Using data provided by NASA’s Spitzer Telescope, astronomers suggests that our galaxy may be inhabited by thousands of hot Neptune-sized planets with clouds of helium, an exotic class of planets in our Milky Way galaxy.
We don’t have any planets like this in our own solar system,
Says Renyu Hu ~ lead author of the paper accepted for publication in the Astrophysical Journal.
But we think planets with helium atmospheres could be common around other stars.
Prior to this study, researchers had been investigating hundreds of candidates of so-called hot Neptune planets in our galaxy. These planets are the size of Neptune or smaller, orbiting their parent stars in orbits closer than our neighbor Mercury revolves around the sun. They can attain temperatures of more than 1,300 ° Fahrenheit (705 ° C) and orbit their stars in as little as one or two days.
Why these exoplanets have an atmosphere consist mostly of helium? Hu says there is one factor behind how these planets developed their atmospheres.
Gas giant planets are composed mainly of hydrogen and helium. But these Neptune planets are very close to their host stars so the intense radiation would boil off hydrogen in the planets’ atmospheres. Hydrogen is lighter than helium and thus more likely to escape. So, after billions of years, the planet would develope an atmospheres rich in helium. (This diagram explain how helium atmospheres might form).
Hydrogen is about 4 times lighter than helium, so it would slowly disappear from the planets’ atmospheres, causing them to become more concentrated with helium over time. The process would be gradual, taking up to 10 billion years to complete.
Said Dr Renyu Hu.
It is believed that ‘hot Neptunes’ to have either rocky or liquid cores, surrounded by gas. If helium is indeed the dominant element in planet’ atmospheres, then they would appear grayish or even white. This is in contrast to Neptune, our solar system’s blue-hued, hydrogen-rich “ice giant”. The methane in its atmosphere absorbs red, giving Neptune its brilliant azure blue color.
Hu’s team was led to this finding by studing one particular warm Neptune called GJ 436b, an exoplanet orbiting the red dwarf Gliese 436 located approximately 33 light-years away in the constellation Leo.
Spitzer Telescope had previously observed GJ 436b and found evidence for carbon but there wasn’t trace of methane. This was puzzling to researchers, as methane is made of one carbon atom linked with four hydrogen atoms (CH4), methane should be present. Why wasn’t the hydrogen linking up with carbon to produce methane?
Although we cannot directly look for helium in these planets, future instruments such as NASA’s planned James Webb Telescope may one day achieve this goal. Meanwhile, Helium planets are just one type of possible exoplanets in a long list that has continued to surprise and amaze astronomers.
Said Sara Seager ~ co-author of the paper.
Any planet one can imagine probably exists, out there, somewhere, as long as it fits within the laws of physics and chemistry,
Planets are so incredibly diverse in their masses, sizes and orbits that we expect this to extend to exoplanet atmospheres.