The Hubble Space Telescope (HST), together with the Spitzer observatory in orbit above Earth have identified the chemical makeup of the atmosphere of a massive world far from the Sun. This discovery marks the first time that the atmosphere of an exoplanet intermediate in size between the Earth and Neptune has been successfully analyzed.
The planet, Gliese 3470 b (GJ 3470 b) is 12.6 times more massive than the Earth, or just under three-quarters of the mass of Neptune. Worlds of this size are not found in our own solar system, but these sub-Neptunes appear to be among the most common type of planets surrounding other stars.
“GJ 3470 b orbits very close to its star and is 30 times less massive than Jupiter but has managed to accrete the same kind of primordial hydrogen/helium atmosphere, largely unpolluted by heavier elements. We don’t have anything like this in the solar system, and that’s what makes it striking,” said Björn Benneke of the University of Montreal in Canada.
Breathe, Breathe in the Air (or not)
Benneke and his team believe GJ 3470 b is made up of a rocky core, surrounded by a dense atmosphere of hydrogen and helium. The chemical makeup of this world is unlike that found around any planet in our Solar System, and it is so dense, it would easily crush almost anything unfortunate enough to find itself on the surface of that world.
“We expected an atmosphere strongly enriched in heavier elements like oxygen and carbon which are forming abundant water vapor and methane gas, similar to what we see on Neptune. Instead, we found an atmosphere that is so poor in heavy elements that its composition resembles the hydrogen/helium rich composition of the Sun,” Benneke explained.
Fortunately for Benneke and his team, the atmosphere of this exoplanet is fairly clear, with just faint haze, allowing them to examine deep within the gaseous envelope.
Somewhere, Over the Rainbow…
The research team spent five years analyzing data from the pair of space telescopes. The instruments provided a wealth of information about the exoplanet, viewing the world in a variety of wavelengths as it orbited between its star and our home world. Light from that star, GJ 3470, was broken up into a spectrum of colors like a rainbow, revealing the chemical makeup of the atmosphere.
By analyzing the atmosphere of GJ 3470 b, the team was able to better understand the history of that alien world. Many of the largest exoplanets, known as hot Jupiters, are believed to form far from their parent stars, and migrate inward toward their stellar companion.
However, this does not seem to be the case with GJ 3470 b, which the team believes developed at around the same distance it is currently seen. This alien world appears to have started as a massive ball of rock, which later collected vast quantities of material from the primordial disk of gas from which the planetary system formed. The gas and dust from which the system formed has long sense dispersed.
“We’re seeing an object that was able to accrete hydrogen from the protoplanetary disk, but didn’t runaway to become a hot Jupiter… The planet got stuck being a sub-Neptune,” Benneke stated.
Red Dwarf is Surprisingly Popular Around the Galaxy
The exoplanet GJ 3470 b orbits 5.3 million kilometers (3.3 million miles) from its parent star once every 80 hours. The star GJ 3470, half the size of our Sun, is a red dwarf star, with a surface temperature less than two-thirds that found on our parent star. Despite the cool temperature of its stellar companion, this exoplanet orbits far too close to its parent star to be considered a likely home for alien life. This world, the only planet so far found in this system, was found in 2012, 100 light years from Earth.
The search for planets around alien stars has revealed that up to 80 percent of alien worlds are sub-Neptunes. This is a puzzle for astronomers attempting to understand why our own family of planets is devoid of worlds this size.
“With no analogues in the Solar System, the discovery of thousands of exoplanets with masses and radii intermediate between Earth and Neptune was one of the big surprises of exoplanet science. These super-Earths and sub-Neptunes probably represent the most common outcome of planet formation,” researchers wrote in a paper detailing the discovery, published in the journal Nature Astronomy.
The James Webb Space Telescope, due for launch in 2021, will have the capability to examine the atmosphere of exoplanets like GJ 3470 b in greater detail than ever before possible with Hubble and other observatories studying alien worlds.
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