in radius than Jupiter but 8 times its mass. This particular star–planet combination has many advantages for figuring out how to look at exoplanet atmospheres. First, white dwarf stars are about the size of Earth, much hotter than our Sun, but so small that they are not nearly as bright. Next, the planet is orbiting at about 2 500 AU—more than 80 times Pluto’s orbit. This combination means the planet is not swamped by starlight and can be seen on its own, and that’s exactly what Dr. Liu’s collaborators did.
They had to use JWST and look in the infrared, but they were able to measure several features of the planet’s atmosphere. They measured carbon monoxide and dioxide, methane, water vapor, and hydrogen sulfide. They were able to determine at what depths the chemistry mixed. So while this is still quite distant from measuring the atmospheres of an Earth‑like planet, it is another advancement in that direction.