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Astronomers Have Found the Perfect Exoplanet to Study Another World’s Atmosphere

TESS (Transiting Exoplanet Survey Satellite) has found a new planet, and the discovery of this sub-Neptune exoplanet has scientists excited about atmospheres. The combination of the planet’s size, its thick atmosphere, and its orbit around a small M-class star close to Earth provides researchers with an opportunity to learn more about exoplanet atmospheres. We’re getting better and better at finding exoplanets, and studying their atmospheres is the next step in understanding them as a whole.

All of our exoplanet-detection strategies have an observation bias. It seems impossible to avoid. Even TESS (Transiting Exoplanet Survey Satellite), probably our most adept planet-finder, has an observation bias. Its predecessor Kepler was biased towards larger planets, and TESS doesn’t share that bias. But TESS still has a sort of blind spot due to how it operates.

No telescope can look everywhere at once, and TESS is no exception. It observes the sky mostly in 28-day chunks. So for one of those chunks, it focuses on one area for 28 days. To be confirmed as an exoplanet, an object must pass in front of its star twice in that 28 days. The end result of all this is that most of the planets TESS finds have orbital periods of less than 14 days.

Most of TESS’s observing is done in 28 day chunk, as the image shows. Image Credit: NASA/JPL

But this new planet, named TOI-1231 b, has a 24-day orbital period. This makes it a great target for the study of exoplanet atmospheres because it’s in front of its star longer and can be more easily studied. Universe Today readers know that studying light as it interacts with things is how we gain most of our knowledge about space. TESS itself won’t study the planet. Other missions like the James Webb Space Telescope (JWST) will take care of that by watching the starlight as it passes through the planet’s atmosphere.

“This new planet we’ve discovered is still weird – but it’s one step closer to being somewhat like our neighborhood planets.”

Jennifer Burt, Paper Lead Author, NASA-JPL.

Since TOI-1231 b spends so much time in front of its star relative to other TESS planets, missions like the JWST will get a much better look at it.

But it’s not only the planet’s orbital period that makes it an ideal target. Its size relative to its star also helps. Since the star is so small, the planet blocks out more of its light than if the planet and star were more similar to Earth and the Sun. “In a sense, this creates a larger shadow on the surface of the star, making planets around M dwarfs more easily detectable and easier to study,” the press release says.

The paper outlining TOI 1321-b’s discovery is titled “TOI-1231 b: A Temperate, Neptune-Sized Planet Transiting the Nearby M3 Dwarf NLTT 24399.” The lead author is NASA JPL scientist Jennifer Burt. The paper will be published in The Astrophysical Journal but is up now on the pre-press site arxiv.org.

“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,” said Burt 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.”

The team that found TOI-1321 b says the planet is similar to Neptune and likely has a similar gaseous atmosphere. Image Credit: NASA/JPL

The new planet has a radius of about 3.65 times that of Earth. It has an orbital period of 24.26 days and mass of about 15.5 Earth masses. The star it orbits is an M-dwarf star about 90 light years away in the constellation Vela.

The planet is a lot closer to its star than the Earth is to the Sun. But 1321 b is about the same temperature because its star is so much cooler than the Sun. That lower temperature also makes it a desirable object for further study with the JWST and other telescopes. Its equilibrium temperature is only about 330 Kelvin, making it one of the coolest small planets available for atmospheric study. For comparison, Earth’s mean temperature is about 288 Kelvin, and an ultra-Hot Jupiter can have a dayside temperature of up to 2700 K, hotter than many stars.

“TOI-1231 b is one of the only other planets we know of in a similar size and temperature range, so future observations of this new planet will let us determine just how common (or rare) it is for water clouds to form around these temperate worlds,” said Burt.

This image from the study shows the transmission spectroscopy metric (TSM) values for some small exoplanets with temperature less than 1000 Kelvin. The four filled-in planets with black circles and labels have undergone follow-up study with the Hubble. TOI-1231 b is next, and gives scientists another opportunity to study the atmospher of small, cooler planets. The horizontal axis shows the J magnitude of the stars the planets orbit. Image Credit: Burt et al 2021.

Adding to its desirability as a target, it has a high systemic radial velocity. Astronomers are especially excited about that because it may permit the observation of low-velocity hydrogen atoms escaping from the atmosphere. TOI-1231 b’s characteristics and relationship with its star are similar to another star named GJ-436 and its planet GJ-436 b. GJ-436 b is well-known for its atmospheric escape, so astronomers think that the newly-discovered exoplanet will also experience atmospheric escape, though at a much lower rate than GJ-436 b. Hydrogen is the most likely escape culprit, but it’s hard to see because of the presence of interstellar gas. But TOI-1232 b is travelling away from Earth very quickly, making the hydrogen more visible.

Diana Dragomir is one of the co-authors of the paper. In the same press release she said, “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. Our upcoming HST observations will begin to answer these questions, and JWST promises an even more thorough look into the planet’s atmosphere.”

It’ll be a while before the JWST can train its sensors on the newly-discovered exoplanet, even though the space telescope is launching soon (™). But the Hubble is ready to go. In fact, one of the paper’s numerous authors is planning to observe TOI-1231 b later this month.

The James Webb Space Telescope in June 2020. We’ve been told it’ll launch soon. Image Credit: NASA/JPL

All of these exoplanet discoveries are showing us the wide variety present in other solar systems. There are some downright weird planets out there, at least compared to Earth. But this one is more similar to Earth than all the Hot Jupiters we’ve found, and while it’s still different, it at least might teach us something about our own Solar System and the planets that reside there.

“One of the most intriguing results of the last two decades of exoplanet science is that, thus far, none of the new planetary systems we’ve discovered look anything like our own solar system,” said Burt. “They’re full of planets between the size of Earth and Neptune on orbits much shorter than Mercury’s, so we don’t have any local examples to compare them to. This new planet we’ve discovered is still weird – but it’s one step closer to being somewhat like our neighborhood planets. Compared to most transiting planets detected thus far, which often have scorching temperatures in the many hundreds or thousands of degrees, TOI-1231 b is positively frigid.”

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