Atmosphere

The Webb Has Confirmed its First Exoplanet, and it’s the Same Size as Earth.

The James Webb Space Telescope is the most powerful telescope ever launched into space. That power has led to a string of observational successes: ancient galaxies, obscured star-forming regions, and an exoplanet atmosphere. Now the telescope has identified its first exoplanet, and it’s a rocky planet the same size as Earth.

The planet is named LHS 475b, and it’s about 41 light-years away. NASA’s TESS (Transiting Exoplanet Survey Satellite) first found indications that the planet was there, but those were only hints. Now the JWST has confirmed it.

The research team that found LHS 475b is led by Kevin Stevenson and Jacob Lustig-Yaeger, both from the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. They presented their findings at the American Astronomical Society (AAS) on Wednesday, 11 January 2023.

“Webb is bringing us closer and closer to a new understanding of Earth-like worlds outside the Solar System, and the mission is only just getting started.”

Mark Clampin, Astrophysics Division director at NASA

Webb’s powerful Near-Infrared Spectrograph (NIRSpec) instrument confirmed the planet after only two transits in front of its star, LHS 475, a red dwarf in the constellation Octans. The confirmation is more confirmation of the James Webb’s power and precision. “There is no question that the planet is there. Webb’s pristine data validate it,” said Lustig-Yaeger. “The fact that it is also a small, rocky planet is impressive for the observatory,” Stevenson added.

A light curve from the NASA/ESA/CSA James Webb Space Telescope’s Near-Infrared Spectrograph (NIRSpec) shows the change in brightness from the LHS 475 star system over time as the planet transited the star on 31 August 2022. The graphic shows the change in the relative brightness of the star-planet system spanning three hours. The spectrum shows that the brightness of the system remains steady until the planet begins to transit the star. It then decreases, representing when the planet is directly in front of the star. The brightness increases again when the planet is no longer blocking the star, at which point it levels out. Image Credit: NASA, ESA, CSA, L. Hustak (STScI), K. Stevenson, J. Lustig-Yaeger, E. May (Johns Hopkins University Applied Physics Laboratory), G. Fu (Johns Hopkins University), and S. Moran (University of Arizona)

One of the JWST’s science objectives is to study exoplanet atmospheres. Its powerful instruments allow it to identify chemicals in the atmospheres of planets light-years away. “These first observational results from an Earth-sized, rocky planet open the door to many future possibilities for studying rocky planet atmospheres with Webb,” said Mark Clampin, Astrophysics Division director at NASA Headquarters in Washington. “Webb is bringing us closer and closer to a new understanding of Earth-like worlds outside the Solar System, and the mission is only just getting started.”

While the telescope has confirmed the planet, it hasn’t yet figured out its atmosphere. “The observatory’s data are beautiful,” said Erin May, also of the Johns Hopkins University Applied Physics Laboratory. “The telescope is so sensitive that it can easily detect a range of molecules, but we can’t yet draw any definitive conclusions about the planet’s atmosphere.” Even more precise future measurements with the Webb will reveal more about its composition, and the research team might take those measurements this summer.

Researchers used the NASA/ESA/CSA James Webb Space Telescope’s Near-Infrared Spectrograph (NIRSpec) to observe exoplanet LHS 475 b on 31 August 2022. As this spectrum shows, Webb did not observe a detectable quantity of any element or molecule. The data (white dots) are consistent with a featureless spectrum representative of a planet that has no atmosphere (yellow line). The purple line represents a pure carbon dioxide atmosphere and is indistinguishable from a flat line at the current level of precision. The green line represents a pure methane atmosphere, which is not favoured since methane, if present, would be expected to block more starlight at 3.3 microns. Image Credit: NASA, ESA, CSA, L. Hustak (STScI), K. Stevenson, J. Lustig-Yaeger, E. May (Johns Hopkins University Applied Physics Laboratory), G. Fu (Johns Hopkins University), and S. Moran (University of Arizona)

While a complete picture of LHS 475b’s atmosphere, if it has one, will have to wait until the summer, the team did make some progress. “There are some terrestrial-type atmospheres that we can rule out,” explained Lustig-Yaeger. “It can’t have a thick methane-dominated atmosphere, similar to that of Saturn’s moon Titan.”

But they can’t rule out a thick, carbon-dioxide-rich atmosphere like Venus’ because those atmospheres can be so compact. “Counterintuitively, a 100% carbon dioxide atmosphere is so much more compact that it becomes very challenging to detect,” said Lustig-Yaeger.

LHS 475b is hotter than Earth, but because it’s orbiting a red dwarf, that doesn’t rule out an atmosphere.

The uncertainty about LHS 475b’s atmosphere extends to uncertainty about small rocky exoplanets in general. Larger gas giants are much easier to detect and characterize, while rocky planets tend to be smaller. They block less light which makes them more difficult to detect via transit, and their atmospheres are also smaller, making them more challenging targets for spectroscopy.

“With this telescope, rocky exoplanets are the new frontier.”

Jacob Lustig-Yaeger, JHUAPL

But this result shows that the JWST can live up to its promise of characterizing exoplanet atmospheres and that it’s only getting started. “We’re at the forefront of studying small, rocky exoplanets,” Lustig-Yaeger said. “We have barely begun scratching the surface of what their atmospheres might be like.”

Rocky exoplanets aren’t rare. They’re common around other stars. But their atmospheres aren’t well-constrained. We don’t know what to expect from them, and the rocky planets in our Solar System don’t necessarily tell us what we can expect.

Based on new evidence from the NASA/ESA/CSA James Webb Space Telescope, this illustration shows the exoplanet LHS 475 b. It is rocky and almost precisely the same size as Earth. The planet whips around its star in just two days, far faster than any planet in the Solar System. Image Credit: NASA, ESA, CSA, L. Hustak (STScI)

Venus’s atmosphere is dominated by CO2, while Earth’s is dominated by nitrogen. Mercury’s atmosphere is extremely thin and consists of atoms blasted off its surface by the Sun and meteoroid impacts. It’s actually an exosphere, and it contains oxygen, sodium, hydrogen, and other elements, but only in tiny amounts. Mars likely used to have a thick atmosphere but now has only a very thin atmosphere that’s 95% CO2.

Characterizing rocky planet atmospheres is important because it’ll help scientists build a model of how rocky planets form and evolve. We’ll grow our understanding of potential habitability. We’re on the cusp of characterizing many of them in the next decade, thanks especially to the JWST.

A transmission spectrum of the hot gas giant exoplanet WASP-39 b, captured by Webb’s Near-Infrared Spectrograph (NIRSpec) on July 10, 2022, reveals the first definitive evidence for carbon dioxide in the atmosphere of a planet outside the Solar System. Credit: NASA, ESA, CSA, and L. Hustak (STScI). Science: The JWST Transiting Exoplanet Community Early Release Science Team

“This rocky planet confirmation highlights the precision of the mission’s instruments,” Stevenson said. “And it is only the first of many discoveries that it will make.” Lustig-Yaeger agreed: “With this telescope, rocky exoplanets are the new frontier.”

Scientists have learned quite a bit about rocky exoplanets in the last few years, even though their atmospheric compositions aren’t well-understood. Researchers think that most rocky planets form with an initial hydrogen/helium atmosphere that is later lost to space. They think while rocky exoplanets might start out the same atmospherically, they can evolve much differently and have widely different compositions.

One of the big goals in the study of rocky exoplanet atmospheres is biosignatures. We have a lot to learn about biotic and abiotic chemistry on other worlds and how to understand what different potential biosignatures might mean in different atmospheric contexts. The JWST will help with that, too.

Ground-based telescopes like the Giant Magellan Telescope will be able to study exoplanet atmospheres. This illustration shows what the Giant Magellan Telescope will look like when it comes online. It’ll have seven glass mirror segments, and each one will weigh 20 tons. The GMT will have 10x the light collecting area and 4x the spatial resolution of the James Webb Space Telescope. Image: Giant Magellan Telescope – GMTO Corporation

Before long, the JWST will have some help studying rocky exoplanet atmospheres. Two ground-based telescopes, the Giant Magellan Telescope (GMT) and the Thirty Meter Telescope (TMT), should see their first light in the next few years, though there are no exact dates. They should yield revolutionary advances in our study of rocky exoplanets.

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Evan Gough

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