66 BriefingAstrobiology The EconomistFebruary 13th 2021
21away. There is, then, plenty to study.
Just because a planet is a rocky world or-
biting within its star’s habitable zone does
not, though, automatically make it a good
candidate. Some such, for instance, are
known to be “water worlds”, with deep
oceans and no continents. These might
support life if it arrived from elsewhere,
but the chances that biology could start
from scratch in such a place seem slim.
How life begins is unknown. But it is a fair
bet that it needs the concentration of cer-
tain chemicals in a way that is difficult to
achieve in the swirling volume of an ocean.
Similarly, just because an object could
have liquid water on its surface in a habit-
able zone does not mean such water actual-
ly exists. Earth’s moon, for example, is
within the habitable zone of the sun and
yet is almost waterless.
An important task, then, is to under-
stand not only where a planet is in its star
system, but what it is made of—whether it
has an all-covering ocean, an ocean inter-
spersed with continents, a covering of ice,
or is all dry land. Also, whether it has an at-
mosphere. And that will be done by a new
generation of instruments.Chart toppers
The first of these, the James Webb Space
Telescope, is the creation of nasa, Ameri-
ca’s space agency. It has a mirror 6.5 metres
across, which gives it a light-collecting
area more than six times greater than that
of the Hubble Space Telescope, currently in
orbit, which has a mirror a mere 2.4 metres
in diameter. The plan is to launch it into or-
bit this October and to use it for detailed
spectrographic observations of exoplane-
tary atmospheres. It will hunt, in particu-
lar, for molecules such as oxygen and
methane that are produced by biological
processes on Earth.
Teegarden b is a prime candidate for
such study, but is by no means the only
one. Potentially habitable planets within
telescope range are turning up thick and
fast. In 2020 alone, four new Earth-size
exoplanets entered the hectop ten.
In January of that year, for example, as-
tronomers using another nasa instru-
ment, the Transiting Exoplanet Survey Sat-
ellite (tess),announced the discovery of
that probe’s first entry into the hec. This
planet, toi-700d, is one of three orbiting a
star called toi-700, which has 40% of the
mass of the sun and is 100 light-years away.
toi-700d is 20% bigger than Earth, com-
pletes an orbit every 37 days and receives
86% of the energy from its star that Earth
gets from the sun. It currently sits in sec-
ond place on the heclist. And in March two
further top-tenners were added by astrono-
mers using the European Southern Obser-
vatory’s instruments at La Silla, in Chile.
They were orbiting in the habitable zone of
a star called gj1061, which, like Teegar-den’s, is 12 light-years from Earth.
The fourth of 2020’s additions to the
hectop ten was announced in April. It was
the result of reanalysis of measurements
taken by Kepler, a now-defunct nasaspace
telescope (it operated from 2009 to 2018)
that, in its heyday, found more than 2,000
new planets. This re-examination uncov-
ered a hitherto-overlooked body, dubbed
Kepler-1649c, which is almost the same
size as Earth, takes 19.5 days to orbit its star,
and receives 75% of the light that Earth gets
from the sun. Kepler-1649c jumped straight
into the charts at number five.
Other top-tenners have been on the list
for longer. One of particular interest is
Proxima Centauri b (number six). It weighs
in at 1.3 times Earth’s mass and has an 11-day
orbit around its star, Proxima Centauri,
which, at a mere 4.2 light-years away, is the
sun’s closest stellar neighbour. But the
most striking star to supply heccandi-
dates is trappist-1. This is 41 light-years
from Earth and is orbited by seven poten-
tially habitable planets of similar density
to Earth, one of which, trappist-1d, is in
the hectop ten, at number four.
At its most recent update, the heclisted
60 worlds, some two dozen of which are
thought to be of rocky composition and
similar in size to Earth. The rest are so-
called “super Earths”, which have masses
larger than Earth’s but less than those of the
solar system’s ice giants, Uranus and Nep-
tune. There are doubts about the habitabil-
ity of super Earths, since they probably
have thick atmospheres and may even be
composed almost entirely of gas. For the
moment, though, it seems sensible to keep
them under scrutiny.
Because of the limits of current observ-
ing technology, which finds it easier to see
signs of planets when they circle dimmer
stars, almost all these promising exopla-
nets orbit what are known as m-type
stars—or, colloquially, red dwarfs. Red
dwarfs are smaller and dimmer than f-, g-
and k-types, known as orange and yellowdwarfs (the sun is g-type). They are by far
the most common stars in the Milky Way
(some estimates suggest they make up
three-quarters of the total), so the easy suc-
cess in finding so many potentially habit-
able worlds circling them suggests that
these sorts of planets are abundant.Atmospheric conditions
Astrobiologists are already making simple
follow-up measurements of some planets.
In 2015 a super Earth called k2-18b turned
up. It is 124 light-years away, has nine times
Earth’s mass and orbits its m-type star once
every 33 days. In 2019 the Hubble Space
Telescope looked at starlight streaming
through this planet’s atmosphere. Spectro-
scopic analysis indicated the presence
there of a fair amount of water vapour—a
first for an exoplanet in a habitable zone.
That was a useful start, says Giovanna
Tinetti, an astrophysicist at University Col-
lege, London who led the study. But, as she
observes, “current instrumentation is just
not good enough really to go beyond say-
ing, ‘Oh, there is some water vapour in the
atmosphere’.” That leaves plenty of ques-
tions. What type of object is k2-18b? Is it a
world covered by an ocean, or perhaps a
thick layer of ice like the icy moons of Jupi-
ter and Saturn? Is the atmosphere pure wa-
ter vapour or mostly hydrogen with a dash
of water and perhaps some other elements?
Future observations with the James
Webb telescope will fill in some of these
gaps. However, Dr Tinetti highlights the
need for catalogues of the properties of ex-
oplanetary atmospheres, against which as-
tronomers can compare their latest finds.
To this end she is working on ariel, a mis-
sion planned by the European Space Agen-
cy (esa) to characterise the properties of a
set of around 1,000 diverse exoplanets.
Once a potentially habitable planet is
found, life on Earth provides clues about
how to detect life on it. For example, as
James Lovelock, a British chemist, pro-
posed in 1965, the presence of gases inHabitable-zoneexoplanetsSources:AbelMendez;“Habitablezonesaroundmain-sequencestars”,byR.K.Kopparapuetal.,AstrophysicalJournalLetters0 20 40 60 80 100 120 140 160 180 200 2203456Startemperature,
K, ’000StartypeFGKM Size=radius
ExcludingJovianplanetsEnergyreceivedbyplanet,%ofEarth’sLess likely to
be habitableMars Earth Venus
Not
habitableHabitableToo cold Conservative habitable zone Optimistic habitable zone Too hotTeegarden b
GJ 1061c
Trappist-1dTOI-700dKepler-1649c