Skip to main content
  1. Home
  2. Space
  3. News

Why are there no hot Neptunes? Atmospheric escape could be the culprit

By

A strange puzzle has been taxing astronomers for many years: where are the hot Neptunes? For unknown reasons, there is an notable absence of planets the size of Neptune which lie close to their respective stars. Scientists looking for exoplanets often find hot large planets the size of Jupiter and “super-Earth” hot planets which are slightly larger than our planet, but they almost never find hot planets that are Neptune-sized. New research from astronomers at the University of Geneva (UNIGE), Switzerland, may shed light on this oddity.

Using data from the Hubble Space Telescope, a few years ago the UNIGE astronomers discovered a warm Neptune-sized planet which was losing its atmosphere. The planet, GJ 436b, was shedding hydrogen from its atmosphere in a manner that suggested that the energy given out by nearby stars could effect the way that the planets evolve. And now the same team has discovered another warm Neptune, GJ 3470b, which is loosing its hydrogen 100 times faster than GJ 436b.

Recommended Videos

GJ 3470b is around 3.7 million kilometers from its star (around 2.3 million miles), which is just one-tenth of the distance between Mercury and the Sun. But the planet is loosing hydrogen at an even more rapid rate because its star is so young and energetic. The atmosphere of GJ 3470b is being lost fast enough that it will effect the way that the planet evolves, and the planet has already lost more than a third of its mass.

An artist’s illustration of hydrogen from the atmosphere of a Neptune-sized planet being pulled away by the energy of a nearby star. NASA, ESA, and D. Player (STScI)

This finding suggests that hot Neptunes do form close to stars, but that the planets are rapidly eroded down to smaller Earth-sized planets or are even degraded completely until all that remains is a rocky core. “Until now we were not sure of the role played by the evaporation of atmospheres in the formation of the desert,” said Dr. Vincent Bourrier, researcher in the Astronomy Department of the Faculty of Science of the UNIGE. “This could explain the abundance of hot super-Earths that have been discovered,” confirmed David Ehrenreich, associate professor in the astronomy department of the science faculty at UNIGE.

Related

In order to confirm whether this theory is correct, the researchers need to observe more exoplanets. The challenge is that escaping hydrogen can only be observed if the planets are less than 150 light-years from Earth, so the team plans to look for evidence of atmospheric escape of heavier elements such as carbon.

Editors' Recommendations

Topics
Georgina Torbet
Georgina Torbet
Space Writer
Georgina is the Digital Trends space writer, covering human space exploration, planetary science, and cosmology. She…
Hubble watches an extreme exoplanet being stripped by its star
This artist's illustration shows a planet (dark silhouette) passing in front of the red dwarf star AU Microscopii.

Of the many strange exoplanets discovered to date, one of the most extreme has to be a world called AU Mic b. This Neptune-sized planet orbits close enough to its star that a year there lasts just over a week, and it is bombarded by dramatic flares from its host star which cook the planet with radiation.

Recently, Hubble observed this system to learn more about the relationship between the exoplanet and its star, technically called AU Microscopii.  The planet's hydrogen atmosphere is blown away by radiation from the star, but there were confusing findings that seemed to show that no atmosphere was being lost at some times, but significant amounts of atmosphere were lost at other times.

Read more
Here’s why scientists think life may have thrived on the ‘hell planet’ Venus
The planet Venus.

When you look at Venus today, it doesn’t seem like a very welcoming place. With surface temperatures hotter than an oven, atmospheric pressure equivalent to being 3,000 feet deep in the ocean, and no liquid water anywhere that we’ve seen, it seems like the opposite of a comfortable environment in which life could emerge.

But in the last decade, scientists have begun to wonder whether this “hell planet” could once have been habitable. Billions of years ago, Venus could have been a cooler, wetter place, with oceans not unlike our own here on Earth.

Read more
This exoplanet is over 2,000-degrees Celsius, has vaporized metal in its atmosphere
This artist impression illustrates how astronomers using the Gemini North telescope, one half of the International Gemini Observatory operated by NSF’s NOIRLab, have made multiple detections of rock-forming elements in the atmosphere of a Jupiter-sized exoplanet, WASP-76b. The so-called “hot Jupiter” is perilously close to its host star, which is heating the planet’s atmosphere to astounding temperatures and vaporized rock-forming elements such as magnesium, calcium and iron, providing insight into how our own Solar System formed.

Astronomers have studied a strange, puffy, scorching-hot planet located 600 light-years away, and have seen elements that would normally form rocks, but are so hot that they have vaporized into the atmosphere.

The planet, named WASP-76b, is around the mass of Jupiter, but orbits its star 12 times closer than Mercury is to the sun. Being so close, its atmosphere its heated to a scorching 2,000- degrees Celsius, which makes it puff up to a large size that's six times the volume of Jupiter. These high temperatures also give astronomers the opportunity to observe elements that would normally be hard to identify in the atmosphere of a gas giant.

Read more