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NASA telescopes reveal the atmosphere of a strange hybrid exoplanet

In space, there is an unusual exoplanet called Gliese 3470 b (GJ 3470 b). It's a strange world, a kind of hybrid between Earth and Neptune. It has a rocky core like Earth but is surrounded by an atmosphere of hydrogen and helium. This combination is unlike anything in our own solar system.

The planet orbits a red dwarf star named Gliese 3470 in the constellation Cancer. GJ 3470 b has about 12.6 Earth masses, which means that it is about halfway between Earth and Neptune. (Neptune consists of about 17 Earth masses.)

Thanks to the Kepler mission, we know that there are many exoplanets in this mass range. It is possible that up to 80% of the planets will fall within this range, although future exoplanet missions will undoubtedly clarify this. So far, astronomers have not studied the atmosphere of one of these planets. Their creation is therefore a mystery.

The Hubble and Spitzer Space Telescopes have teamed up to take a close look at the atmosphere of GJ 3470 b. It's the first time astronomers have been able to identify the chemical fingerprint of a planet's atmosphere like this. They found that the planet has an almost untouched, original atmosphere of hydrogen and helium without heavier elements.

And that is a bit puzzling.

"We do not have such a thing in the solar system, and that makes it so noticeable."

Björn Benneke from the University of Montreal in Canada

Half Star, Half Planet?

GJ 3470 b, with its atmosphere of hydrogen and helium, is more akin to a star than a planet. Our own sun consists of 73% hydrogen, and the remainder is almost entirely helium. Only a small part of the sun is composed of heavier elements such as oxygen, neon, iron and carbon. The gas giants Jupiter and Saturn consist mainly of hydrogen and helium, but also contain other compounds such as methane and ammonia as well as heavier elements. These compounds are missing in GJ 3470 b.

almost. "This is a great discovery from the perspective of planet formation. The planet orbits very close to the star and is far less massive than the Jupiter (31

8 times the mass of the earth). He has, however, succeeded in acclimating the original hydrogen / helium atmosphere, largely due to heavier elements & # 39; unpolluted & # 39; "said Björn Benneke of the University of Wisconsin Montreal in Canada, in a NASA press release. "We do not have that kind of thing in the solar system, and that makes it so noticeable."

The illustration by this artist shows the theoretical inner structure of the exoplanet GJ 3470 b. It's different than any planet in the solar system. At 12.6 Earth masses, the planet is more massive than Earth, but less massive than Neptune. Unlike Neptune, which is 3 billion miles from the Sun, GJ 3470 b may have formed very close to its red dwarf star as a dry, rocky object. Then, by gravity, it pulled hydrogen and helium from a circumstellar disk to build up a thick atmosphere. The disk broke up many billions of years ago and the planet stopped growing. The illustration below shows the disk as the system may have looked like a long time ago. NASA Hubble and Spitzer Space Telescope observations have chemically analyzed the composition of the very clear and deep atmosphere of the GJ 3470 b and provided clues to the planet's origin. Many planets of this mass exist in our galaxy. Photo credits: NASA, ESA and L. Hustak (STScI)

The astronomers behind this work combined the multiwavelength capabilities of both space telescopes to gain a good insight into the atmosphere of the GJ 3470 b. They measured the absorption of starlight as the exoplanet went through in front of its host star. They also measured the loss of reflected light as the exoplanet passed behind the star. In total, the pair of space telescopes observed 12 transits and 20 eclipses.

The Spitzer Space Telescope is the last of four telescopes in NASA's Great Observatories program. It is observed in the infrared. The mission will be over in January 2020. Credit: NASA

Astronomers use spectroscopy to identify the chemical fingerprints of hydrogen and helium in the atmosphere, and the nature of the Earth's atmosphere has made this all possible. It's mostly clear with very little haze, meaning they could look deep into the atmosphere. "For the first time, we have a spectroscopic signature of such a world," said Benneke.

However, this spectroscopy revealed something unexpected. The astronomers thought they would find a similar chemical composition to the planet Neptune, with heavier elements like oxygen and carbon. Instead, they found an atmosphere that resembled the sun.

"We found an atmosphere so poor in heavy elements that its composition resembles the hydrogen / helium-rich composition of the sun."


"We have one Atmosphere that is heavily enriched with heavier elements such as oxygen and carbon and forms abundant water vapor and methane gas, similar to what we see on Neptune, "said Benneke. "Instead, we found an atmosphere so poor in heavy elements that its composition resembles the hydrogen / helium rich composition of the sun."


Now that astronomers have a good grip on the exoplanets, thanks to the combined power of Hubble and Spitzer space telescopes, they can begin to understand how this strange planet could have formed.

GJ 3470 b is in sharp contrast to other exoplanets. Astronomers believe that other exoplanets, for example, hot Jupiter, form at a great distance from their sun and then migrate inward. However, astronomers believe that this exoplanet has formed very close to its red dwarf star, close to where it is today.

  Artist concept of a Jupiter-sized exoplanet circling relatively close to its star (also known as
Artist concept of a Jupiter-sized exoplanet that circles relatively close to its star (also known as "hot Jupiter"). NASA / JPL-Caltech)

It probably first formed as a tiny rocky object that is at about the same time as the star in the center of the protoplanetary disk. It would have accumulated its atmosphere from the same source material in the disk that formed the star. And that would explain his hydrogen / helium atmosphere and why he lacks heavier elements.

"This is a great discovery from the point of view of planet formation."


We see an object that could accumulate hydrogen from the protoplanetary disk but did not run away become hot Jupiter, "said Benneke. "This is a fascinating regime."

What has possibly happened is that it still collects matter from the disk, but the star grows faster and the disk dissolves. This prevented GJ 3470 b from becoming larger and resembling the gas giant in our solar system with heavier elements in the atmosphere.

For now, here is our understanding of this fascinating, strange exoplanet. Once the James Webb Space Telescope (JWST) is up and running, we'll learn more.

The JWST is a powerful space telescope that can look into the infrared with unprecedented sensitivity. It will be able to investigate the atmosphere of GJ 3470 b and other exoplanets and reveal things that have not been seen before. In particular, it will observe wavelengths that make fogged cloudiness almost transparent.

Then our understanding of all exoplanets, not just this one, will skyrocket.


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