New 'super-Earth' found with water and the right temperature for life

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This massive range stems from the fact that, with Hubble observations, researchers can only identify a water signature, the "fingerprint" observed using transit spectroscopy; they can't tell how much water is there, Giovanna Tinetti, a researcher on this study and a professor of astrophysics at UCL, said during a September 10 news conference. At the moment, one extremely important piece of information that is missing is the presence, composition and structure of their atmospheres.

As of now, the researchers have concluded that there is some quantity of water and likely hydrogen in the planet's atmosphere.

"Over 4,000 exoplanets have been detected but we don't know much about their composition and nature", said Tinetti.

The new research was published September 11 in Nature Astronomy.

A planet's atmosphere plays a vital role in shaping the conditions inside it - or on its surface, if it has one.

The primary method that we use when examining exoplanets is transit spectroscopy.

'It's orbiting a completely different star, so it doesn't look like Earth. "The atmosphere of the planet leaves a characteristic fingerprint on the light ... this is what we try to observe". Further analysis can then help us match this footprint to known elements and molecules, such as water or methane. It has never before been seen in smaller planets - until now. "This is not only because super-Earths like K2-18b are the most common planets in our Milky Way, but also because red dwarfs - stars smaller than our Sun - are the most common stars".

"This is the only planet right now that we know outside the solar system that has the correct temperature to support water, it has an atmosphere, and it has water in it, making this planet the best candidate for habitability that we know right now", lead author Angelos Tsiaras, an astronomer at University College London, said in a press conference.

Although K2-18 b flaunts some of the most Earth-like features observed in an exoplanet so far - water, habitable temperatures, and a rocky surface - the researchers point out the world is still far from Earth-like.

They made the discovery studying data captured by the Hubble Space Telescope developed by Marshall Space Flight Center in Huntsville.

That's because this 33-day orbit is right smack-bang in the middle of the star's habitable zone - not too hot that liquid water would evaporate from the surface, and not so cold that it would totally freeze.

In order for an exoplanet to be defined as habitable, there is a long list of requirements that need to be satisfied.

The watery planet is not particularly Earth-like, and it's doubtful that it has a rocky surface like that of our planet, Seager and Shaefer say. It is also necessary that the planet has an atmosphere to protect the planet from any harmful radiation coming from its host star.

Tsiaras and his team think the planet is likely a rocky "super-Earth" in possession of an atmosphere that's either very water dominant, heavily mixed with a transparent gas like nitrogen, or features significant cloud formation.

"Models are really essential for the planning, but of course in all the observations we have to be willing to accept really unexpected and new things", Schaefer says. Our data are limited to an area of the spectrum - this shows how light is broken down by wavelength - where water dominates, so other molecules can unfortunately not be confirmed.

The prospective launch of Nasa's much delayed James Webb Space Telescope (JWST) in 2021, and the European Space Agency's Ariel mission seven years later, will enable astronomers to study in detail the atmospheres of the varied worlds that have been detected so far. This will help us understand just how habitable it is.

The researchers studied Hubble data to analyze K2-18 b's transit, or its movement across its host star's face, using a technique known as transit spectroscopy.

"K2-18b is very different from anything we know", says Sara Seager, a professor of physics and planetary science at MIT not involved in the research, in an email.

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