James Webb Space Telescope Takes Closest Look Yet Of A 'Mini-Neptune', A Mysterious World Outside Solar System

The mini-Neptune, called GJ 1214b, is a highly reflective world with a steamy atmosphere, and is too hot to harbour liquid-water oceans. The atmosphere probably contains water.

NASA's James Webb Space Telescope, also called Webb, has taken the clearest look yet of a 'mini-Neptune', a mysterious planet outside the solar system which other telescopes had not been able to observe clearly in the past. The mini-Neptune, called GJ 1214b, is a highly reflective world with a steamy atmosphere, and is too hot to harbour liquid-water oceans. The planet's atmosphere probably contains water in vaporised form.

The study, led by researchers at University of Maryland and University of Chicago was recently published in the journal Nature. 

What are sub-Neptunes?

The authors noted in the study that no planets intermediate in size between the Earth and Neptune are found in the solar system, but these objects are found around several stars other than the Sun. These planets, which have sizes greater than Earth but less than Neptune, are called sub-Neptunes.

The sub-Neptunes are distinguished from other planets through their hydrogen-dominated atmospheres that are a few per cent of the total mass of the planets.

The "sub-Neptune" category of planets is the most common in the Milky Way galaxy. 

Why have telescopes other than Webb not been able to observe the planet clearly?

The study said that GJ 1214b is an archetype sub-Neptune that has been observed using transmission spectroscopy to test the hypothesis that sub-Neptunes have hydrogen-dominated atmospheres. Transmission spectroscopy is a technique which allows researchers to know the chemical composition of the atmospheres of transiting exoplanets. The technique studies radiation that has been transmitted through a planet's atmosphere. 

Since high-altitude aerosols are present in the atmosphere of GJ 1214b, the spectra measured by telescopes and observatories other than Webb are featureless. 

How did Webb obtain the planet’s spectra?

Webb has obtained the spectra of the planet's atmosphere in mid-infrared wavelength, using the Mid-Infrared Instrument. Water is present on the dayside as well as nightside of the planet. 

The mini-Neptune has a high metallicity atmosphere which is blanketed by a thick and highly reflective layer of clouds or haze. 

According to a statement released by the University of Chicago, the exoplanet is too hot to be habitable. 

What elements are present in the planet’s atmosphere?

The planet is located 40 light-years away, and is not too far from us, but a thick layer of haze blanketing it is the biggest obstacle for people on Earth attempting to study it.

The planet's atmosphere also likely contains elements heavier than oxygen. A large proportion of the planet's elements are slightly heavier than hydrogen. The compounds include water, methane and mixtures of the two.

In the statement released by the University of Chicago, Eliza Kempton, the lead author on the paper, said the findings mark a "turning point" in the study of planets like GJ 1214b. She explained that the light from the planet disappeared when it went behind its host star, something that had never been seen before for this planet or any other sub-Neptune. 

At least around 50 per cent of all stars similar to the Sun in the Milky Way have sub-Neptunes around them, the statement said. 

The planet was discovered in late 2009, but after more than two decades, scientists have unravelled its mysteries, thanks to Webb. 

Professor Jacob Bean, one of the co-authors on the paper, said the planet is extremely frustrating to study because it has a lot of clouds in the atmosphere, which block the ability of people and telescopes on Earth to see anything else. 

However, Webb observed the planet using infrared light, for about 40 hours, which is equal to the orbital period of the planet. 

The planet is tidally locked

The planet is tidally locked, which means that one side of it is permanently turned towards the star while the other remains in darkness. Despite this, the planet's thick atmosphere transfers heat all around the planet. 

After extracting the data collected by Webb, the researchers made a map of the planet's temperature as it orbited the star. The team measured the flow of heat to determine what the atmosphere is made up of. There is a flow of heat because heavier and lighter molecules transport heat differently. 

The planet is reflective

Another important discovery is that the planet is reflective, and the clouds reflect sunlight, rather than absorbing it. Earlier, it was believed that the planet's clouds are dark, soot-like substances that absorb light. 

In a NASA statement, Kempton said that the night side is cooler than the dayside. 

He also said that the planet is hot by human standards, but much cooler than expected, a fact the team deduced from the planet's unusually shiny atmosphere, which reflects a large portion of light from the planet's star. 

What type of star does the planet orbit?

According to NASA, the planet probably formed farther from its star, which is a red dwarf, and then spiralled inward to its present, close orbit. The planet takes 1.6 Earth days to complete one orbit around its star.