Scientists Find 'Exciting' Rare Exoplanet in Habitable Zone of Twin Stars

A planet has been discovered lurking in the habitable zone of not one, but two, stars.

This far-off planet, situated around 310 light-years from Earth, has been found to exist within the two-star binary system, orbiting its host star at just the right distance that liquid water could exist on its surface, according to a new study in The Astronomical Journal.

This unique planet—named TOI 4633 c, or “Percival”—was spotted thanks to the efforts of citizen scientists, who aided astronomers in sifting through data collected by NASA’s Transiting Exoplanet Survey Satellite (TESS).

TOI 4633 c was spotted as it passed in front of its star, with TESS picking up the momentary but regular dimming of the star’s glow. This is known as the “transit method”, and is how we have discovered many of the thousands of exoplanets known outside of our solar system. This planet in particular was determined to take 272 days to orbit its star, making it unusually far out. According to the paper, this makes it the second-longest orbit of any TESS exoplanet, and one of five to have orbits longer than 100 days.

This distance from its bright star host puts the planet squarely in the habitable zone, which is defined by astronomers as the region around a star where conditions are just right for liquid water to exist on a planet’s surface—considered essential for life as we know it. The exact range of this zone varies depending on the size and temperature of the star.

For a star like our sun, the habitable zone is roughly between the orbits of Venus and Mars. For smaller, cooler stars, such as red dwarfs, the habitable zone is much closer to the star. Conversely, for larger, hotter stars, the habitable zone is further out.

After further investigation, the star in this distant solar system was determined to be two stars orbiting each other, though the astronomers can’t tell which of the two TOI 4633 c is orbiting. Planets are about half as likely to form around binary stars as they are around single stars like our own sun.

This makes TOI 4633 c’s star by far the brightest seen to date hosting a transiting planet in the habitable zone.

“Finding planets in multi-star systems is crucial for our understanding of how you can make different planets out of the same material,” study author Nora Eisner, a research fellow at the Flatiron Institute’s Center for Computational Astrophysics in New York City and principal investigator of Planet Hunters TESS, said in a statement. “It’s quite exciting that we found this one.”

NASA’s Transiting Exoplanet Survey Satellite (TESS). This satellite spotted this far-off planet. NASA

TOI 4633 c is thought to be a gas giant, somewhat like Neptune, with a thick water vapor, methane and hydrogen atmosphere and no solid surface.

“This planet is remarkable in many aspects,” Eisner says. “It’s remarkable in its orbit, it’s remarkable for being in the habitable zone and it’s remarkable for orbiting a bright star.”

While TOI 4633 c is not likely to host life due to its lack of surface, long-orbit planets are more likely to have moons, which could be a candidate for life.

“If this planet were to have a moon, that moon would likely have a solid surface, which could then be a great place to find water,” Eisner said.

Additionally, a second planet was spotted orbiting in the same system but is far outside the habitable zone with a mere 34-day orbit.

Scientists are thrilled at these discoveries but anticipate up to a 30-year wait until the two stars in the binary system move far enough apart to tell which star each of the two planets are orbiting.

“If we were able to constrain where the planets orbit, it would really offer a stepping stone to open up our understanding of exoplanet formation,” Eisner said. “It could also possibly help us someday be able to look at a star and its properties and make some guesses about what planets are potentially orbiting in that system.”

The TESS Citizen Science program has discovered over 25 million objects so far.

“The human brain has a really incredible ability to recognize patterns and to filter out noise,” Eisner said. “While our algorithms struggle to identify these longer-period planets, the citizen scientists don’t.”

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