IN THE HABITABLE ZONE

The TRAPPIST-1 system has at least three potentially habitable planets

A key focus of exoplanet science is the question of habitability. This doesn’t necessarily refer to worlds where humans could survive without life support, but any planet where Earth-like life might be able to develop. A major factor is the ability for water to exist in liquid form, which is only possible in the so-called ‘habitable zone’ around a star where the temperature is just right. The TRAPPIST-1 system – relatively close to Earth at just 39 light years away – is remarkable in that three or four of its seven known planets appear to lie in the habitable zone.

1 PLANET DENSITIES

The average densities of all the TRAPPIST-1 planets are similar to each other, but about eight per cent lower than Earth’s.

2 UNIFORM COMPOSITION

We only know the average density, not the internal structure. The planets may be a uniform mix of material throughout.

3 MOON-LIKE STRUCTURE

The planets may have a relatively simple layered structure, like Earth’s Moon.

4 SMALL IRON CORE

The core would have to be proportionately smaller than Earth’s to explain the lower average density.

5 OCEAN WORLD

The surface might be covered with a deep ocean of low-density water, allowing the core to be larger.