Weird Exoplanets Fill the Cosmos. Here’s How Astronomers Find Them

The Strangest Alien Worlds Are beyond Astronomers’ Wildest Dreams

By Paul M. Sutter

Planets raining iron, planets being consumed by their parent star or planets featuring globe-spanning oceans—the variety of known exoplanets has far exceeded even the most fantastical ideas from science fiction. And we’re barely scratching the surface; only several thousand worlds have been confirmed out of an estimated trillion or more in our galaxy alone. But while we have gotten quite good at detecting exoplanets, characterizing them—discerning what their environment is actually like—is another, far more difficult feat. Yet using mere trickles of light collected through telescopes, astronomers are able to combine clever modeling, reasonable assumptions and serious detective work to uncover these riches. And it will take all these tools and more to claim the real prize: the discovery of life on another world.

An Incandescent World

For example, take the case of HD 104067. This is an orangish star that sits about 66 light-years away and is somewhat smaller than our own sun. In 2011 astronomers announced the discovery of a Jupiter-class planet in a tight 55-day orbit around this star. More recently, astronomers followed up with both ground- and space-based observatories to combine their studies with a smattering of datasets going back to 1997. The results of all that Herculean effort are a few lines on a plot that indicate how quickly the star is wobbling back and forth along our line of sight.

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That’s all an astronomer needs to play a game of gravitational whodunit. Planets move in a regular orbit that will influence the motion of the star in repeatable ways—such as a stellar to-and-fro wobble that astronomers often associate with an accompanying, unseen world. What may seem like a single wobble, however, can be better seen as overlapping multitudes over time that are caused by the combined influence of multiple planets that all march to their own orbital beat around a star. Given enough data, astronomers can tease out the architectures of an entire planetary system from one wobbly star—revealing not only new worlds but also the absence of planets. For example, using thousands of toy-model planetary configurations for HD 104067, astronomers were able to conclusively say what sorts of planets are not orbiting this star.

But the astronomers found something else buried in the data: the stellar wobble from a new Uranus-mass planet sitting inward of the orbit of its larger Jupiter-scale sibling. Within that secondary wobble there was something more: the first faint hints of a terrestrial world on a 2.2-day orbit.

What would life be like on this innermost terrestrial world? This is where another layer of modeling comes in. Given a planet’s estimated mass from the strength of the wobble it induces on its star, and assuming that the planet is vaguely Earth-like (in the sense that it is largely made of solid rock and metal rather than liquid or gas), astronomers can simulate how it responds to the gravitational pulls and tugs of its parent star and its two larger, uncomfortably close siblings.

In the case of the newfound small world circling HD 104067 every two days or so, such modeling suggests that this planet experiences significant tidal stretching and squeezing that delivers more than enough energy to melt its surface. This is essentially a much more intense version of what happens to Jupiter’s hypervolcanic moon Io in our own solar system. In fact, that energy equates to a surface temperature of around 2,600 kelvins, making this little quasi-cousin of Earth literally incandescent, like a planet-sized glowing lightbulb.

Data Mining for Distant Exoplanets

Yes, this analysis could be wrong, but if it is wrong, it’s not in an obvious way. Every statement made is based on either the raw data or well-understood assumptions about how physics operates in the universe.

But perhaps our greatest feats of understanding come from even fewer data: measurements of just a tiny dot of light, without some sort of multiyear, long-duration observation campaign. Under the right conditions, astronomers can measure the spectrum of a planet to rapidly get an estimate of its atmospheric composition.

For example, astronomers used the James Webb Space Telescope (JWST) to identify methane, carbon dioxide and the potential presence of a compound called dimethyl sulfide in the atmosphere of the exoplanet K2-18 b. Based on modeling of this kind of atmosphere, some astronomers believe that this planet is a “hycean” world—one that features a globe-spanning liquid water ocean under a thick hydrogen atmosphere.

Other planets are far stranger. Another recent JWST investigation centered on the “hot Jupiter” world WASP-43 b, a bloated gassy orb twirling in a hellishly hot close-in orbit around its star. This planet transits across the face of its star, as seen from Earth, and is “tidally locked,” meaning that it always presents the same hemisphere to its star. Previous observations by the Hubble and Spitzer space telescopes had used this quirk of orbital geometry to measure the temperatures of the planet’s nightside (seen around the time of its transit) and its dayside (seen when the planet is about to pass behind its star, as viewed from Earth). But JWST’s sharper vision has allowed astronomers to discern more subtle details and effectively create a weather map of WASP-43 b. And the forecast is for metal-melting temperatures, winds blowing faster than 5,000 miles per hour and clouds composed not of water vapor but of molten rock.

Brimstone Biospheres

All of this detective work will be necessary in the coming decades as astronomers build the tools and techniques to hunt for life outside the solar system. NASA’s upcoming Habitable Worlds Observatory, slated for launch sometime in the 2040s, will target several dozen nearby exoplanets with the goal of directly imaging them, but most of the useful information will come from high-resolution spectra. The key evidence will likely come in the form of data points known as atmospheric biosignatures—essentially one or more chemicals in an alien world’s air that can best be explained by life thriving on the planet.

On our own world, our atmosphere would probably have far less oxygen and methane, for example, without photosynthesis and anaerobic microbes. But Earth’s atmosphere has undergone radical changes across the past few billion years; some of our own biosignatures would be discoverable by an alien equivalent of the Habitable Worlds Observatory, and some of them would not. Needless to say, astronomers have spent years putting together a rich catalog of potential biosignatures, abiotic false positives and detection criteria to design as wide a survey program as possible—after all, we’re not exactly sure what alien life will look like or do to its atmosphere—while increasing our chances of knowing life when we see it.

Still, most work in this direction is speculative and subject to change. For example, a preprint paper recently accepted for publication in the Astrophysical Journal Letters used simulations of global climates on hycean exoplanets, along with some reasonable assumptions about how efficiently ultraviolet light from a parent star could destroy life-generated sulfur gases, to model the detectability of sulfur biosignatures. On Earth such sulfur gases don’t last long, but it may not have always been so in the past, and the thick, hazy atmospheres of hycean worlds may allow sulfur to build up in the nightside to a detectable degree. Our first hint of alien life may be a subtle spectral whiff of brimstone—or not. The reliability of this hypothesis depends on how well you trust our models and assumptions of how these worlds behave.

No matter what, understanding exoplanets will be a tough job requiring many parallel developments in technology, methods and physical understanding. Our first signs of exoplanet life may only be in the form of a whisper, a slight bump in a spectrum or an odd result from the output of a model. Like many great scientific discoveries in the past, this result will not be heralded by a “Eureka!” but rather a “Hmm, that’s weird.”