Sky & Telescope - June 2022 - 8

NEWS NOTES
for such stellar activity in order to find
Proxima Cenaturi d, as announced in
the February Astronomy & Astrophysics.
The candidate planet wobbles its
star by 40 cm/s, a detection that is
possible because of a new instrument:
the ESPRESSO spectrograph on the
Very Large Telescope (VLT) in Chile. In
addition to taking incredibly high-resolution
radial velocity measurements,
the instrument also detects emission
lines associated with stellar activity.
Faria's team found that none of these
indicators had the same five-day signal
as the planet. However, the team still
labels the discovery a candidate for now.
" There is always a chance that we were
fooled by the star, " Faria says.
The candidate, which is at least twice
EXOPLANETS
Third Candidate Planet Around Proxima Centauri
A NEW INSTRUMENT on a powerful
telescope has enabled astronomers
to discover another planet around
our nearest stellar neighbor, Proxima
Centauri. The find brings the star's
exoplanet tally to three candidates (one
of them confirmed).
The newest of the worlds, Proxima
Centauri d, would make an unpleasantly
hot place to live, orbiting its faint
red star every five days. But the method
by which the astronomers detected the
planet, by measuring the minute wobble
of its host star, might well lead to the
discovery of more habitable worlds.
pAn artist's concept shows Proxima Centauri
d, the third candidate planet orbiting our nearest
stellar neighbor.
More massive and/or closer-in planets
exert greater gravitational influence
on their stars, creating stronger radial
velocity signals. Earth, on the other
hand, induces a motion of only 10 cm/s
on the Sun. To find other Earths,
improving instrument precision is only
the first step, as activity on stars' boiling
surfaces can produce radial-velocity
" noise " of meters per second.
João Faria (University of Porto,
Portugal) and colleagues accounted
the mass of Mars, orbits Proxima Centauri
eight times closer than Mercury
circles the Sun. Since the star radiates
only 0.2% of the Sun's luminosity, the
planet wouldn't be quite as scorched
as Mercury is. Nevertheless, a rough
estimate of the surface's equilibrium
temperature is 360K (190°F), close to
water's boiling point.
Guillem Anglada-Escudé (Institute of
Space Sciences, Spain), who discovered
the first confirmed planet around this
system, Proxima Centauri b (S&T: Dec.
2016, p. 10), says the discovery indicates
that Earth-mass planets on somewhat
longer orbits (tens of days) are well
within ESPRESSO's reach.
¢MONICA YOUNG
SOLAR SYSTEM
Meteorite Evidence: Earth Was Born with Its Water
ASTRONOMERS LONG THOUGHT
that the terrestrial planets formed too
close to the newborn Sun to harbor any
water. So the inner planets must have
formed dry, their water instead delivered
later on by comets and/or asteroids
from the outer solar system. But results
over recent years have shown that the
chemistry of outer solar system objects
- in particular, the isotopic composition
of comets - don't match that of
Earth's oceans.
8
JUNE 2022 * SKY & TELESCOPE
The difference lies in water's hydrogen
atoms, some small fraction of
which are deuterium, which contains
an extra neutron. Lighter hydrogen is
more likely to react or be stripped away
by radiation, while heavier deuterium
tends to stay put. The deuterium-tohydrogen
ratio (D/H) thus provides a
clue to an object's chemical history.
Jérôme Aléon (National Museum of
Natural History, France) and colleagues
pondered this conundrum while analyzing
the Efremovka meteorite found
in Kazakhstan in 1962. It contains
several calcium-aluminum-rich inclusions
(CAIs), rocks that long ago trapped and
preserved minerals from the hot solar
nebula out of which the planets formed.
Those minerals' D/H ratios, Aléon and
colleagues reported February 3rd in
Nature Astronomy, give us a glimpse of
that past environment.
To their surprise, Aléon and his team
found that the minerals trapped within
the meteorite had two distinct D/H
ratios. Some components had almost no
deuterium, having interacted only with
ESO / L. CALÇADA
Sky & Telescope - June 2022

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