Habitable Worlds May Hide in the Dusty Regions Around Stars

A team of astronomers has proposed peering into the inconspicuous region between host stars and their soot lines to find habitable worlds beyond our solar system.

There are few known habitable worlds—planets that reside in the so-called Goldilocks Zone of their star systems—compared to the number of exoplanets known to science. But there may be as many as 300 million habitable worlds in our galaxy alone, according to the SETI Institute.

Habitable worlds are of importance not just to planetary scientists and those who hope to get humankind extraplanetary, but for those involved in the search for extraterrestrial intelligence (or SETI). Habitable worlds clue researchers into how unique Earth’s situation might be, and the sorts of conditions needed to foster life in the universe.

Now, a team of researchers has proposed a new model for exoplanet searches that focuses on the region around the soot line, a region where great volumes of dust orbit a given system’s star. Focusing on the soot line will bring exoplanet searches closer to the host stars of systems under investigation. The research team’s findings were published in The Astrophysical Journal Letters.

“It adds a new dimension in our search for habitability. It may be a negative dimension or it may be a positive dimension,” said Ted Bergin, a planetary scientist at the University of Michigan and the study’s lead author, in a university release. “It’s exciting because it leads to all kinds of endless possibilities.”

The team posits that worlds forming in and around the soot line would have carbon-rich surfaces distinct from Earth’s. They’d also be lacking in water compared to our illustrious blue marble. Between the soot line and a star, organic compounds sublimate into gasses, reducing the amount of that material that would otherwise contribute to a planet’s formation.

The team modeled how a silicate-rich planet might develop in the soot line, and found that such a world would have an atmosphere rich in methane, similar to the large moon Titan. They would sit between water-rich worlds and rocky worlds, the team expects.

“Planets that are born within this region, which exists in every planet-forming disk system, will release more volatile carbon from their mantles,” Bergin said. “This could readily lead to the natural production of hazes. Such hazes have been observed in the atmospheres of exoplanets and have the potential to change the calculus for what we consider habitable worlds.”

Indeed, life beyond Earth may look very different from what we see on Earth. It might look like organisms that produce phosphine, as scientists recently thought might be happening in Venus’ sulfurous atmosphere. Or something entirely different.

While the team’s model offers an intriguing proposition for a new birthplace of habitable worlds, it does not a habitable world make. In other words, these models are only as useful as the observational tools astronomers have for detecting such planets.

Thankfully, the hunt for exoplanets is moving at a real clip, bolstered by the launch of the Webb Space Telescope in December 2021. Besides looking at some of the universe’s oldest light, Webb’s unflinching gaze is revealing previously unknown exoplanets as well as new details on known worlds.

Everything about exoplanets—from their noxious atmospheres to their silty clouds—will be of use in astrophysicists’ figuring out the true extent of their diversity and their development. If sooty worlds like the ones predicted by the team’s model exist, our tools for finding them are only getting better.

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