by Citizen scientists searching through data collected by NASA’s Transiting Easioplanet Survey Satellite (TESS) discovered a world of excellence, and gave it a memorable nickname to match: Percival, after the father of Albus Dumbledore, headmaster of Hogwarts in the Harry Potter books. .
In more official terms, however, the extrasolar planet, or “exoplanet,” is designated as TOI 4633 c.
But there is more to Percival than his connection to the Dumbledores. The planet, which is about three times the size of Earth, orbits TOI 4633 A, a solar star in a binary system located about 309 light years from Earth. Earth also happens to sit in that star’s habitable zone, a region where temperatures are neither too hot nor too cold for 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 that it is half as likely that planets form in systems with more than one star. And that’s not all.
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In addition, Percival appears to have the longest orbit of its planetary category, namely Neptune-like planets. The exoplanet also has the second-longest orbit in all TESS data so far, and is one of five worlds with orbits longer than 100 days seen by NASA’s exoplanet-hunting spacecraft since it launched into space in 2018.
“This planet is amazing in many aspects,” Nora Eisner of the Center for Computational Astrophysics and principal investigator of Planet Hunters TESS, said in a statement. “It’s spectacular in its orbit, it’s spectacular in the habitable zone and it’s spectacular in the orbit of a bright star.”
What do we know about this magical star system?
Percival was seen because it crosses, or transits, the face of the star TOI 4633, causing a tiny dip in light that TESS is sensitive enough to see. The transit mode is usually best for seeing planets that orbit their stars closely. That’s because planets hugging a star are more likely to transit between Earth and its star while scientists’ instruments are watching, allowing us to detect a light-blocking arrangement more often.
However, Percival is extremely far from its star for a transit-detectable world, taking about 272 Earth days to orbit its star.
Ironically, scientists think that there is another planet in this system, TOI 4633 b, whose orbit is closer to its parent star and which only takes Earth 34 days to complete a revolution. However, this information has not been confirmed because, despite its proximity to its star, TOI 4633 b does not pass between its star and TESS. Therefore, it does not move its star from our view and our instruments have difficulty analyzing it.
Also part of the binary system is the star TOI 4633 B (capital letters representing stars and lowercase letters representing planets), which takes about 203 years for Earth to orbit TOI 4633 A and its orbiting planets.
A binary star in a star system forms from the same cloud of gas and dust when cool, overdense regions “collide” together and gather enough mass to collapse under their own gravity. If there is enough material, two stars can form. That’s a binary system.
Planets are born from material left over after star formation, explaining why, if a cloud of material has given birth to two stars, planets are less likely to be born as well.
“Finding planets in multiple star systems is critical to our understanding of how you can make different planets from the same material,” Eisner said. “It’s pretty exciting that we got this one.”
Although Percival is located in the habitable zone of its star, to be clear, it is unlikely to be very friendly to life as we know it. The planet probably does not have a solid surface; instead, it appears 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 eemons that are more favorable for life.
“If this planet had a moon, that moon would probably have a solid surface, which could be a great place to get 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 considered advantageous for exomoon detection.
Discover a new life right at home
Citizen scientists of the Planet Hunters TESS program help astronomers by sifting through vast amounts of data that researchers could not handle on their own. The program is paying dividends; Its 43,000 volunteers from 90 countries have helped astronomers catalog around 25 million objects to date.
The huge team of TESS Planet Hunters are particularly skilled at spotting the faint signals of planets in long orbits, which computer programs struggle to identify.
“The human brain has an incredible ability to recognize patterns and filter out noise,” Eisner said. “While our algorithms struggle to identify these long-period planets, the citizen scientists do not.”
Percival’s discovery occurred 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 looked at the TOI 4633 system and saw tiny “wobbles” in the motion of one of the stars that indeed indicated the gravitational pull of the orbiting planets.
Delving deeper into the system using archival data collected over nearly 120 years, the team then discovered that TOI 4633 is made up of two stars, not just one. However, these stars are currently so close together that they cannot be identified individually.
It will be three decades before the stars TOI 4633 A and TOI 4633 B are wide enough for astronomers to determine the exact location of this binary star system. That data could help determine whether the two planets orbit the two stars, which could help us better understand the star systems we’ll target as we continue our exoplanet hunt.
“If we could constrain where the planets orbit, it would offer a step forward in opening up our understanding of exoplanet formation,” explained Eisner. “It could also help us maybe someday to be able to look at a star and its properties and make some guesses about the planets that could be orbiting in that system.”
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In the meantime, until 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 see a possible transit, I can feel my heart beat faster, and my excitement rises widely,” Simon Bentzen, a citizen scientist from Denmark who has volunteered with Planet Hunters TESS since 2018, said in the statement . “I’m really happy that I helped found the new system.
“I hope the new planets can help 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
