A giant, boiling, shrinking planet has a tail stretching 350,000 miles into space, astronomers discover

• Astronomers have observed a shrinking, Jupiter-sized planet with a comet-like tail in its wake.
• The planet orbits so closely to its host star that its atmosphere is boiling and shedding away.
• Scientists say studying WASP-69b can teach us about how planets and stars evolve over time.

Astronomers have nabbed a rare opportunity to watch a planet shrink in real-time.

The Jupiter-sized planet, located 160 light years away, is orbiting its host star so closely that the star is boiling the planet's atmosphere.

The boiling is so intense, that the world has developed a comet-like tail stretching 350,000 miles behind it, scientists announced on Tuesday.

Don't retire on this planet

The planet, called WASP-69b, circles very closely to its star, completing an orbit in under four days. The star's intense radiation is evaporating the helium gas in the planet's atmosphere.

"This is an extreme environment," Dakotah Tyler, a UCLA doctoral student who led the research, said in a press conference on Tuesday at the meeting of the American Astronomical Society.

"I would suggest that you do not consider retiring on this planet," he added jokingly.

As a result, WASP-69b is shedding the equivalent of one Earth every billion years, or about 200,000 tons per second, the scientists calculated.

The star's "wind" — a stream of charged particles and magnetic fields that constantly buffets the planet — is shaping that shedding helium gas into a tail, which stretches at least seven times the planet's width.

This kind of shedding may be common in the universe. Scientists think it explains a mysterious gap in the sizes of exoplanets.

Shrinking worlds with comet-like tails could explain a planetary mystery

As NASA has cataloged more than 5,000 alien worlds, it's become clear that two of the most common types of planets in the universe are super-Earths (which are up to 1.6 times as wide as Earth) and sub-Neptunes (about two to four times Earth's width).

But there are hardly any planets in between those sizes. Scientists think it's because the planets in between shrink so quickly that they turn into super-Earths.

That could be because radiation from the planets' cores pushes their atmospheres away, or because their stars boil the planets' outer layers away — evaporating mass over time.

WASP-69b is much bigger than a sub-Neptune, but it's demonstrating the exact boiling behavior that scientists suspect.

Because it's so large, it takes much longer to lose its atmosphere, giving scientists plenty of time to study it.

Studying this Jupiter-sized world and its long comet-like tail can teach us more about the physics of space, including how planets and their host stars evolve together over time.

"The WASP-69b system is a gem because we have a rare opportunity to study atmospheric mass-loss in real time and understand the critical physics that shape thousands of other planets," Erik Petigura, study co-author and UCLA professor of physics and astronomy, said.