BBC Knowledge June 2017

The star is relatively small, just 8 per cent the mass

of the Sun, and would appear to glow salmon pink

when observed from the surface of the planets,

the researchers say.

Now that astronomers know that the planets are

there, the next job is to find out what they are really like.

The first step is to make an accurate determination

of their densities. When searching for habitable worlds,

rocky planets are the clear preference because –

put simply – they provide a surface for life forms

to walk, slither or otherwise move across.

The European Space Agency (ESA) will launch

CHEOPS (CHaracterising ExOPlanet Satellite) in 2018.

The main science goals of the mission are to measure

the densities of planets with radii between one

and six times of Earth. The TRAPPIST-1 system

will be high on the list. 

The next step will be to analyse the planets’

atmospheres to see if any look like they could be

habitable. “The main goal will be trying to detect

the signature of water,” said CHEOPS scientist

Dr Vincent Bourrier. Water vapour in a planet’s

atmosphere could betray widespread oceans and a

water cycle. Its signature appears in the infrared region of

the spectrum and this is where the NASA-built James

Webb Space Telescope (JWST) comes in.

ESA will launch the JWST in the same year as CHEOPS.

With its 6.5m-diameter infrared mirror, JWST will make

analysing exoplanet atmospheres easier than ever.

One of its first targets is likely to be the seven worlds

of the TRAPPIST-1 solar system.

“ROCKY

PLANETS ARE

THE CLEAR

PREFERENCE

BECAUSE THEY

PROVIDE

A SURFACE FOR

LIFE FORMS”

While finding water vapour would increase the belief

that the planet under investigation is potentially habitable,

there are other factors that could affect a planet’s ability

to support life.

To investigate those, Bourrier and his team have

already used the Hubble Space Telescope to look

at the ultraviolet signature of TRAPPIST-1’s two

innermost planets. His work shows that those planets

could have had their atmospheres completely

eroded away by the radiation from the star –

rendering the planets barren. Could this have

happened to the other worlds of TRAPPIST-1?

Actually proving that a planet is habitable may

be really tough. Astronomers will have to look for

‘biomarkers’. These are gases that only exist together

in an atmosphere because they are being replenished

by the metabolisms of living creatures. Oxygen and

methane are good examples in our own atmosphere.

So far, there are no firm plans to build a space telescope

capable of making such an exacting measurement,

although NASA and ESA have both studied

engineering concepts.

NASA’s next exoplanet mission, the Transiting

Exoplanet Survey Satellite (TESS), also launching

next year, could reveal many more solar systems

like TRAPPIST-1. This mission will survey 200,000 stars

and is expected to discover thousands of exoplanets,

from the size of Earth up to Jupiter and larger.”

Stuart Clark is an astronomy writer. His most recent book

is the The Search For Earth’s Twin (Quercus).

As the planets in

the TRAPPIST-1 system are

so close together, they’d be

visible in each other’s skies,

as seen in this illustration

PHOTOS:

NASA X3

EXPERT COMMENT

24 June 2017

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