Artist rendering of LHS 475 b, an Earth-sized exoplanet recently identified using the James Webb Space Telescope. This was the first planet of its size detected by the JWST. {NASA / ESA / CSA / Leah Hustak (STScI)}
In the search for life on distant planets, scientists generally focus on identifying Earth-sized, rocky planets, finding planets in their host star’s habitable zone, and having available the telescope power to read the chemical make-up of the atmospheres.
A relatively small number of Earth-sized exoplanets discovered by telescopes in space and on Earth have meet some of the key characteristics. But now with the James Webb Space Telescope in operation, with its 21-foot high-precision mirror, scientists have been looking forward to finding small, rocky planets that meet all the key criteria.
And during its first year of operation, the JWST has already found and studied one small planet that meets at least some or those criteria. The planet identified, called LHS 475 b, is nearly the same size as Earth, having 99% of our planet’s diameter, scientists said, and is a relatively nearby 41-light-years away.
The research team that detected the small planet is led by Kevin Stevenson and Jacob Lustig-Yaeger, both of the Johns Hopkins University Applied Physics Laboratory.
The team chose to observe this target with Webb after reviewing targets of interest from NASA’s Transiting Exoplanet Survey Satellite (TESS), which hinted at the planet’s existence. Webb’s Near-Infrared Spectrograph (NIRSpec) captured the planet easily and clearly with only two transit observations.
“There is no question that the planet is there,” said Lustig-Yaeger. “Webb’s pristine data validate it.”
“With this telescope, rocky exoplanets are the new frontier.”
The TRAPPIST-1 system contains a total of seven known Earth-sized planets orbiting a weak red dwarf star. Three of the planets — TRAPPIST-1e, f and g — are located in the habitable zone of the star (shown in green in this artist’s impression), where temperatures are potentially moderate enough for liquid water to exist on the surface. As a comparison to the TRAPPIST-1 system the inner part of the Solar System and its habitable zone is shown. (NASA)
Earth-sized exoplanets have been found earlier. The Trappist-1 system, only 39 light-years away, is famously known to include seven small, rocky planets, and it was detected by a small, ground-based telescope.
The Kepler Space Telescope also detected a debated but significant number of Earth-sized planets during its nine-year survey of one small section of the distant sky last decade. Based on data from the Kepler mission, NASA scientists concluded that billions of exoplanets existed in our galaxy and that many millions of them are Earth-sized and likely rocky.
Indeed, a study published in The Astronomical Journal in 2020 predicts that at least 300 million rocky planets in the habitable zones of their stars exist in the Milky Way.
But predicting the existence or so many small, rocky planets, or even detecting some thousands of light-years away, is quite different from being able to identify and then study any of them.
And that is why the JWST exoplanet detection is so promising. The telescope has the capability, under certain conditions, to find quite small exoplanets. But more important is its capability to begin characterizing them. In particular, this means learning if the planets have an atmosphere and if they do, what molecules will be found in them.
As lead author Stevenson explained, the purpose of his team’s JWST observation was precisely to determine whether LHS 475 b for certain existed and whether it had an atmosphere.
The results regarding an atmosphere so far are equivocal, except to conclude that there is no methane atmosphere present, as there is on Saturn’s moon Titan. The team hopes to obtain additional data this summer that will tell them whether the planet has a carbon dioxide atmosphere or none at al.
But whatever the makeup of its atmosphere, the planet is clearly not habitable. As NASA reported, the planet is close to its sun and “a couple of hundred degrees warmer than Earth.”
Researchers used NASA’s James Webb Space Telescope’s Near-Infrared Spectrograph (NIRSpec) to observe exoplanet LHS 475 b on August 31, 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 favored since if methane were present, it would be expected to block more starlight at 3.3 microns.
The detection of LHS 475 b is significant, but is not really what JWST is designed for in terms of exoplanets. Its exoplanet instruments will be used primarily to examine the atmospheres of distant planets large and small, and to better understand what molecules (if any) can be found on particular types of planets.
The Trappist-1 planets offer some of the most exciting possibilities and JWST observations of all of them are already planned. The science goals involve whether some, or all, have atmospheres, whether some might be water worlds, whether they have cloud cover, and if they have elements heavier than hydrogen and helium in atmospheres, if they exist.
The Trappist-1 planets, like LHS 475 b, orbit a red dwarf star — the least hot and most common type of star in our galaxy.
The planets detected orbiting dwarf stars so far are almost always much closer to their star than planets in our solar system are to our Sun, often orbiting in days. This makes them potentially at risk of being sterilized by the solar flares known to be common in the early phases of a red dwarf evolution.
So the question of whether Trappist-1 planets have atmospheres at all is a crucial early one that scientists will use JWST to answer. If atmospheres exist, especially on the three Trappist-1 planets in the system’s habitable zone, then the rush will be on to determine what their chemical compositions might be.
This is a crucial question for astrobiology because it is through knowing those atmospheric molecules (if they exist) that the question of habitability and extraterrestrial life can be addressed.
Just as the presence of substantial amounts of oxygen in our atmosphere is a clear sign of biological activity below, the presence of other molecules, or combinations of molecules could suggest biology on Trappist-1 planets, or one of the many millions of other small rocky planets known to be out there but not yet found.
TOI 700 e believed to be 95 percent the size of Earth and mainly rocky, according to a recent NASA announcement. It was detected by NASA’s Transiting Exoplanet Survey Satellite, which had identified a second small planet in the system in 2020. (NASA/JPL-Caltech/Robert Hurt)
Last week NASA offered up another Earth-sized planet for future study, TOI 700 e. (The TOI stands for TESS Object of Interest.)
This planet was detected orbiting inside the habitable zone of its star, TOI 700, about 100 light-years away.
Another small planet orbiting that star, TOI 700 d, was discovered several years ago. It too is located in the solar system’s habitable zone, the region of a solar system where water on a planet’s surface would likely to be liquid, rather than ice or vapor.
Exoplanet scientists generally see the placement of a planet within a habitable zone as a necessary condition for life to be present. But it is hardly sufficient, since many other conditions need to be met to make a planet actually habitable.
(Adding to the limits of a habitable zone designation, scientists see the potential for life beneath the ice crusts of moons such as Europa and Enceladus. They orbit Jupiter and Saturn respectively, so are nowhere near the solar system’s habitable zone.)
TOI 700 e takes 28 days to complete a single orbit, whereas TOI 700 d – which is a little further out than its neighbor – takes 37 days. As TOI 700 e is smaller than TOI 700 d, it took more data to confirm the silhouette really did represent a new planet.
“If the star was a little closer or the planet a little bigger, we might have been able to spot TOI 700 e in the first year of TESS data,” said astrophysicist Ben Hord from the University of Maryland. “But the signal was so faint that we needed the additional year of transit observations to identify it.”
TESS is monitoring around 100 million stars that are the brightest and nearest to Earth, with the goal of identifying transiting exoplanets that can be further characterized with ground- and space-based telescopes. This is precisely what happened with the Hopkins team and LHS 475 b.
Both TOI 700 e and TOI 700 d are thought to be tidally locked, with one side of the planet always facing its star — in the same way that the same side of the Moon is always visible from Earth.
There is a debate in astrobiology circles about whether having one side of a planet constantly baking in the sunlight eliminates any possible emergence of life, or if the circulation of that heat around the planet could establish zones where life is possible.
An illustration of the cold side of the Webb telescope, where the mirrors and instruments are positioned. (Northrop Grumman)
Among all operating telescopes, only Webb is capable of characterizing the atmospheres of Earth-sized exoplanets. This is done through transmission spectroscopy — reading the spectral lines of molecules produced by starlight passing through the atmosphere as the planet transits in front of the star.
The basic premise of spectroscopy is that different materials emit and interact with different wavelengths (colors) of light in different ways, depending on properties like temperature and composition. Scientists can use spectra—the detailed patterns of colors—to figure out properties such as how hot or cold a planet is, and exactly what elements and compounds it consists of.
The Johns Hopkins team that detected and then studied LHS 475 b is part of the JWST Early Release Program. Scientists associated with JWST several years ago selected teams and their projects for Cycle 1 observations. The goal was to have observations that could be completed relatively quickly and their results published, giving other potential JWST users insight into how the observatory works and what it can produce.
The Hopkins team was certainly impressed with the results.
“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.”
That it was able to collect data from the atmosphere of such a small planet was “impressive for the observatory,” Stevenson added.
And so, as fellow team member Lustig-Yaeger said with enthusiasm, their findings will surely be only the first of many small exoplanet discoveries made possible by the JWST.
