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Image: Earth transiting sun
When Earth passes in front of the sun, it blocks a small part of the sun’s light. Potential observers outside our solar system might be able to detect the resulting dimming of the sun and study Earth’s atmosphere. (Credit: Axel Quetz / MPIA / NASA)

The past decade has brought about a revolution in astronomers’ ability to detect potentially habitable planets, and there’s much, much more to come. The problem will be identifying the likeliest places for life to lurk, and two newly published studies address that problem from two dramatically different perspectives.

One study takes an inward-looking perspective: If we were the aliens, how would we know about Earth?

The best planet-detection method that’s currently available to earthly astronomers looks for the telltale dimming of light as a planet crosses the disk of its parent star. But that dimming, known as a planetary transit, can be seen only when the planet and the alien star are lined up with Earth.

That suggests that Earth is most likely to be detected by observers on alien planets in a narrow strip of the sky where our planet can be seen crossing the sun.

Transit zone
This image shows the narrow transit zone from which distant observers could see Earth pass in front of the sun. (Credit: Axel Quetz for MPIA / Axel Mellinger for Central Michigan University)

The study, which is being published in the March issue of the journal Astrobiology, estimates that Earth’s transit zone amounts to only 0.2 percent of the sky as seen from our planet. Potentially habitable planets that lie in that strip should be put at the top of the list in the search for extraterrestrial intelligence, or SETI, the study’s authors say.

“The key point of this strategy is that it confines the search area to a very small part of the sky,” Rene Heller, an astronomer at the Max Planck Institute for Solar System Research, said in a news release. “As a consequence, it might take us less than a human life span to find out whether or not there are extraterrestrial astronomers who have found the Earth. They may have detected Earth’s biogenic atmosphere and started to contact whoever is home.”

Heller and the study’s other co-author, McMaster University’s Ralph Pudritz, estimate that about 100,000 nearby stars could harbor planets with inhabitants who could be trying to contact us. And they list 82 sunlike stars in the transit zone that are known to lie within 3,260 light-years of Earth and have a good chance of hosting habitable planets.

How can we be sure which of those planets harbor life? The other study suggests a way to do a reality check on readings that NASA’s James Webb Space Telescope is expected to get after its launch in 2018.

The Webb telescope should be good enough to determine the ingredients in the atmospheres of thousands of planets far beyond our solar system, including oxygen. On Earth, oxygen is considered a “biosignature” associated with life’s presence – but on extrasolar planets, that ain’t necessarily so.

For example, astronomers already have detected oxygen as well as carbon in the atmosphere of a “hot Jupiter” called HD 209458 b. However, the conditions there are in no way conducive to life. In this example, the detection of oxygen is a false positive for habitability.

“We wanted to determine if there was something we could observe that gave away these ‘false positive’ cases among exoplanets,” Edward Schwieterman, a doctoral student in astronomy at the University of Washington, said in a news release.

Schwieterman and his colleagues lay out their strategy in a paper published last week in Astrophysical Journal Letters. They focused on a couple of scenarios that could produce atmospheric oxygen through non-biological means – particularly on planets orbiting low-mass stars.

In one scenario, the star’s ultraviolet light causes carbon dioxide molecules to break down, freeing up oxygen atoms to form atmospheric O2 molecules. Computer modeling showed that such a process should produce significant amounts of carbon monoxide as well. “So if we saw carbon dioxide and carbon monoxide together in the atmosphere of a rocky planet, we would know to be very suspicious that future oxygen detections would mean life,” Schwieterman said.

In the other scenario, starlight causes atmospheric water vapor to break down, turning H2O into hydrogen and oxygen. The hydrogen atoms escape into space, but the oxygen stays behind. This process should produce lots of O4 molecules as well as O2 molecules. “Seeing a large O4 signature could tip you off that this atmosphere has far too much oxygen to be biologically produced,” Schwieterman said.

UW astronomer Victoria Meadows, principal investigator for the university’s Virtual Planetary Laboratory, said such insights will help astronomers analyze and prioritize the flood of data that’s expected to come from the James Webb Space Telescope.

“This research is important because biosignature impostors may be more common for planets orbiting low-mass stars, which will be the first places we look for life outside our solar system in the coming decade,” she said.

In addition to Schwieterman and Meadows, the authors of “Identifying Planetary Biosignature Impostors: Spectral Features of CO and O4 Resulting From Abiotic  O2/O3  Production” include Shawn Domagal-Goldman, Drake Deming, Giada Arney, Rodrigo Luger, Chester harman, Amit Misra and Rory Barnes.

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