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Astronomers have discovered an exoplanet with a highly elongated orbit that experiences large temperature swings. The planet may be transitioning to another type of world.
The exoplanet, called TIC 241249530 b, orbits a star about 1,100 light-years from Earth. The star is one of a binary system, so the planet orbits the primary star, while the primary star orbits a secondary star.
Interactions between the two stars, which have misaligned orbits, could be responsible for putting this planet on a path to becoming a “hot Jupiter,” researchers reported in a study published Wednesday in the journal Nature.
Astronomers have found more than 5,600 confirmed exoplanets, and 300 to 500 of them are “hot Jupiters.” These planets are huge, Jupiter-like bodies of gas that orbit their host stars closely, heating them to scorching temperatures.
While Jupiter takes 4,000 Earth days to complete one orbit around the Sun, a hot Jupiter completes one orbit every few days.
Scientists believe the giant planets start out orbiting their stars at a distance but migrate closer over time. But they have long puzzled over how the massive worlds end up in such tight orbits, which are much closer to their stars than Mercury is to our sun.
Observations of TIC 241249530 b, first captured by NASA’s planet-hunting TESS satellite in January 2020, provide rare, revealing insights into what could be a planet on its way to becoming a hot Jupiter.
“Astronomers have been looking for exoplanets that are likely progenitors of hot Jupiters, or intermediate products of the migration process, for more than two decades, so I was very surprised — and excited — to find one,” said lead researcher Arvind Gupta, a postdoctoral researcher at NOIRLab who discovered the planet as a doctoral student at Penn State, in a statement.
Recognizing a Changing World
On January 12, 2020, the Transiting Exoplanet Survey Satellite collected data indicating something was passing in front of the parent star, TIC 241249530. TESS monitors the brightness of nearby stars to look for dips in starlight that could indicate the presence of exoplanets.
Gupta and his colleagues followed up on the data and determined that a Jupiter-sized planet was passing in front of the star. They then took measurements using instruments on the WIYN 3.5-meter telescope at Kitt Peak National Observatory in Arizona to determine the star’s radial velocity, or how much the star wobbles back and forth as the planet’s gravity tugs on it.
The radial velocity data also confirmed the presence of the same planet and helped researchers clarify that it was about five times more massive than Jupiter and had what astronomers call a highly eccentric orbit.
Astronomers use “eccentric” to refer to the shape of a planet’s orbit on a scale of zero to 1. Zero is equivalent to a perfectly circular orbit. In our solar system, Earth has an eccentricity of 0.02, while Pluto’s highly oval orbit around the sun is considered to be 0.25.
The newly discovered exoplanet has an eccentricity of 0.94, which is more elongated than any other passing exoplanet astronomers have ever found, the researchers said. The strange world takes about six months to complete one orbit around its host star, coming extremely close to the star before flaring out and then returning to a narrow, oval orbit shaped like a cucumber.
“We caught this huge planet making a sharp hairpin turn as it passes its star,” study co-author Suvrath Mahadevan, the Verne M. Willaman Professor of Astronomy at Penn State, said in a statement. “Such highly eccentric transiting planets are incredibly rare — and it’s truly astonishing that we were able to discover the most eccentric.”
The planet is just 3 million miles from its star, more than 10 times closer to the star than Mercury gets to the sun. For reference, Mercury is on average 36 million miles (58 million kilometers) away from the sun, according to NASA.
The extreme orbit causes “huge temperature swings” throughout the planet’s year, said study co-author Jason Wright, a professor of astronomy and astrophysics at Penn State.
“The temperature at the cloud tops gets high enough to melt titanium during the few days it passes the star,” Wright said via email. “For most of its orbit it is farther away, and at its farthest point the cloud top temperature during the day is like a warm summer day on Earth.”
The research team also discovered that the planet is rotating in the opposite direction to the rotation of its star, a rare phenomenon that has not been observed in most exoplanets and does not occur in our solar system.
All the oddities observed in TIC 241249530 b help astronomers understand how the planet formed.
“While we can’t really press rewind and watch the process of planetary migration in real time, this exoplanet serves as a kind of snapshot of the migration process,” Gupta said. “Planets like this are hard to find, and we hope it can help us unravel the story of how hot Jupiter formed.”
A ‘dance of jobs’
The team performed simulations to determine how the planet ended up in such an unusual orbit and how it might evolve over time. The simulations included modeling the gravitational interactions between TIC 241249530 b and its host star and the secondary star.
The research team estimated that the planet likely formed far from the host star and started out in a wide, circular orbit, similar to Jupiter. But the host star has a misaligned orbit with the second star, which exerted gravity on the planet and stretched out its orbit, the researchers said.
“Over the course of many orbits, the gravitational influence of that outer star changed the orbit of TIC 241249530 b, causing it to become increasingly longer,” Wright said.
With each pass by its parent star, the planet’s orbit loses energy. Astronomers estimate that in hundreds of millions of years, the orbit will become smaller and more stable, lasting only a few days instead of the 167 days it currently lasts.
Then the planet will become a true hot Jupiter, said study co-author Sarah Millholland, an associate professor of physics at the Massachusetts Institute of Technology’s Kavli Institute for Astrophysics and Space Research.
“It’s quite an extreme process, because the changes in the planet’s orbit are enormous,” Millholland said. “It’s a big dance of orbits that takes billions of years, and the planet just goes along for the ride.”
Two Hot Jupiters
Before TIC 241249530 b, the only other known early hot Jupiter was an exoplanet called HD 80606 b, discovered in 2001. HD 80606 b was considered the planet with the most eccentric orbit until its recent discovery.
HD 80606 b has an eccentricity of 0.93 and a shorter orbit of 111 days, orbiting in the same direction as its star. But otherwise the planets are practically twins, Wright said. Finding two planets at such a brief stage of planetary orbital evolution is like “happening to stumble upon a butterfly just as its cocoon is opening,” he said.
The discovery of a second hot Jupiter progenitor helps astronomers confirm the idea that massive gas giants transform into hot Jupiters when they migrate from eccentric to circular orbits, the researchers said.
The team hopes to observe TIC 241249530 b with the James Webb Space Telescope to discover the dynamics of its atmosphere and see how it responds to such rapid heating. And the search continues for more planets like it that transform into hot Jupiters.
“This system highlights how incredibly diverse exoplanets can be,” Millholland said. “They’re mysterious other worlds that can have wild orbits that tell a story about how they got there and where they’re going. For this planet, the journey isn’t quite over yet.”
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