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K2-33b planet transit
An artist’s conception shows the planet K2-33b crossing the disk of its parent star. (Credi: NASA / JPL-Caltech)

Scientists say they’ve detected a giant planet circling a star that’s only 5 million to 10 million years old, which would make it the youngest exoplanet ever identified.

The super-Neptune-sized planet traces a super-close-in orbit, making a complete swing around its parent star every 5.4 Earth days, according a team of astronomers associated with NASA’s repurposed Kepler space telescope. The star, known as K2-33, is in the Upper Scorpius stellar association, about 500 light-years from Earth.

Infrared observations of K2-33 indicate that the star is still surrounded by the remnants of gas and dust from a protoplanetary disk. Such disks form around stars as they’re born and give rise to planets, but the disks are thought to dissipate after a few million years. That’s how astronomers figured out that the planet was so young.

“At 4.5 billion years old, the Earth is a middle-aged planet — about 45 in human-years,” Caltech astronomer Trevor David said in a news release. “By comparison, the planet K2-33b would be an infant of only a few weeks old.”

David is the first author of a paper on the discovery published online today by the journal Nature.

The planet was discovered by analyzing the faint dimming of starlight as it passed over K2-33’s disk, as recorded during Kepler’s K2 extended mission. The data analysis led David and his colleagues to conclude that the planet is about six times as wide as Earth, or 50 percent wider than Neptune. The astronomers also figured out that K2-33b’s orbit is about 20 times closer in than Earth’s orbit around the sun. That translates to an orbital distance of 4.6 million miles.

Astronomers were intrigued by the fact that a planet so big and so young can have an orbit so close to its parent star. Some theories suggest that giant planets have to form farther out in a stellar system and then circle inward over a long period of time. K2-33b’s existence supports alternate hypotheses: either that giant planets can coalesce close-in to begin with, or that they can migrate inward soon after their formation.

Caltech’s Erik Petigura, a study co-author, characterized the find as “a remarkable milestone in exoplanet science.”

“The newborn planet K2-33b will help us understand how planets form, which is important for understanding the processes that led to the formation of the Earth and eventually the origin of life,” he said.

K2-33 system
This diagram shows the K2-33 system, and its planet K2-33b, compared to our own solar system. Click on the image for a larger version. (Credit: NASA / JPL-Caltech)

In addition to David and Petigura, the authors of “A Neptune-Sized Transiting Planet Closely Orbiting a 5-10-Million-Year-Old Star” include Lynne Hillenbrand, John Carpenter, Ian Crossfield, Sasha Hinkley, David Ciardi, Andrew Howard, Howard Isaacson, Ann Marie Codie, Joshua Schlieder, Charles Beichman and Scott Barenfield.

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