Spitzer Spies Surface of Exoplanet for Signs of Atmosphere

The Cosmic Companion
Aug 20 · 5 min read

The Spitzer Space Telescope examined LHS 3844b, an unusual world orbiting a dim star. This is the smallest planet yet to have its atmosphere studied through infrared light reflecting from its planetary surface.

The exoplanet LHS 3844b has a mass 30 percent greater than the Earth, and it orbits a small, dim, M dwarf star. The surface of this world is covered in dark, volcanic material, similar to the darker mare seen on the Moon and Mercury. This new study shows there is little to no atmosphere enveloping this alien world.

An artist’s illustration of the exoplanet LHS 3844b. Image credit: NASA/JPL-Caltech/R. Hurt (IPAC)

“We’ve got lots of theories about how planetary atmospheres fare around M dwarfs, but we haven’t been able to study them empirically. 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,” Laura Kreidberg of the Harvard and Smithsonian Center for Astrophysics said.

Small M dwarf stars, like the one around which LHS 3844b orbits, are the most common, and longest-lived, stellar bodies in the galaxy. Because of this, these types of stars are thought to host a large percentage of the exoplanets in the Milky Way.

The planet LHS 3844b revolves close to its sun, once every 11 hours. With an orbit so tight around its sun, this world is likely tidally locked to its parent star, with one face eternally facing the star, like the Moon orbiting the Earth. Temperatures on the side facing the star were measured as high as 770 degrees Celsius (1,410 Fahrenheit), hot enough to melt aluminum.

This means the planet radiates brightly in infrared light, and this energy is not entirely drowned out by its relatively cool star. This provided astronomers a chance to study the surface of the world using the Spitzer space telescope, an infrared telescope exploring the galaxy in wavelengths invisible to the naked eye.

Will this be on the TESS?

This alien world, LHS 3844b, was discovered in 2018 by the Transiting Exoplanet Survey Satellite (TESS), as the planet orbited in front of its parent star as seen from Earth.

Detecting Light from Exoplanet LHS 3844b. Image credit: NASA/JPL-Caltech/L. Kreidberg (CfA | Harvard & Smithsonian)

Viewing planets close to their stars can be difficult, as the light from the star drowns out the light reflected from the surface of the world. This effect is analogous to trying to see detail on the surface of an insect flying near a searchlight. However, this system offered the optimal conditions of a dim star and bright planet, making this study possible.

The Spitzer Space Telescope, an orbiting observatory examining heavenly bodies in infrared light, was utilized to examine the surface of this world, 48.6 light years from Earth. The IRAC camera, designed by the Center for Astrophysics (CfA), captured the infrared data needed to explore the surface — and atmosphere — of LHS 3844b.

“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 Center for Astrophysics explains.

The Life of the Party

The atmosphere of Earth is one of the critical ingredients of our planet that make life possible. Air not only provides terrestrial lifeforms with gasses to breathe, but it also helps moderate planetary temperatures.

Mars was once replete with water, and primitive life may have evolved on its surface. But, billions of years ago, the Red Planet lost most of its atmosphere, and all the water on its surface.

Astronomers measured temperature differences on the “day” and “night” sides of this world, in order to determine if LHS 3844b holds onto a significant atmosphere. They found the difference in temperatures on each side of this exoplanet was dramatic, suggesting there is little to no atmosphere to carry heat from the hot side of the planet to the dark side.

“The temperature contrast on this planet is about as big as it can possibly be. That matches beautifully with our model of a bare rock with no atmosphere,” Kreidberg stated.

By studying atmospheres around exoplanets, researchers hope to better understand how some worlds might support life, while others develop in such a way that the development of life, as we know it, would be nearly impossible.

We all have our Talents

The class of M dwarf stars are dim in visible light, but they emit vast amounts of ultraviolet radiation. They are also highly-active when they are young, potentially stripping away atmospheres surrounding planets in their systems.

“I’m still hopeful that other planets around M dwarfs could keep their atmospheres. The terrestrial planets in our solar system are enormously diverse, and I expect the same will be true for exoplanet systems,” Kreidberg said.

An artist’s conception of the Spitzer Space Telescope, carrying out its mission of exploration. Image credit: NASA/JPL

Researchers believe the dark outer layer of LHS 3844b is basalt, a form of volcanic rock that makes up most of the ocean floor here on Earth. This material also forms the dark regions (mare) on the Moon, which were the product of vulcanism in the ancient past. Researchers believe a similar process, detailed in Nature, may have formed the surface of LHS 3844b.

Studying the surface of an exoplanet in order to examine a planetary atmosphere is extremely challenging, and LHS 3844b is the smallest world for which this has yet been done.

Astronomers have examined the atmospheres of gas giants around other stars, including the fascinating worlds of the TRAPPIST-1 system, but LHS 3844b is much smaller than those targets.

The Spitzer Space Telescope, carrying out science in space since 2003, will come to an end in 2020, when engineers will command the spacecraft to shut down forever.

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The Cosmic Companion

Exploring the wonders of the Cosmos, one mystery at a time

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Written by

James Maynard is the author of two books, and thousands of articles about space and science. E-mail: thecosmiccompanion@gmail.com

The Cosmic Companion

Exploring the wonders of the Cosmos, one mystery at a time

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