Citizen scientists searching through data collected by NASA's Transiting Exoplanet Survey Satellite (TESS) have discovered a record-breaking world, and gave it a memorable nickname to match: Percival, after the father of Albus Dumbledore, the headmaster of Hogwarts in the Harry Potter books.
In more official terms, however, the extrasolar planet, or "exoplanet," is designated TOI 4633 c.
But there is more to Percival than its association with the Dumbledores. The planet, which is around three times the size of Earth, orbits TOI 4633 A, a sun-sized star in a binary system located around 309 light-years from Earth. The world also happens to sit in that star's habitable zone, a region with temperatures neither too hot nor too cold to allow liquid water to exist, hence its other moniker: The "Goldilocks zone."
That's why Percival is so special. Not only is it serendipitously in the Goldilocks zone, but scientists also believe planets are half as likely to form in systems with more than one star. And that's not all.
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Additionally, Percival appears to have the most elongated orbit of its planetary category, namely Neptune-like planets. The exoplanet also has the second-longest orbit found in all TESS data thus far, and is one of just five worlds with orbits longer than 100 days that the NASA exoplanet-hunting spacecraft has spotted since it launched to space in 2018.
"This planet is remarkable in many aspects," Nora Eisner of the Center for Computational Astrophysics and principal investigator of Planet Hunters TESS, said in a statement. "It's remarkable in its orbit, it's remarkable for being in the habitable zone and it's remarkable for orbiting a bright star."
What do we know about this magical star system?
Percival was spotted because it crosses, or transits, the face of the star TOI 4633, causing a tiny dip in light that TESS is sensitive enough to spot. The transit method is usually better at spotting planets that closely orbit their stars. That's because star-hugging planets are more likely to make a transit between Earth and their star while scientists' instruments watch, thus allowing us to detect a light-blocking arrangement more frequently.
Still, Percival is unusually distant from its star for a transit-method detected world, taking around 272 Earth days to orbit its star.
Ironically, scientists think there is another planet in this system, TOI 4633 b, that orbits closer to its parent star and takes just 34 Earth days to complete a revolution. However, this information hasn't been confirmed because, despite its proximity to its star, TOI 4633 b doesn't pass between its star and TESS. Thus, it doesn't transit its star from our vantage point and our instruments have difficulty in analyzing it.
Also part of the binary system is the star TOI 4633 B (notice capital letters denoting stars and lower case letters representing planets), which takes around 203 Earth years to loop around TOI 4633 A and its orbiting planets.
Binary stars form in a star system from the same cloud of gas and dust when cool, overly dense regions "clump" together and gather enough mass to collapse under their own gravity. If there is enough material, then two stars can form. That's a binary system.
Planets are born from material leftover after the creation of a star, explaining why if a cloud of matter has birthed two stars, it is less likely to birth planets, too.
"Finding planets in multi-star systems is crucial for our understanding of how you can make different planets out of the same material," Eisner said. "It's quite exciting that we found this one."
Though Percival is located in the habitable zone of its star, to be clear, it isn't likely to be very friendly to life as we know it. The planet likely lacks a solid surface; instead, it seems to be composed of a thick atmosphere of water vapor, hydrogen and methane, making it more like a solar system gas giant than a rocky Earth-like world. However, just as Jupiter and Saturn are proposed to have moons of ice and rock like Enceladus that could support life, Percival could be orbited by exomoons that are more favorable to living things.
"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.
There is no sign of these exomoons yet, but the system would be a good choice for future exomoon detection campaigns thanks to the brightness of its stars and the long orbit of Percival, factors thought to be advantageous to exomoon detection.
Discovering new worlds from right at home
The citizen scientists of the Planet Hunters TESS program assist astronomers by filtering through vast amounts of data that researchers couldn't possibly handle alone. The program has been paying dividends; its 43,000 volunteers from 90 countries have helped astronomers catalog around 25 million objects so far.
The huge Planet Hunters TESS team is particularly adept at spotting the faint signals of planets on elongated orbits, which computer programs have trouble identifying.
"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."
The discovery of Percival happened when 15 Planet Hunter TESS citizen scientists first identified a signal in data that looked to indicate a transiting planet. Eisner and her team then took a closer look at the TOI 4633 system and spotted tiny "wobbles" in one of the stars' motion that indeed indicated the gravitational tug of orbiting planets.
Delving deeper into the system using archival data collected over almost 120 years, the team then discovered that TOI 4633 is comprised of two stars, not one. These stars are currently so close to each other, however, that they can't be separately distinguished.
It will be three decades before the stars TOI 4633 A and TOI 4633 B are widely spaced enough for astronomers to determine the exact arrangement of this binary star system. That data could help determine if the two planets actually orbit both stars, which in turn could help us better understand what star systems to focus on as we continue to hunt exoplanets.
"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 explained. "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."
In the meantime, before more can be learned about this system, the Planet Hunter TESS team will continue their search for fascinating planets beyond the solar system.
"Every time I spot a possible transit, I can feel my heart beat faster, and my excitement rises extensively," Simon Bentzen, a Danish citizen scientist who has volunteered with Planet Hunters TESS since 2018, said in the statement. "I'm very happy that I helped find the new system.
"I hope that the new planets can help contribute to our understanding of planet formation and help answer other interesting planetary questions."
The team's research was published on Tuesday (April 30) in The Astrophysical Journal
