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.

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…
Hellishly hot planet orbits so close to its star that a year lasts a few days
tess exoplanet star orbit image 7543 hot jupiter 1

A group of astronomers from the Physical Research Laboratory (PRL) in Ahmedabad, India has discovered a scorching new exoplanet where a year lasts just a few days, according to the Indian Space Research Organization.

The team, led by Prof. Abhijit Chakraborty of PRL, spotted the planet orbiting a star that is around 1.5 times the mass of our sun and is located 725 light-years away.

Read more
This Neptune-like planet has a thick atmosphere and maybe even a tail
Artist’s rendering of TOI-1231 b, a Neptune-like planet about 90 light-years away from Earth.

Artist’s rendering of TOI-1231 b, a Neptune-like planet about 90 light years away from Earth. NASA/JPL-Caltech

Discovering planets beyond our solar system is one thing -- we've discovered over 4,000 exoplanets so far -- but discovering if they might be habitable or if they might have an atmosphere is another. That's why it's exciting when researchers discover an exoplanet with a substantial atmosphere, as a group from NASA’s Jet Propulsion Laboratory and The University of New Mexico did recently.

Read more
This weird exoplanet is regrowing its atmosphere
This image is an artist’s impression of the exoplanet GJ 1132 b. For the first time, scientists using the NASA/ESA Hubble Space Telescope have found evidence of volcanic activity reforming the atmosphere on this rocky planet, which has a similar density, size, and age to that of Earth.

This image is an artist’s impression of the exoplanet GJ 1132 b. For the first time, scientists using the NASA/ESA Hubble Space Telescope have found evidence of volcanic activity reforming the atmosphere on this rocky planet, which has a similar density, size, and age to that of Earth. NASA, ESA, and R. Hurt (IPAC/Caltech)

Scientists know that the atmospheres of planets change over time -- Mars, for example, is gradually losing its atmosphere as it evaporates into space. The examples we know of suggested this was a one-way process, where an atmosphere developed and then was subsequently lost. But now, researchers using the Hubble Space Telescope have discovered a very odd planet that seems to be regrowing its atmosphere after having lost it in the past. This is the first time such a thing has been observed.

Read more