40 AUSTRALIAN SKY & TELESCOPE July 2017
LEAH TISCIONE /S&T, SOURCE: ESO / O. FURTAKAre these worlds habitable?
TRAPPIST-1 is quiet for anMdwarf — notably less active
that Proxima Centauri, which also has a planet. It’s what’s
called anultracool dwarf, a common type of star that lives for
trillions of years.
Unfortunately, astronomers don’t know how old the star
is.Theyknowit’satleast500millionyearsold,thetime
neededto‘settle’intobeinganadult.TheratioofitsX-ray
to ultraviolet emission, which changes with age, suggests it
is “relatively young,” Vincent Bourrier (University of Geneva
Observatory, Switzerland) and colleagues posit in the March
2017 issue ofAstronomy & Astrophysics.What“relatively
young” means is an open question.
Jeffrey Linsky (University of Colorado, Boulder), who
has worked extensively onMdwarfs, says that TRAPPIST-1
seemsbotholdandyoung.Starsarebornspinningquickly,
then slow as they age. This star whirls around every 1½ days,
whichatfacevaluewouldpointtoitbeingyoung,hesays—
but astronomers don’t know how fast ultracool dwarfs spin
down. Conversely, the star’s fast motion through space usually
would indicate it’s a member of the old stellar population that
comprisesthegalaxy’shalo,butmaybethat’safluke.
Bourrier’s team also speculated on how TRAPPIST-1’s X-ray
and ultraviolet levels would matter for habitability. Although
thestar’sultravioletoutputislessthanhalfthatfromother
cool, exoplanet-hostingMdwarfs, the radiation overall is
stillhighenoughthatitcouldstripanEarth-likeatmosphere
fromtheinnertwoplanets,bandc,in1to3billionyears.
Fortheplanetsd,e,fandg(thelatterthreeareinthestar’s
habitablezone),theprocesscouldtakeanywherefrom5to22
billion years. The team does see a hint of atmospheric escape
frombandc,basedondipsinstarlightaroundthetimeof
their transits, but this effect might instead be due to coronal
variabilityinthestar.
Thenextgoalistolookforatmospheres.Ifanyofthese
worlds hosts life, then it might leave chemical fingerprints
in the atmosphere. No single compound is a smoking gun —
forexample,oxygencancomefromphotosynthesisorfrom
water molecules broken up by starlight into their constituent
hydrogen and oxygen. But certain combinations of chemical
compounds (such as methane, carbon dioxide and molecular
oxygen) would be highly suggestive.
Gillon’s team hopes to use the Hubble Space Telescope
to look at the starlight passing through the planets’ (maybeextant) atmospheres as they transit, to detect any compounds
that might have absorbed light. Follow-up will come with the
James Webb Space Telescope, more apt for this project because
itfocusesoninfraredwavelengths,inwhichTRAPPIST-1puts
out most of its light. Meanwhile, NASA has released three
months of observations by the Kepler space telescope and
invited astronomers to dive into the data.
Co-discovererAmauryTriaud(InstituteofAstronomy,UK)
favours planet f as the most promising for life. With a girth
of 1.05 Earths and about 60% Earth’s density, TRAPPIST-1f
might be rich in liquid water or ice. It receives about as much
energyfromitsstarasMarsdoesfromtheSun,andwitha
good atmosphere it could be habitable. (Mars is technically in
the Sun’s habitable zone.)
During a February 21st press briefing, Triaud painted this
picture of what we might see from one of these worlds:
The amount of light reaching your eye would be something
like ½ 00 as much as you receive from the Sun on Earth —
similar to what you experience at the end of sunset. However,
it’d still be quite warm, because there’s still about the same
amount of energy reaching you from the star as Earth
receives from the Sun — it’s just that most of that comes in
infrared, which you can’t see but your skin can feel. The star
would be a salmon-like colour. Viewed from TRAPPIST-1f, he
estimates, the star would appear three times wider in the sky
than the Sun is to us.
“The spectacle would be beautiful,” he said.Science EditorCAMILLE M. CARLISLE’s favourite planet is
the remarkable world called Earth.Orbital separation (astronomical unit)0.00 0.02 0.04 0.06fhgbecdSHABITABLE? This diagram shows the relative sizes of the orbits of the
seven planets circling TRAPPIST-1. The shaded area is the habitable zone.
Although drawn here, planet h’s orbit is not well known. The dotted lines
show alternative limits to the habitable zone based on different theoretical
assumptions. Note that a more optimistic definition includes planet d.TEMPERATE VS. HABITABLE
All of TRAPPIST-1’s planets lie in what the discoverers
call the ‘temperate zone’ — orbits with enough incoming
starlight that, with the right conditions, the planets might
at least sometimes have liquid surface water. It’s a
looser definition than that for the more commonly used
‘habitable zone’.EXOPLANET NEWS