Hubble Spots A ‘Warm Neptune’ Evaporating At A Frantic Speed

NASA / ESA / D. Player (STScI)

The Hubble Space Telescope has discovered a very intriguing exoplanet, NASA announced yesterday. The intrepid spacecraft has laid eyes on a Neptune-sized planet that is rapidly losing its atmosphere, evaporating at a frantic pace.

Dubbed GJ 3470b, the exoplanet orbits a very young red dwarf star located 97 light-years away in the general direction of the Cancer constellation (“The Crab”), notes Phys.org — citing Johns Hopkins University, which participated in the discovery.

The detection was made as part of a new Hubble project called the Panchromatic Comparative Exoplanet Treasury (PanCET), a survey of 20 exoplanets that comparatively examines their atmospheres in ultraviolet, visible, and infrared light. Led by David Sing, Bloomberg Distinguished Professor at Johns Hopkins, the program is the first large-scale endeavor of this kind and the largest exoplanet observation project to be run with NASA’s Hubble.

The newfound planet is now described as a “very warm” Neptune and was picked up by Hubble after the telescope’s spectrograph saw the UV-light fingerprint of hydrogen wafting away from its atmosphere and leaking into space. According to NASA, the hydrogen had amassed in a gaseous cocoon enveloping the planet and was spotted as GJ 3470b passed in front of its star, blocking out some of its starlight.

“These results are interpreted as evidence of the planet’s atmosphere bleeding off into space.”

The Curious Case Of GJ 3470b

The case of GJ 3470b was compared to that of another warm Neptune called GJ 436b, discovered by Hubble a few years ago and is also losing gases from its atmosphere — a phenomenon known as atmospheric escape.

Similar to the newfound warm Neptune, this planet also orbits a red dwarf star — albeit a little older one, estimated to be between 4- and 8-billion-years-old.

Artist’s illustration of the warm Neptune GJ 436b next to its red dwarf star, which blasts the planet with radiation, heating its atmosphere to about 980 degrees Fahrenheit.
Artist’s illustration of the warm Neptune GJ 436b next to its red dwarf star, which blasts the planet with radiation, heating its atmosphere to about 980 degrees Fahrenheit.Featured image credit: NASA/JPL-CaltechWikimedia Commons/Resized

Both GJ 3470b and GJ 436b sit at about the same distance from their star, circling the red dwarfs from roughly 3.7 million miles away.

“That’s one-tenth the distance between our solar system’s innermost planet, Mercury, and the sun,” NASA points out.

Due to this close proximity to their young, active stars, the planets are constantly battered with radiation, which gnaws away at their atmospheres. However, the main difference between GJ 3470b and GJ 436b is that, while both of them are leaking atmospheric gases, the latter is in no danger of evaporating away.

Astronomers believe that this is because the planet is denser, which increases its gravitational pull and allows it to better hold on to its atmosphere. At the same time, its star is a little older and, therefore, less virulent.

The interior structure of exoplanet GJ 436b, a warm Neptune discovered by Hubble a few years ago.
The interior structure of exoplanet GJ 436b, a warm Neptune discovered by Hubble a few years ago.Featured image credit: Dr. Jason WrightWikimedia Commons / Resized

Meanwhile, the newfound planet is losing its atmosphere 100 times faster and has already depleted 35 percent of its mass. If GJ 3470b continues to evaporate at such as high pace, the planet will shrink down to a mini-Neptune in just a few billion years.

“This is the smoking gun that planets can lose a significant fraction of their entire mass,” said Sing.

“GJ 3470b is losing more of its mass than any other planet we seen so far; in only a few billion years from now, half of the planet may be gone.”

These incredible results were published yesterday in Astronomy & Astrophysics and make the focus of a study led by Vincent Bourrier of the University of Geneva in Switzerland.

“I think this is the first case where this is so dramatic in terms of planetary evolution,” said Bourrier. “It’s one of the most extreme examples of a planet undergoing a major mass-loss over its lifetime.”

As it turns out, the peculiar case of GJ 3470b could have profound implications for other planets as well. This warm Neptune could explain why there is such a scarcity of hot Neptunes in the universe, reports Science Daily, citing the University of Geneva.

What Happened To All The Hot Neptunes?

In the hunt for new exoplanets, astronomers have often come across a class of planets known as hot Jupiters — giant exoplanets about the same mass as Jupiter found in tight orbit around their stars. As the Inquisitr previously reported, these star-hugging planets get cooked by their stars, which heat up their atmospheres to a whopping 3,600 degrees Fahrenheit. Most of the planets surveyed by PanCET are hot Jupiters, notes NASA.

Hot super-Earths are also found in abundance by our telescopes. These planets are bigger than Earth but smaller than Neptune, usually measuring no more than 1.5 times the width of our planet. Similar to hot Jupiters, hot super-Earths also orbit extremely close to their parent star and get pummeled by radiation.

Judging by their size, hot Jupiters and hot super-Earths are two extremes that should be met in the middle by another class of planets described as hot Neptunes — gas giants clinging close to their stars, which heat their atmospheres to more than 1,700 degrees Fahrenheit. Nevertheless, there seems to be a mysterious deficit of hot Neptunes in the universe.

Their apparent absence has been described by astronomers as a “desert of hot Neptunes.” Scientists infer that these alien worlds are either very rare or were plentiful at one time but have since disappeared.

Given its distance to its star, the newfound warm Neptune sits at the border of this “desert,” as shown in the infographic below. Its discovery has led scientists to speculate that its current fate could explain the puzzling lack of hot Neptunes.

Info-graphic depicting the so-called ‘desert of hot Neptunes,’ comparing their prevalence to super-Earths and Jupiter-sized exoplanets orbiting both close and far away from their stars.
Info-graphic depicting the so-called ‘desert of hot Neptunes,’ comparing their prevalence to super-Earths and Jupiter-sized exoplanets orbiting both close and far away from their stars.Featured image credit: NASA, ESA and A. Feild (STScI)

“The question has been, where have the hot Neptunes gone?” said Bourrier. “If we plot planetary size and distance from the star, there’s a desert, a hole, in that distribution. That’s been a puzzle.”

“We don’t really know how much the evaporation of the atmospheres played in forming this desert. But our Hubble observations, which show a large amount of mass loss from a warm Neptune at the edge of the desert, is a direct confirmation that atmospheric escape plays a major role in forming this desert.”

One possible explanation is that their close proximity to their star has boiled away the atmospheres of these planets, either shrinking hot Neptunes into mini-Neptunes or completely eroding their atmosphere and leaving only their rocky core.

As the Inquisitr recently reported, this hints at the possibility that super-Earths may be the remnants of hot Neptunes whose atmospheres have completely evaporated.

Going forward, astronomers plan to look even further out into the cosmos and scour the atmospheres of warm Neptunes in search of helium signatures indicating atmospheric escape. Since hydrogen gas seeping out of these planets can’t be detected in UV light farther away than 150 light-years from Earth, looking for other atmospheric elements could expand the survey beyond this border and hopefully shed more light into the mystery of hot Neptunes.