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Astronomers discover nearby exoplanet with substantial atmosphere

An artistic rendering shows TOI-1231 b, a Neptune-like planet located roughly 90 light years from Earth. Image by NASA/JPL-Caltech
An artistic rendering shows TOI-1231 b, a Neptune-like planet located roughly 90 light years from Earth. Image by NASA/JPL-Caltech

June 8 (UPI) -- Astronomers have discovered a temperate, sub-Neptune-sized exoplanet orbiting a nearby M dwarf star.

Initial observations of the Earth-like planet, described Wednesday in the Astronomical Journal, suggest the alien world boasts a substantial atmosphere -- which is sure to inspire followup studies for years to come.

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The exoplanet, TOI-1231 b, was initially spotted using photometric data from the Transiting Exoplanet Survey Satellite, or TESS.

Followup observations captured using the Planet Finder Spectrograph on the Magellan Clay telescope at Las Campanas Observatory in Chile allowed scientists to work out the planet's radius, mass and density.

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TESS trains its gaze on sections of the night sky for nearly a month at time, capturing images of thousands of stars.

Astronomers, citizen scientists and algorithms scan the data for dimming patterns made by exoplanets as they orbit across the face of their host stars.

Because M dwarf stars are smaller and dimmer than stars like the sun, the transits of nearby stars have a more pronounced dimming effect.

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Still, astronomers were lucky that TESS picked up on the presence of TOI-1231 b, which takes 24 days to complete an orbit around its host star.

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To confirm the presence of an exoplanet, TESS typically needs to capture two transits, or the completion of an orbit.

Because TESS only stares at one section fo the sky for 28 days, the average orbital period of exoplanets spotted by TESS is 14 days.

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Scientists said TESS caught TOI-1231 b at just the right time.

"Working with a group of excellent astronomers spread across the globe, we were able to assemble the data necessary to characterize the host star and measure both the radius and mass of the planet," lead study author Jennifer Burt said in a press release.

"Those values in turn allowed us to calculate the planet's bulk density and hypothesize about what the planet is made out of. TOI-1231 b is pretty similar in size and density to Neptune, so we think it has a similarly large, gaseous atmosphere," said Burt, a scientist at NASA's Jet Propulsion Laboratory.

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In addition to enabling a more robust exoplanet transiting signal, an M dwarf star's small stature also makes it easier for astronomer to calculate the masses of newly discovered exoplanets, as the ratio of planet to stellar mass is larger.

Astronomers calculate an exoplanet's mass by measuring the planet's slight gravitational effect on its host star -- the smaller the star, the larger an exoplanet's gravitational effect will be.

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"Even though TOI-1231 b is eight times closer to its star than the Earth is to the sun, its temperature is similar to that of Earth, thanks to its cooler and less bright host star," study co-author Diana Dragomir said in the release.

"However, the planet itself is actually larger than earth and a little bit smaller than Neptune -- we could call it a sub-Neptune," said Dragomir, an assistant professor of astronomy at the University of New Mexico.

What has astronomers most excited about TOI-1231 b is its atmosphere.

Previous studies suggests small, cool planets similar to the newly discovered exoplanet are capable of holding water in the upper layers of their atmosphere.

"The low density of TOI-1231 b indicates that it is surrounded by a substantial atmosphere rather than being a rocky planet. But the composition and extent of this atmosphere are unknown!" said Dragomir.

"TOI-1231 b could have a large hydrogen or hydrogen-helium atmosphere, or a denser water vapor atmosphere. Each of these would point to a different origin, allowing astronomers to understand whether and how planets form differently around M dwarfs when compared to the planets around our sun, for example," Dragomir said.

The researchers said they hope future observations will help them work the exoplanet's atmospheric composition, as well as determine how rare or common such exoplanets are.

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