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The system of planets found orbiting nearby dwarf star TRAPPIST-1 OC[ DG QWT DGUV EJCPEG [GV QH ƂPFKPI CNKGP NKHG
TRAPPIST-1, an ultracool dwarf star located just 40 lightyears from Earth in the Aquarius constellation, was first detected by researchers from Liege using the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile, and later confirmed by NASA’s Spitzer Space Telescope and the Very Large Telescope, also in Chile.
The planets were detected by observing dips in the star’s light output caused by each of the seven planets passing in front of it, events known as transits.
The researchers found that all of the planets are comparable in size to the Earth, while density measurements suggest that the innermost six are rocky.
Current climate models suggest the three innermost planets are probably too hot to support liquid water, and the one furthest from the star is too cold. However, the remaining three sit comfortably within the habitable zone and could host oceans of surface water – a feature thought to be essential for the existence of life.
“The energy output from dwarf stars like TRAPPIST-1 is much weaker than that of our Sun. Planets would need to be in far closer orbits than we see in the Solar System if there is to be surface water,” said researcher Dr Amaury Triaud. “Fortunately, it seems that this kind of compact configuration is just what we see around TRAPPIST-1.”
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 YKNN DG JKIJ QP VJG NKUV |
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.
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.”
“ROCKY PLANETS ARE THE CLEAR PREFERENCE BECAUSE THEY PROVIDE A SURFACE FOR LIFE FORMS”
Stuart Clark is an astronomy writer. His most recent book is the The Search For Earth’s Twin (£20, Quercus).
EXPERT COMMENT 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