Astronomers Watch Giant Planet TOI-2535 b Vanish From View

"The planet's orbit is tilting so quickly that its transit disappeared while we were watching," said Samuel Yee, a UCLA postdoctoral fellow and the study's corresponding author, in a statement.

What happened to the orbit

TESS never saw the planet directly. It infers a planet by catching the small drop in starlight when one passes in front of its star.

The fading crossings showed the planet's path sliding from the middle of the star toward its edge. The planet still circles its star, but from Earth it now passes above or below it.

UCLA astrophysicist and co-author Erik Petigura compared it to eclipses. "If the moon's orbit around the Earth was perfectly aligned with the Earth's orbit around the sun, we would see an eclipse every month," he said.

The unseen second planet

The early and late arrivals pointed to something unseen tugging on the planet. The team combined those timing clues with measurements of how the star wobbles as its planets pull on it.

Everything pointed to a second planet, likely at least as massive as Saturn, circling nearby on its own tilted orbit. Nobody has seen it. Its path carries it above or below the star from TESS's vantage point, so it never blocks detectable starlight.

The two planets keep pulling on each other, so their orbits wobble slowly like spinning tops. The researchers expect TOI-2535 b to swing back into view in 30 to 50 years. Its hidden neighbor may show up sooner.

A clue to how hot Jupiters form

TOI-2535 b sits about 1,000 light-years from Earth. It is a "hot Jupiter," a gas giant that orbits so close to its star that it finishes a lap in about eight days. Mercury needs 88 days.

Scientists think most such planets reached their tight orbits violently, perhaps after a close encounter with another object knocked them onto a stretched-out path. This pair likely took a gentler route and spiraled in slowly together, according to the new paper.