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A 2007 image from the APL-built STEREO B spacecraft of the moon crossing in front of the sun. A new conceptual study proposes to observe the Earth in a similar way, to learn if the transit method can determine a planet’s habitability.
If astronomers were to observe Earth from another solar system, could they tell that our planet is teeming with life? By examining Earth in the same way that we look at exoplanets — planets orbiting other stars — we might improve our chances of detecting alien organisms on distant worlds, researchers recently suggested.
Since 1999, a process for spotting exoplanets, known as the transit method, has revealed thousands of worlds by measuring fleeting dips in the brightness of the stars that the planets orbit. No one knows whether or not these worlds host life, but if scientists were to peer at Earth using the transit method, they would likely spot definitive signatures of life.
Once those signatures have been identified from Earth observations, experts could then look for those same clues in exoplanets. Scientists recently described this approach as a mission concept called Earth Transit Observer (ETO), presenting it on March 17 at the 52nd Lunar and Planetary Science Conference 2021, held virtually this year due to the COVID-19 pandemic.
Most of the exoplanets we know about were found through the transit method, according to NASA. Powerful telescopes, such as the Kepler Space Telescope and the Transiting Exoplanet Survey Satellite (TESS), can detect when the passage of an orbiting planet briefly dimmed a star's light, even for stars that are thousands of light-years away. Scientists can estimate how big such a planet is, based on how much light it blocks. They can also calculate the size of its orbital path by measuring how much time passes between dimming events.
A host star's size and temperature, and how near or far the planet is from the star, provide more clues about how hospitable to life an exoplanet might be. Transits can also hint at an exoplanet's atmosphere. During a transit, a star's light filters through atmospheric molecules, which absorb certain frequencies. The result can help researchers to identify elements such as oxygen and methane. However, such signatures are typically so small that astronomers need dozens of transit observations to confirm that these elements are present, the scientists said in a statement.
However, other factors on the planet and star can also affect readings of atmospheric molecules. For example, planets experience shifts in their seasons, weather patterns and ocean currents, while solar activity — such as the ebb and flow of solar winds and the formation of powerful solar storms — is also highly variable. Any of these conditions could shape atmospheric behavior during different transits, potentially affecting ratios of atmospheric molecules and elements, according to the statement.
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In order to understand those variables, "you need to know your stars as well as anticipate what your planet is going to look like," said Laura Mayorga, lead author on an upcoming paper about the mission proposal in The Planetary Science Journal.
This can be challenging when both the star and exoplanet are unfamiliar, she added.
"It’s a very hard problem," Mayorga, an exoplanet astronomer at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, said in the statement.
Luckily, scientists already have all those answers for an inhabited planet/star pair: Earth and the sun. For the ETO mission, a small satellite with equipment capable of imaging the light spectrum from near-violet to near-infrared would watch Earth as it passed in front of the sun. The spectrograph would check for signs of water and carbon dioxide, as well as biosignature pairs — oxygen and methane, and ozone and methane — which together indicate conditions that are favorable for hosting life (of course, it remains to be seen if such signatures are just unique to life on Earth).
"The transit technique used by such an investigation would be the same that will be used by the James Webb Space Telescope (JWST) to study some of the thousands of known exoplanets transiting their host stars," the scientists wrote in the presentation. The ETO satellite would peer at Earth from a distance of 930,000 miles (1.5 million kilometers), about where the JWST will orbit the sun, after it launches on Oct. 31.
Because the climate variabilities on Earth and the activity patterns of our sun are well-known, scientists can observe how they affect readings of atmospheric molecules, and then apply that to observations of "new Earths," according to the report.
“The solar system is the only place where we know all the right answers to things. We can test our techniques, figure out their limitations and make connections between the results," Mayorga said in the statement.
"Can we then connect that with the unresolved observations we normally make of exoplanets, and test the method of stacking up low-signal observations? That’s really where we want to go," Mayorga added.
The scientists plan to submit the ETO proposal to NASA’s Astrophysics Pioneers Program in the fall of 2021, according to the statement. This program develops astrophysics missions that use smaller equipment and require smaller budgets than missions in the agency's Explorers Program, according to NASA.
Mindy Weisberger is a science journalist and author of "Rise of the Zombie Bugs: The Surprising Science of Parasitic Mind-Control" (Hopkins Press). She formerly edited for Scholastic and was a channel editor and senior writer for Live Science. She has reported on general science, covering climate change, paleontology, biology and space. Mindy studied film at Columbia University; prior to LS, she produced, wrote and directed media for the American Museum of Natural History in NYC. Her videos about dinosaurs, astrophysics, biodiversity and evolution appear in museums and science centers worldwide, earning awards such as the CINE Golden Eagle and the Communicator Award of Excellence. Her writing has also appeared in Scientific American, The Washington Post, How It Works Magazine and CNN.
