Strange exoplanets fill the cosmos. Here’s how astronomers find them

The strangest alien worlds are beyond astronomers’ wildest dreams

By Paul M. Sutter

Planets that rain iron, planets that are eaten by their parent star or planets with oceans that span the entire globe: the variety of known exoplanets far exceeds even the most fantastic ideas from science fiction. And we’re barely done; in our Galaxy alone, out of an estimated trillion or more, only a few thousand worlds have been confirmed. But while we’ve gotten pretty good at detecting exoplanets, characterizing them—distinguishing what their environment actually looks like—is another, much more difficult feat. But by using just a little bit of light collected by telescopes, astronomers are able to combine smart models, reasonable assumptions and serious detective work to uncover these riches. And all these tools and more are needed to claim the real prize: the discovery of life on another world.

A glowing world

Take for example the case of HD 104067. This is an orangish star located about 66 light-years away and slightly 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 have contacted both ground-based and space-based observatories to combine their studies with a few data sets dating back to 1997. The results of all that Herculean effort are a few lines on a graph showing how fast 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 gravity whodunit. Planets move in a regular orbit that will affect the star’s motion in repeatable ways, such as the wobbling of stars that astronomers often associate with a companion, invisible world. However, what looks like a single swing is better thought of as overlapping masses over time, caused by the combined influence of multiple planets each marching toward their own orbits around a star. With enough data, astronomers can tease out the architecture of an entire planetary system from a single wobbly star, revealing not only new worlds but also the absence of planets. For example, by using thousands of planetary configurations in toy models for HD 104067, astronomers were able to say with certainty what kind of planets not orbits around this star.

But the astronomers found something else hidden in the data: the stellar wobble of a new Uranus-mass planet that lies within the orbit of its larger Jupiter-scale sibling. Inside That secondary fluctuation, there was something more: the first vague indications of an Earth world in a 2.2-day orbit.

What would life be like on this inner earthly world? This is where a new layer of modeling comes into play. Given the approximate mass of a planet based on the strength of the wobble it produces on its star, and assuming the planet is vaguely Earth-like (in that it is mostly solid rock and metal rather than liquid or gas), astronomers can simulate how it responds to the gravity of its parent star and its two larger, uncomfortably close siblings.

In the case of the newly discovered small world that orbits HD 104067 about every two days, such models suggest that this planet experiences significant tidal stretching and squeezing, providing 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 about 2,600 Kelvin, making this little cousin of Earth literally glowing, like a glowing light bulb the size of a planet.

Data mining for distant exoplanets

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

But perhaps our greatest insights come from even less data: measurements from just a tiny point of light, without some kind of multi-year, long-term observation campaign. Under the right conditions, astronomers can measure a planet’s spectrum to get a quick estimate of its atmospheric composition.

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

Other planets are much stranger. Another recent JWST study focused on the “hot Jupiter” world WASP-43 b, a bloated gaseous sphere orbiting its star in a fiercely hot, close orbit. This planet moves straight through the surface of its star as seen from Earth and is ‘tidally locked’, meaning 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 temperatures on the planet’s nightside (seen around the time of its transit) and on its dayside (seen when the planet is about to pass behind its star). to visit, to measure). as seen from Earth). But JWST’s sharper vision has allowed astronomers to pick out subtler details and effectively create a weather map of WASP-43 b. And the prediction is that the temperatures of the metal will melt, there will be wind speeds of more than 8,000 kilometers per hour and there will be clouds that are not made of water vapor but of molten rock.

Sulfur biospheres

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

On our own world, our atmosphere would probably contain much less oxygen and methane, without photosynthesis and anaerobic microbes. But Earth’s atmosphere has undergone radical changes over the past few billion years; some of our own biosignatures might be discovered by an alien equivalent of the Habitable Worlds Observatory, and some might not. Needless to say, astronomers have spent years compiling a rich catalog of potential biosignatures, abiotic false positives, and detection criteria to design the broadest possible research program. After all, we don’t know exactly what alien life will look like or do. to its atmosphere – while increasing our chances of knowing life when we see it.

Yet most work in this direction is speculative and subject to change. For example, a preprinted article was recently accepted for publication in the Astrophysical diary letters used simulations of the global climate on hycean exoplanets, along with some reasonable assumptions about how efficiently ultraviolet light from a host star could destroy life-generated sulfur gases, to model the detectability of sulfur biosignatures. On Earth, such sulfur gases do not last long, but in the past this may not always have been the case, and the thick, hazy atmospheres of hycean worlds can cause sulfur to accumulate in the nightside to a noticeable extent. Our first hint of alien life may be a subtle ghostly smell of sulfur – or it may not. The reliability of this hypothesis depends on how well you trust our models and assumptions about how these worlds behave.

Whatever happens, understanding exoplanets will be a tough job, requiring many parallel developments in technology, methods and physical understanding. Our first signs of life on exoplanets may only take the form of a whisper, a small bump in a spectrum, or a strange result from a model’s output. Like many great scientific discoveries in the past, this result will not be heralded by a “Eureka!” but rather a “Hmm, that’s weird.”