In newly published research, CU Boulder scientists study a rocky exoplanet outside our solar system, learning more about whether and how planets maintain atmospheres

In June 2019, Harvard astrophysicists discovered a rocky exoplanet 22 light years from Earth. Analyzing data from the Transiting Exoplanets Survey Satellite (TESS), they and other scientists around the world learned key details about the rocky exoplanet named LTT 1445 A b: It is almost 1.3 times the radius of Earth and 2.7 times Earth’s mass and orbits its M-dwarf star every 5.4 days.

What they couldn’t ascertain from those data, however, was whether LTT 1445 A b has an atmosphere, “and that’s a big general question even in our own solar system: What sets how much atmosphere a planet has?” says Zach Berta-Thompson, a University of Colorado Boulder assistant professor of astrophysical and planetary sciences. “Atmospheres matter for life, so before we go searching for life on other planets, we need to understand a very basic question—why does a planet have atmosphere or not have atmosphere?”

 

Pat Wachiraphan (left), a PhD student in the CU Boulder Department of Astrophysical and Planetary Sciences, and Zach Berta-Thompson (right), an assistant professor in the department, collaborated with colleagues around the country to study JWST data about rocky exoplanet LTT 1445 A b.

Now, after detailed analysis of data from the James Webb Space Telescope (JWST), a lot more is known—and was recently published—about LTT 1445 A b, whether it has an atmosphere and what its atmosphere might be if it has one. CU Boulder researchers partnered with astrophysicists around the country to build on previous research that ruled out a light hydrogen/helium-dominated atmosphere but could not distinguish between a cloudy atmosphere, an atmosphere composed of heavier molecules like carbon dioxide or a bare rock.

The paper’s first author, Pat Wachiraphan, a PhD student studying astrophysical and planetary sciences, Berta-Thompson and their colleagues analyzed three eclipses of LTT 1445 A b from the JWST, watching the planet disappear behind its star and measuring how much infrared light the planet emits. From this, they were able to rule out the presence of a thick carbon dioxide atmosphere like the one on Venus, which has about 100 times more atmosphere than Earth. This highlights an important aspect of science: Sometimes just as much is learned from understanding what something isn’t as from defining what it is.

“What I think should be the next step, naturally, is to ask whether we might detect an Earth-like atmosphere?” Wachiraphan says.

Not like Venus

LTT 1445 A b is one of the closest-to-Earth rocky exoplanets transiting a small star, Wachiraphan notes, and thus one of the easiest to target when studying whether and how it and similar rocky exoplanets hold atmospheres.

The JWST is more sensitive to atmospheres of transiting exoplanets around smaller stars, and LTT 1445 A b transits one of the smallest known type stars—about 20 to 30% the radius of Earth’s sun.

In November 2020, Berta-Thompson and several colleagues submitted a proposal to the Space Telescope Science Institute, the international consortium that decides where JWST is pointed and for how long, “before the telescope had even launched,” he says. “Scientists from all over the world send in anonymized proposals where we make our case for why (JWST) should spend  hours looking at this particular patch of the sky and what we would be able to learn from that.

“A panel reads through the proposals, ranks them, from which a lucky 5% to 10% will be selected as the best possible scientific use of the telescope. It is such a precious resource that we care really deeply that the choices about who gets to use the telescope are made fairly; every minute of its time is accounted for.”

 

Rocky exoplanet LTT 1445 A b is in a three-star system; the star it orbits is an M-type star, also known as a red dwarf. (Artists' illustration: Luis L. Calçada and Martin Kornmesser/European Southern Observatory)

Studying data from three eclipses sent back by JWST, Wachiraphan, Berta-Thompson and their colleagues were able to chart thermal emission consistent with instant reradiation of incoming stellar energy from a hot planet dayside. “This bright dayside emission is consistent with emission from a dark rocky surface, and it disfavors a thick, 100-bar, Venus-like CO2 atmosphere,” the researchers noted.

“So, you can imagine that if you have a planet that is just a rock, with no atmosphere, it would be hot on day side and cold on the night side, but if it has atmosphere, then the atmosphere could redistribute heat from day to night,” Wachiraphan says.

In the case of LTT 1445 A b, “we were basically putting an infrared thermometer up to the planet’s forehead and learned its average temperature is around 500 Kelvin,” Berta-Thompson says. “The whole planet is like the inside of a hot oven, basically.

Based on the data sent back by JWST, there could be several ways to detect atmosphere on LTT 1445 A b. “We came up with an observation with this planet passing behind its star. When the planet is behind its star, we’d just get light from the star itself, but before and after the eclipse we’d get a little contribution from the planet itself, too.” Wachiraphan explains. “But you can also detect an atmosphere when a planet passes in front of its star. “The starlight coming out could pass through the atmosphere of the planet and get absorbed, and we could observe that absorption.”

More observations are currently planned for LTT 1445 A b, led by other scientists and using this complementary method of observation, Berta-Thompson says—of collecting data as the planet transits in front of its star. “There’s a lot more we can learn using different wavelengths of light and different methods that allow us to more sensitively probe these thinner atmospheres.”

Like the inside of a hot oven

One of the most fascinating questions for researchers studying exoplanets, Berta-Thopson says, is “what does it take for a planet to retain or maintain atmosphere? Learning more about that is an important step in the process toward finding a planet maybe like this one—that has a surface, has an atmosphere, is a little farther away from its star, where you can imagine it has liquid water at the surface. Then you’re asking, ‘Is this a place where life could potentially thrive? Is there a place where life is thriving?”

These questions are so interesting, in fact, that they’ve prompted the formation of the Rocky Worlds Program, with which Wachiraphan and Berta-Thompson will work closely, to support international collaboration on the next phases of exploration of rocky exoplanets using satellite data.

“Using this really magnificent telescope that is the collective effort of thousands of people over decades, let alone the broader community that found this planet, is the kind of thing that is under threat right now,” Berta-Thompson says. “All of this science and this discovery requires a really long, big, sustained investment in telescopes, in scientists, in education.”

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