A rare look at the surface of a rocky exoplanet

A new study using data from the IRAC camera on NASA's Spitzer Space Telescope provides a rare glimpse at the conditions on the surface of a rocky planet around another star. The exoplanet very likely has little to no atmosphere, according to the data, and could be covered in the same cooled volcanic material that comprises the dark lunar regions known as mare. The exoplanet therefore might be similar to Mercury, or to Earth's Moon.

This exoplanet discovery, published in the journal Nature, is just the latest in a series of nearly 700 refereed exoplanet publications relying on IRAC since 2009 when Spitzer's Warm Mission began and IRAC became its only operating instrument. The IRAC camera's PI-team is based at the CfA and is led by Giovanni Fazio.

The planet, LHS 3844b, was discovered in 2018 by NASA's Transiting Exoplanet Satellite Survey (TESS) mission, is located 48.6 light-years from Earth, and has a radius 1.3 times that of Earth. It orbits a small, cool type of star called an M dwarf - especially noteworthy because, as one of the most common and long-lived types of stars in the Milky Way galaxy, M dwarfs may host a high percentage of the total number of planets in the galaxy.

TESS found the planet via the transit method which detects when the observed light from a parent star dims as its orbiting exoplanet crosses the line-of-sight between the star and Earth. TESS's Director of Science is CfA astronomer Dave Latham, and CfA astronomers are key members of the TESS Science Office Core and other TESS teams.

During follow-up observations, IRAC was able to detect light from the surface of LHS 3844b. The planet makes one full revolution around its parent star in just 11 hours. With such a tight orbit, LHS 3844b is most likely "tidally locked" with one side of the planet permanently facing the star. The star-facing side, or dayside, is about 1,410 degrees Fahrenheit (770 degrees Celsius).

Being relatively hot, the planet radiates copious amounts of infrared light which IRAC, an infrared camera, is able to measure. This observation marks the first time IRAC data have been able to provide information about the atmosphere of a terrestrial-sized world around an M dwarf.

The Search for Life
By measuring the temperature difference between the planet's hot and cold sides, the team concluded that there is a negligible amount of heat being transferred between the two. If an atmosphere were present, hot air on the dayside would naturally expand, generating winds that would transfer heat around the planet. On a rocky world with little to no atmosphere, like the Moon, there is no air present to transfer heat.

"The temperature contrast on this planet is about as big as it can possibly be," said CfA researcher Laura Kreidberg, lead author of the new study. "That matches beautifully with our model of a bare rock with no atmosphere."

Understanding the factors that could preserve or destroy planetary atmospheres is part of how scientists plan to search for habitable environments beyond our solar system. Earth's atmosphere is the reason liquid water can exist on the surface, enabling life to thrive. On the other hand, the atmospheric pressure of Mars is now less than 1% of Earth's, and the oceans and rivers that once dotted the Red Planet's surface have disappeared.

"We've got lots of theories about how planetary atmospheres fare around M dwarfs, but we haven't been able to study them empirically," Kreidberg said. "Now, with LHS 3844b, we have a terrestrial planet outside our solar system where for the first time we can determine observationally that an atmosphere is not present."

Compared to Sun-like stars, M dwarfs emit relatively high levels of ultraviolet light, which is harmful to life and can erode a planet's atmosphere. They're particularly violent in their youth, belching up a large number of flares - bursts of radiation and particles that can strip away budding planetary atmospheres.

The IRAC observations rule out an atmosphere with more than 10 times the pressure of Earth's. An atmosphere between 1 and 10 bars on LHS 3844b has been almost entirely ruled out, although the authors note a slim chance it could exist if the stellar and planetary properties were to meet some very specific and unlikely criteria. (Measured in units called bars, Earth's atmospheric pressure at sea level is about 1 bar.)

They also argue that with the planet so close to a star, a thin atmosphere would be stripped away by the star's intense radiation and winds. "I'm still hopeful that other planets around M dwarfs could keep their atmospheres," Kreidberg said. "The terrestrial planets in our solar system are enormously diverse, and I expect the same will be true for exoplanet systems."

A Bare Rock
The authors of the new study went one step further, using LHS 3844b's surface albedo (or its reflectiveness) to try to infer its composition. The Nature study shows that LHS 3844b is "quite dark," according to co-author Renyu Hu, an exoplanet scientist at NASA's Jet Propulsion Laboratory in Pasadena, California, which manages Spitzer. He and his co-authors believe the planet is covered with basalt, a kind of volcanic rock. "We know that the mare of the Moon are formed by ancient volcanism," Hu said, "and we postulate that this might be what has happened on this planet."

IRAC/Spitzer and NASA's Hubble Space Telescope have previously gathered information about the atmospheres of multiple gas planets, but LHS 3844b appears to be the smallest planet for which scientists have used the light coming from its surface to learn about its atmosphere (or lack thereof). IRAC previously used the transit method to study the seven rocky worlds around the TRAPPIST-1 star (also an M dwarf) and learn about their possible overall composition; for instance, some of them likely contain water ice. NASA plans to terminate the Spitzer/IRAC operations in February, 2020, as a cost-savings measure.

Headquartered in Cambridge, Mass., the Center for Astrophysics | Harvard and Smithsonian (CfA) is a collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe. This release is based on Spitzer-JPL release 2019-113.

Research Report: "Absence of a Thick Atmosphere on the Terrestrial Exoplanet LHS 3844b"

Related Links
Center For Astrophysics | Harvard and Smithsonian
Lands Beyond Beyond - extra solar planets - news and science
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