Astronomers have discovered an Earth-size exoplanet, a world beyond the solar system, that may be carpeted with volcanoes and could potentially support life. Called LP 791-18 d, the planet could undergo volcanic outbursts as often as Jupiter’s moon Io, the most volcanically active body in the solar system.
LP 791-18 d orbits a small red dwarf star about 90 light-years away in a southern constellation called Crater. The team behind the discovery estimates it is only slightly larger and more massive than Earth. It is tidally locked, which means the same side constantly faces its star. The side facing the star would probably be too hot for liquid water to exist on the surface, but the team suspects that the amount of volcanic activity potentially taking place all over the planet could sustain an atmosphere. These conditions may allow water to condense on the dark side of the planet.
“Only a small proportion of the exoplanets discovered so far are thought to be able to support life,” said Karen Collins, astronomer at the Center for Astrophysics. “Our discovery of LP 791-18 d gives us more hope that we might one day find signs of life on another planet.”
Discovery of the planet was made possible through a combination of space-based and ground-based observations. The researchers discovered and studied the planet using data from NASA’s Transiting Exoplanet Survey Satellite (TESS) and retired Spitzer Space Telescope, as well as a suite of ground-based observatories organized by Collins.
The team initially estimated the planet’s mass by measuring tiny differences in the time it takes the planet to orbit its host star from one orbit to the next, which are caused by the gravitational tug of the other planets in the system.
The planet’s volcanic activity might arise from other planets in the same system. Before the discovery of LP 791-18 d, astronomers already knew about two other worlds in the system, including LP 791-18 c. The outer planet, c, is much larger and more massive than planet d.
During each orbit, planets d and c pass very close to each other. During each close pass, the more massive planet c produces a gravitational tug on the newly discovered planet d, making its orbit somewhat elliptical. On this elliptical path, planet d deforms slightly more every time it goes around the star. These deformations can create enough internal friction to substantially heat the planet’s interior and produce volcanic activity at its surface. Jupiter and some of its moons affect Io in a similar way.
Planet d sits on the inner edge of the habitable zone, the traditional range of distances from a star where scientists hypothesize liquid water could exist on a planet’s surface. If the planet is as geologically active as the research team suspects, it could maintain an atmosphere. Temperatures could drop enough on the planet’s night side for water to condense on the surface.
Researchers have already received approval to study planet c using the recently launched James Webb Space Telescope, and the team thinks planet d is also an exceptional candidate for atmospheric studies by the mission.
“A big question in astrobiology, the field that broadly studies the origins of life on Earth and beyond, is if tectonic or volcanic activity is necessary for life,” said co-author Jessie Christiansen, a research scientist at NASA’s Exoplanet Science Institute at the California Institute of Technology (Caltech). “In addition to potentially providing an atmosphere, these processes could churn up materials that would otherwise sink down and get trapped in the crust, including those we think are important for life, like carbon.”
"This discovery is just a first step,” Collins said. “With the potential to continue studying this planet with the James Webb Space Telescope, we will be able to fine-tune our observations and learn more about the planet’s likely volcanically fueled atmosphere. Future discoveries will help us understand how the ingredients of life might have come to be on worlds other than our own.”
TESS is a NASA Astrophysics Explorer mission led and operated by MIT in Cambridge, Massachusetts, and managed by NASA's Goddard Space Flight Center in Greenbelt, Maryland. Additional partners include Northrop Grumman, based in Falls Church, Virginia; NASA’s Ames Research Center in California’s Silicon Valley; MIT’s Lincoln Laboratory; and the Space Telescope Science Institute in Baltimore. More than a dozen universities, research institutes and observatories worldwide participate in the mission.
Spitzer’s observations of the system where LP 791-18 d resides were among the last the satellite collected before it was decommissioned in January 2020. The entire body of scientific data collected by Spitzer during its lifetime is available to the public via the Spitzer data archive, housed at the Infrared Science Archive at IPAC at Caltech in Pasadena, California. NASA’s Jet Propulsion Laboratory, a division of Caltech, managed Spitzer mission operations for the agency’s Science Mission Directorate in Washington. Science operations were conducted at the Spitzer Science Center at IPAC at Caltech. Spacecraft operations were based at Lockheed Martin Space in Littleton, Colorado.
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