The 21st Century is an exciting time for those who study exoplanets. As the number of confirmed worlds in orbit around far-away stars grows almost daily—as of this writing the NASA Exoplanet Archive lists 3,489 of them—Lucianne Walkowicz says it’s becoming more than just a tally.
“We are starting to get to know these planets not just as a new marble in the bin, but something that you can understand what its environment might be like,” Walkowicz said. “We’re right around the corner from being able to really understand these places as worlds, and not just compare their size or their orbit to the planets in our solar system.”
Walkowicz is an astronomer at the Adler Planetarium in Chicago who earned her master’s and doctoral degrees at the University of Washington. She was back in Seattle recently to give a talk at the Pacific Science Center about the latest in exoplanet research and the prospects for figuring out if life exists on any of them. The talk complemented the Mission: Find Life! astrobiology exhibit that is presently running in the center’s Portal to Current Research space. (Check our previous post and podcast for more about the exhibit.)
While a majority of the early exoplanet finds by the Kepler mission were of gas giants on the order of Jupiter, Walkowicz said that was because bigger planets are easier to find because they block more light when they pass in front of, or transit, their host stars.
“The smaller planets that are more like Earth that might be out taking a full year to go around their star are even harder (to find) because you have a very small signal that takes years and years to repeat,” she said. As time went on, more and more smaller exoplanets were detected.
It turns out that, “The things that are a little bit bigger than Earth and little bit smaller than Neptune are super common,” Walkowicz noted. This threw planetary formation theorists for a loop. They’d spent careers constructing formation models that did not result in planets of that size because we don’t have any in our solar system. While journalists tend to rush toward a declaration of some of these planets as “Earth-like,” Walkowicz said that’s typically premature.
“It’s not the discovery of life around other stars,” she said of the finding exoplanets in this size range, “but it does mean that there is a lot of real estate for us to look.”
Goldilocks lives in the habitable zone
Walkowicz discussed the concept of the so-called habitable zone around stars—an area that could be at the right temperature for liquid water to exist on a planet’s surface. This is a ballpark figure; she noted that distant astronomers looking at our solar system would declare Venus, Earth, and Mars all to be within our Sun’s habitable zone. Yet these are very different worlds and two of them are not particularly hospitable at present. The type and kind of starlight, a planet’s orbit, tidal heating, radiation protection, and the actual existence of water, ice, haze, or clouds all can make big differences for a planet’s potential habitability.
As we move forward, astronomers will look not just at exoplanets’ location, but also at their biosignatures, which Walkowicz explained are the, “signatures of the chemical species that make up a planet’s atmosphere and whether they mean that life can be there.” Finding oxygen, ozone, water, carbon dioxide, and methane all could be positive indicators for life. On the other hand, finding carbon monoxide in a planet’s atmosphere would mean life is less likely. But even the right ingredients don’t necessarily mean life exists.
Local help from VPL
The Virtual Planetary Laboratory (VPL) at the University of Washington is leading the way in looking at such biosignatures and interpreting what they could mean for varying planets around different types of stars. Having so many possible combinations can be on the confusing side.
“Planets are complicated,” Walkowicz said. “They’re not just spheres orbiting around a star that’s getting some illumination at some distance.”
“It’s usually just not one thing, its the balance of things overall. You have to be able to look at the complete picture in order to interpret what you find,” she added.
That is difficult to do at present, but a couple of future space telescopes will be able to help. The Transiting Exoplanet Survey Satellite (TESS) is scheduled to launch next March, and the James Webb Space Telescope is targeted for launch in October 2018. TESS will look for more exoplanets by watching for their transits in front of some 200,000 nearby stars. The Webb, according to Walkowicz, will look into the infrared and explore the plentiful small, red stars in the galaxy and try to take the spectra of the atmospheres of planets in orbit around them.
“In some cases, we’ll be able to get this chemical fingerprint of what the planet’s atmosphere is made up of,” Walkowicz enthused. She added that the VPL’s work will help astronomers decide which planets to study with the new space telescopes—it will be important to winnow the field down to the most promising candidates, as competing demands for time on these scopes will limit the observing opportunities for any one project.
We might well have a better idea if there’s life elsewhere in the not-too-distant future.