By Detecting exoplanets used to be so difficult that scientists spotted the first black hole, detected the leftover radiation from the Big Bang and took snapshots of countless distant galaxies before discovering the first planet beyond our solar system in 1992. Plenty of exoplanet astronomers began their careers before the field of exoplanet astronomy even existed.
Now though, astronomers have identified well over 5,000 — and continue to find more (SN: 3/22/22). Given the stream of discoveries, it can be easy to forget how little we still know about these distant worlds. The colorful volcanoes, oceans and cloud-streaked atmospheres that appear in artists’ renditions are speculative fantasies often inspired by the only information scientists have about most worlds: their mass, radius and distance from their star.
But the James Webb Space Telescope is revealing exoplanets in dazzling new detail (SN: 10/6/21). Scientists are using the telescope, launched at the end of 2021, to study the atmospheres of hundreds of worlds, from gas giants to rocky worlds about the size of Earth.
As an exoplanet orbits its star, starlight passes through the planet’s atmosphere, and light at specific wavelengths is absorbed depending on the gases present. This leaves traces in the spectrum of starlight, which scientists can use to figure out which chemicals the light passed by. JWST is sensitive to infrared wavelengths where these traces are strongest and has already detected water, carbon dioxide, methane and more on other worlds.
Given these detections, Laura Kreidberg of the Max Planck Institute for Astronomy in Heidelberg, Germany, says she often gets asked one question in particular about what JWST might reveal.
“There’s a lot of excitement about finding signatures of alien life,” she says. “And I’m excited about that also.” But, she adds, scientists have a lot to learn about planets before they’ll be able to detect life on other worlds with confidence. And due to technical limitations, the telescope’s gaze will be mostly restricted to exoplanets that are very hot, very big or both — not conditions thought to be suitable for life.
Most of what we know about exoplanets today comes from the eight planets in our solar system. JWST’s planned 10-year lifetime could reveal a lot, perhaps answering fundamental questions including what exoplanets are made of, how they form and whether our solar system is an oddball within our galactic neighborhood (SN: 5/11/18).
Here are five big planetary puzzles that scientists hope to solve with JWST.
Why do some rocky planets have atmospheres and others don’t?
If a rocky planet is going to host life, it needs an atmosphere. But scientists still aren’t sure what determines whether a rocky body can hold onto a gaseous outer shell.
Astronomers are searching for what they call the “cosmic shoreline,” a conceptual dividing line that separates worlds with and without atmospheres. In 2017, scientists identified such a shoreline within our solar system, set by the balance between the amount of radiation a planet or moon receives from the sun and the strength of the world’s surface gravity. Sunlight provides gas particles with the energy needed to escape from the upper atmosphere, while gravity holds atmospheric gases to the planet.
To test whether this type of cosmic shoreline exists throughout the galaxy, scientists need to figure out which exoplanets have atmospheres and which don’t. This question may sound incredibly basic, but it’s only just now becoming possible to answer thanks to JWST.
Renyu Hu, an astronomer at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., says he and colleagues have settled the atmosphere question for 55 Cancri e, a planet that orbits a sunlike star some 40 light-years from Earth (SN: 11/18/07). 55 Cancri e is a super-Earth, a bit bigger than Earth but much smaller than Neptune (SN: 5/11/15). In a paper published May 8 in Nature, Hu and colleagues present JWST data suggesting that 55 Cancri e has an atmosphere of either carbon monoxide, carbon dioxide or a mix of the two with nitrogen. It’s the first detection of an atmosphere shrouding a terrestrial, or rocky, exoplanet.
Air or no air
The cosmic shoreline is a theoretical dividing line between planets with and without atmospheres based on the relationship between how much gas-stripping radiation a planet gets from its star and the strength of its gravity. Solar system planets are shown.
Likelihood that a planet has an atmosphere
But scientists are pessimistic about the existence of atmospheres on the other rocky worlds JWST is observing — specifically those orbiting M-dwarf stars. These small, dim stars are easiest for JWST to see. They also tend to spew bursts of atmosphere-stripping radiation more often than stars like our sun. So some scientists doubt that rocky planets around these stars can hold onto atmospheres.
According to JWST observations of LHS 3844b, a super-Earth orbiting such a star, the planet is almost certainly a bare rock. JWST observations of the planets TRAPPIST-1b and TRAPPIST-1c, which orbit the M-dwarf TRAPPIST-1, suggest that these planets are bare too (SN: 3/27/23). But it’s also possible that they could have very thin atmospheres, says astronomer Elsa Ducrot of the Paris Observatory. Follow-up work with JWST will help settle the question.
As scientists use JWST to identify more examples of rocky planets with and without atmospheres, our understanding of the cosmic shoreline can be put to the test.
“There are some colleagues of mine who just want an atmosphere to be there so badly. They’re just heartbroken if it isn’t there,” Kreidberg says. “But for me, if we learned that a planet doesn’t have an atmosphere, we learned a lot about it already.”
What is exoplanet geology like?
Discovering exoplanets without atmospheres also will allow astronomers to study something impossible to probe directly before JWST: exoplanet geology.
“I’m really excited about this,” Kreidberg says. “Of course, I want to see the atmospheres. But I think there’s a lot you can learn from the surface also.”
Kreidberg and her team plan to use JWST to look for the chemical fingerprints of specific rocks in the infrared light cast by the rocky, airless super-Earth LHS 3844b. Learning what the planet’s surface is made of would be a powerful clue about the planet’s geologic history and ongoing processes.
Finding signs of granite would be especially intriguing. Granite is a common rock on Earth that forms from recycled and remelted rock. On Earth, this process depends in part on plate tectonics. But beyond Earth, granite appears to be vanishingly rare — probably because plate tectonics is too. Right now, there’s no more evidence for plate tectonics on other worlds than there is for alien life. Finding granitelike rock on an exoplanet would be a major discovery.
Super discoveries
Astronomers are hungry to learn more about a type of planet not found in our solar system: super-Earths, which are bigger than Earth but smaller than Neptune (planets are shown to scale). The James Webb telescope has already snooped on at least two. The searingly hot LHS 3844b lacks an atmosphere, JWST data suggest. That should allow JWST to gather intel on surface geology. 55 Cancri e, another sizzler, has an atmosphere — the first detected around a rocky exoplanet.
Astronomers are also seeking signs of rocks that are more common in our solar system. For example, a surface covered in the black rock basalt would hint at volcanic processes. And rocks more like those in Earth’s mantle, such as peridotite, could point to a recently frozen magma ocean or exotic, high-temperature volcanism.
JWST might even reveal the textures of rocks on exoplanet surfaces.
In our solar system, radiation from the sun wears down rocks on worlds without atmospheres. The result is a crumbly material called regolith that creates a ragged, rough planetary surface. Kreidberg and colleagues plan to look for regolith on LHS 3844b by measuring how the planet’s brightness changes as it orbits its star. Compared with a rough surface, a smooth one should appear to reflect less of the sunlight that comes in at shallow angles. Smoothness could hint at a process like volcanism that refreshes the surface with new rock. Or astronomers might find that radiation from the planet’s M-dwarf star doesn’t weather planets the same way our sun’s radiation does.
What are rocky exoplanets made of?
While JWST will help astronomers learn about the surfaces of exoplanets, it also might offer a glimpse at their geologic guts thanks to a particularly extreme type of terrestrial world.
Hotter than scorched Mercury, lava worlds orbit so close to their stars that their years are best measured in hours, not days or months. This proximity causes the planets to become tidally locked, meaning the same side of the planet always faces its sun. As a result, one hemisphere freezes in endless night while the other’s rocky surface melts into lava.
The magma oceans on the daysides of lava planets offer about as close to a window into the interior of a planet as astronomers could hope to find. Gases escaping from the magma might give clues to the composition of the planet’s deep interior. And learning what planets are made of can tell astronomers a lot about how these bodies form, and whether their compositions and histories are similar to or different from the way rocky planets form in our solar system.
A “lava planet is a special case of planetary formation. And oftentimes, some of the most extreme cases are the most revealing.”
Lisa Đặng, exoplanet scientist
“You might be probing really deep — which is something that I think is hard to do even on Earth,” says Lisa Đặng, an exoplanet scientist at the University of Montreal who studies these blazing hot planets using JWST.
Because they should have magma oceans, lava planets are expected to have atmospheres; even if part of the atmosphere is lost over time, it would be constantly replenished by gas released from magma. Scientists haven’t yet detected whiffs of such gases. But Đặng is trying. She’s observing the lava world K2-141b, a super-Earth 200 light-years away that orbits a K-type star, also called an orange dwarf.
A “lava planet is a special case of planetary formation. And oftentimes, some of the most extreme cases are the most revealing,” Đặng says.
Sub-Neptunes are the most common planets in our galaxy. What are they?
While what we know about Earth, Mercury and Mars can help astronomers imagine what alien rocky planets are like, the most common type of planet in our galaxy can’t be found in our solar system. Sub-Neptunes, so named because the planets’ radii are just a bit smaller than Neptune’s, seem to be everywhere scientists look (SN: 9/8/11). But scientists still know very little about these worlds. For example, are they gas giants, rocky planets or something else entirely?
“They seem to be incredibly common, statistically,” says exoplanet scientist Joshua Krissansen-Totton of the University of Washington in Seattle. “We also really have no idea what they’re made of.”
Based on their masses and radii alone, sub-Neptunes might be miniature ice giants rich in ammonia, methane and water, like Neptune and Uranus. But the same data could describe planets with very different structures, such as rocky cores wreathed in hydrogen and helium, or exotic water worlds made mostly of different forms of water, not necessarily liquid (SN: 7/6/20).
Using JWST, scientists plan to study the atmospheres of sub-Neptunes to distinguish between these possibilities. JWST observations of the sub-Neptune K2-18b made headlines last year after researchers detected carbon dioxide and methane but no ammonia — an expected component of gas planets — in its atmosphere. The team interpreted this gas mix as evidence for a water world since ammonia dissolves easily in water and would get trapped in an ocean if it were there. But other researchers, including Krissansen-Totton, think the same data could fit a Neptune-like composition with a thick gas envelope over a rocky core. A definitive answer will require follow-up observations.
Alien atmosphere
Using the James Webb Space Telescope, astronomers discovered methane, carbon dioxide and dimethyl sulfide in the atmosphere of exoplanet K2-18b. The team studied starlight that passed through the planet’s atmosphere and looked at the patterns of wavelengths of light that had been absorbed to indicate the presence of different gases.
Gases present in the atmosphere of K2-18b
If sub-Neptunes turn out to be gas-wreathed rocks, that conclusion could explain another mystery about the variety of planet types in our galaxy.
When astronomers look across the range of planet sizes, there’s a dip in the number of planets with radii somewhere between those of Earth and Neptune. There are many sub-Neptunes just smaller than Neptune and many super-Earths just bigger than Earth, but very few planets right in between.
One possible explanation for this radius valley is that super-Earths and sub-Neptunes are actually the same types of planets, just spotted at different points in their lifetimes, says astrophysicist Collin Cherubim of Harvard University.
Super-Earths might simply be the leftover rocky cores of sub-Neptunes that lost their hydrogen-rich atmospheres. That process would dramatically shrink the planets’ radii. If true, scientists may have made the planetary equivalent of mistaking a juvenile animal for a new species.
To explore this possibility, Kreidberg and colleagues are using JWST to study the atmosphere of a planet called WASP-47e, which sits smack-dab in the middle of the radius valley. They want to determine what the planet is made of, and if it might be in the process of losing its atmosphere.
How do gas planets form?
Despite having four gas giants in our solar system, scientists still aren’t sure how these enormous worlds form and evolve — and whether our four are oddballs or not.
“Essentially, it’s three questions: How do gaseous planets form? How do they evolve? And what are they made of?” says planetary scientist Ravit Helled of the University of Zurich, who studies gas giants. These are “fundamental questions in planetary science that we still haven’t answered.”
In particular, scientists want to know whether gas giants form where we find them or whether they tend to wander over time, as they seem to have done in our solar system (SN: 3/15/16). Planets can wander due to gravitational interactions with other objects, including the disks of gas and dust that orbit young stars and provide the raw materials for planets. Gas planet migration can wreak gravitational havoc, knocking other planets out of their orbits and flinging around small bodies like comets and asteroids. The resulting chaos can have serious implications for the stability and potential habitability of smaller worlds.
Inventory of exoplanets
As of May 9 astronomers have discovered 5,616 confirmed exoplanets. The majority are Neptune-like (including the slightly smaller sub-Neptunes), gas giants or super-Earths (larger than Earth but smaller than Neptune). Far fewer small terrestrial planets like Earth have been found.
JWST could provide astronomers with a vital clue to this mystery — the composition of gas giant atmospheres. The abundance of elements heavier than hydrogen and helium in gas giant atmospheres should depend on where the planet formed relative to its star. In general, the heavier the elements found in an atmosphere, the farther out the planet formed. Observing enough gas giant planets to start identifying trends — and planets that buck them — could reveal the general rules governing how these planets form and migrate.
Scientists also want to find out whether warm gas giants form in the same way as cool ones do. JWST is mostly restricted to observing planets close to their stars, so the gas planets it can observe are much toastier than Jupiter, Saturn, Uranus and Neptune. It’s not yet clear whether these toastier planets are just hotter versions of the gas giants in our solar system, or if they’re a different class.
There’s reason to be hopeful that some of these questions might be settled soon. Since gas giants are so big, they’re much easier to study than small, rocky planets. Helled says that with JWST, astronomers will soon characterize the atmospheres of enough gas giants to have the statistical power to test hypotheses about their formation, compositions and evolution.
“The key is that we are going to have a large number of planets,” Helled says. “Until JWST, it was a handful of objects. But once we have more and the measurements are accurate, we can start to understand trends in the statistics. And this is the power of JWST.”
