Astronomers have discovered yet another exoplanet to add to their growing list of other worlds that exist outside of our solar system. While thirty years ago, nobody knew if exoplanets existed, astronomers have quickly realized that they are quite common in the universe. Now the biggest question is how many exoplanets exist that are similar to Earth and might even harbor extraterrestrial life? This is why astronomers are so intrigued by a new exoplanet dubbed LP 791-18d.
Astronomers documented the details of a newly discovered exoplanet, published last week in the journal Nature, thanks to observations using data from NASA's Transiting Exoplanet Survey Satellite (TESS) and 127 hours of data from the retired Spitzer Space Telescope. According to the paper, the exoplanet is located in our very own Milky Way about 90 lightyears away from us. Unlike Earth, the exoplanet is tidally locked, meaning that the same side constantly faces its host star, which is a small red dwarf in the Crater constellation.
Despite these differences, LP 791-18d does have some Earth-like qualities. For example, the group of astronomers estimate that it's slightly more massive and larger than Earth. It's also in its host star's habitable zone, meaning that liquid water could exist on its surface. Perhaps most intriguing is there is also compelling evidence to suggest that it's partly covered in volcanoes.
"What if it didn't have volcanoes? Would that mean it's bad for life?"
While the astronomers didn't see volcanoes directly — it's pretty far away to get a clear photograph — observations on how LP 791-18d interacts with nearby exoplanets has provided strong data pointing to volcanic activity on the planet. This important observation has surfaced a debate on how important volcanic activity is on exoplanets, and whether or not it can be a strong indicator of life elsewhere in the universe.
"The thing that makes this one so special is that it's in the habitable zone, so that's a positive," Stephen Kane, a professor of planetary astrophysics at the University of California, Riverside and co-author of the paper, told Salon. "The huge amounts of volcanoes, that's what makes this one stand out."
Kane emphasized that it's important for planets to be geologically active, but quickly cautioned that the mere presence of volcanoes might not be enough evidence to strongly suggest that life could exist. Nonetheless, it's an important detail, especially given that volcanoes factor in some leading theories on the origin of life. But whether they are enough is an open debate.
"Back in March was the discovery that Venus has active volcanoes, and it is the opposite of habitable, so you need to be careful about things being described as 'this is great, but is it everything that you need?'" Kane said, adding that another way to pose the question around volcanoes, exoplanets and the possibility of life, is: "'What if it didn't have volcanoes? Would that mean it's bad for life?'"
Kane elaborated by saying if an exoplanet doesn't have volcanoes that means it's not contributing to its atmosphere anymore.
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"The Earth's atmosphere is actually getting blown away by the Sun, always, constantly, especially when the Sun is very active," Kane said. "Those periods of solar activity can blow away parts of the Earth's atmosphere, so the atmosphere needs to be rebuilt and the thing that's doing that is the Earth being volcanically active."
Without volcanoes, Kane said, Earth would look a lot like Mars, which has an atmosphere that's primarily composed of carbon dioxide.
"[Mars] has had its atmosphere blown away, and it doesn't have any active volcanoes, so it's not able to replace it," Kane said, though the red planet does have earthquakes, so it's not entirely dead geologically. "So that's why volcanoes are important; it's important that you have an ecosystem of the atmosphere in which it's been blown away and then it's been restored."
Two billion years ago, before the rise of oxygen, Earth would be unrecognizable
In other words, volcanoes on exoplanets could suggest that the exoplanet has an atmosphere. However, Kane said volcanoes can be somewhat of a "double-edged sword," meaning that too many of them would not be favorable to the possibility of life.
Seth Shostak, a senior astronomer at the SETI Institute who was not involved in the study, said volcanoes are "nature's way of cooling down."
"But does it tell you that there's life there? No," Shostak said. "It just tells you that the usual chemical processes that you would find on Earth, you can on find any place where you have, oceans, or liquids, if you will."
Shostak pointed to Jupiter's moon Io, which is the most volcanically active world in our solar system.
"It's got lots of volcanic activity too, but it's not clear that there's anybody alive on Io," Shostak said. "Having lots of volcanic activity is a predictable consequence of being in an orbit that's very close to a much larger body because that much larger body is going to be pushing and pulling anything that's orbiting that close."
Indeed, LP 791-18d is orbiting very close to exoplanets LP 791-18b and c. Kane told Salon he is "quite happy to be open" to the possibility of life existing on LP 791-18d.
"But the reason I say that is because we know so little about it," Kane said. "My suspicion, in this case, is that you have a planet that may have some surface liquid water, which is what then in the habitable zone means, but if it has a ton of volcanoes, it means it could have a very carbon dioxide rich atmosphere."
While this sounds like it's bad news for the possibility of life, Kane said not necessarily.
"You have to remember that Earth's early atmosphere was carbon dioxide dominated," he said. " The way a friend of mine put it really well, he said, 'past Earth was not Earth-like,' which is a very kind of profound statement."
He said two billion years ago, before the rise of oxygen, Earth would be unrecognizable.
"Having a carbon dioxide atmosphere doesn't preclude life, if in that case, as it was earlier, life just existed in the oceans," he said. "And so that's the sort of scenario that I imagine for this planet, potentially."