TheEconomistFebruary 13th 2021 651T
eegarden’s staris a tiny, dim object in
the zodiacal constellation of Aries. It
has a tenth of the sun’s mass and emits
most of its light in the infrared part of the
spectrum. That makes it too faint to see
with the naked eye, even though it is only 12
light-years away. So far, so unremarkable.
But when astronomers at Calar Alto Obser-
vatory, in Spain, started scrutinising it,
they spotted tiny wobbles in its motion. In
2019, after three years of careful measure-
ment, they concluded that these are a con-
sequence of the gravitational fields of two
planets tugging the star around. The inner-
most, Teegarden b, has roughly the same
mass as Earth, receives a similar amount of
illumination from its host star and is prob-
ably rocky.
At the moment, Teegarden b tops the
Habitable Exoplanets Catalogue (hec), a
list of planets beyond the solar system
maintained by Abel Méndez and his col-
leagues at the University of Puerto Rico,
Arecibo. They are looking for Earthlike
worlds which, among other things, lie in
the zones around stars that might support
terrestrial-type life. This means, in prac-
tice, planets that are the correct distance
from their parent stars to be able to main-
tain liquid water on their surfaces without
that water either freezing or boiling. For
water, the medium in which biochemical
reactions take place on Earth, is assumed to
be a precondition for the existence of life
elsewhere, too.Warm bodies
The existence of the hec, compilation of
which began in 2011, is one manifestation
of a renewed surge of interest in answer-
ing, once and for all, the question of wheth-
er life is a phenomenon that exists only on
Earth, or is widespread. Until recently, that
question was unanswerable. But astrobiol-
ogists, as the diverse band of scientists
now trying to do so describe themselves,
are increasingly confident that an answer
will be found within a couple of decades.
There are, broadly, three ways of doing
this. One is to look from a distance, usingtelescopes to examine systems such as that
orbiting Teegarden’s star. The second, if the
object of interest is close enough, is to visit
it, as is happening with the arrival this
month of Mars-bound craft launched by
America, China and the United Arab Emir-
ates. And the third is to search for radio sig-
nals or other signs of technology, on the as-
sumption that at least some life elsewhere
has followed the trajectory of life on Earth
and generated technically adept species.
The first two exoplanets were discov-
ered in 1992. Now, more than 4,000 are
known. It seems likely that every one of the
billions of stars in the Milky Way, Earth’s
home galaxy, is the centre of a system that
includes one or more planets, and also that
lots of these planets orbit in habitable
zones. Because they are big, and therefore
easy to spot, a lot of the planets discovered
so far are gas giants larger even than Jupi-
ter. These seem unlikely places for life to
establish itself. But an increasing number
of bodies close in size to Earth are turning
up, too (see chart on following page).
A study published recently in the Astro-
nomical Journalsuggests that about half of
the sun-like stars in the Milky Way are cir-
cled by at least one rocky planet capable of
sustaining liquid water on its surface. This
amounts to 300m potentially habitable
worlds. That calculation implies the pres-
ence of at least four such favoured orbs
within 30 light-years of the solar system,
with the closest at most 20 light-yearsCome out, come out, wherever you are!
If life exists beyond Earth, science may find it soon
Briefing Astrobiology