24 | New Scientist | 10 August 2019
I
N THE past 100 years,
astrophysicists have deduced
that space-time is expanding,
that this expansion is accelerating,
that the universe is about 14 billion
years old and that there are at least
4000 planets beyond those in our
solar system, called exoplanets.
When I wrote my junior
undergraduate thesis on exoplanet
atmospheres back in 2002, all
we knew was that simulations
suggested we should see sodium
in the atmospheres – and we did,
but we saw less of it than expected.
Today, our simulations are
more sophisticated and we have
moved beyond basic details
about atmospheres to thinking
through how to learn more
about exoplanet surfaces.
Increasingly, our research
on stars has become entangled
with research on exoplanets too.
In particular, we have become
very interested in a type of star
known as a red dwarf or M dwarf.
These are pretty fun because
they are the coolest, smallest
stars that exist on the main
sequence, which comprises all
of the hydrogen-burning stars
(as opposed to neutron stars
and white dwarfs, which are
just compact collections of
particles). M dwarfs are totally
different from our own sun, with
surface temperatures that are
often about half that of our star.
They also tend to be less than
half the mass of the sun, and
the smallest ones have masses
and radii that are less than
10 per cent of the sun’s.
Importantly, simulations
show that Earth-sized planets
with extensive oceans could be
abundant around M dwarfs. For
this reason, they have become
subjects of intense interest, but
observing them isn’t simple. They
are also known as red dwarfs for
a reason: unlike the sun, M dwarfsaren’t bright in the visible parts
of the spectrum.
To get a really good look
at them, we must use infrared,
the part of the light spectrum
on the red side that is just beyond
the capacity of the human eye to
see. Anyone who has seen action
heroes in films use night-vision
goggles is familiar with it.
Effectively, to study M dwarfs,
we must use telescopes that are
fitted with technology similar
to that of these goggles.
In many ways, astronomy
is a “wait and see” science – but
thanks to advances in physics
and centuries of informationgathering, we have become
extremely good at doing that
in an intelligent way. Now, we
specifically design instruments
that do targeted scans of the sky,
surveying with specific objects
in mind.
Part of our smart searching
involves building specific
instruments designed to
capture information about the
kinds of objects we want to see.
The Kepler space telescope spent
nine years surveying the sky, and
the information it collected was
photometric in nature, meaning
that it measured the brightness
of the stars in different parts of
the light spectrum. Scientists thensearched these measurements
for signs of periodic dimming –
evidence of a planet eclipsing the
star. More than 2000 confirmed
planets were discovered this way.
Today, Kepler is retired, but
the search from space goes on
with NASA’s Transiting Exoplanet
Survey Telescope and the
European Space Agency’s Gaia
observatory. The information
collected by these instruments
is made all the more interesting
by simulation work that is under
way. For example, last year
Aomawa Shields and Regina
Carns published a paper that
looked at the presence of sodium
chloride dihydrate (hydrohalite)
on the surfaces of M dwarf-
orbiting exoplanets.
Hydrohalite can condense into
sea ice at low temperatures. Unlike
salt-free ice, this ice can be highly
reflective in the infrared spectrum,
which is exactly where we are
already looking for M dwarfs. This
presents the challenge of telling
the difference between light
coming directly from the star and
starlight being reflected from the
planet’s surface, but it also gives us
an opportunity. Shields and Carns
found that this reflectivity also
enhanced the build-up of carbon
dioxide, which is a key ingredient
for planet habitability.
Of course, it is unclear
what it means for humanity
to find habitable planets. As I
mentioned in a previous column,
the possibility of long-distance
travel to other stars during a
single human lifetime is low.
At the same time, as we refine
data collection mechanisms, we
strengthen our ability to stumble
across indicators of life in far-flung
places. Perhaps, more than
knowing how old space-time is,
this is the kind of information that
could revolutionise humanity’s
M. WEISS/CFA relationship with the universe. ❚This column appears
monthly. Up next week:
Graham Lawton“ To study M dwarfs,
we must use
telescopes with
technology similar
to night-vision
goggles”Star spotting science Exoplanets are abundant near M dwarfs, the
galaxy’s smallest stars. Infrared technology may help us learn more
about these planets’ surfaces, writes Chanda Prescod-WeinsteinField notes from space-time
What I’m reading
A lot about space ethics
on the JustSpace Alliance
website.What I’m watching
I have been enjoying a
lot of Women’s National
Basketball Association
and National Women’s
Soccer League games.What I’m working on
I have been balancing a
project on neutrinos with
work to support Native
Hawaiian sovereignty.Chanda’s week
Chanda Prescod-Weinstein
is an assistant professor of
physics and astronomy, and
a core faculty member in
women’s studies at the
University of New Hampshire.
Her research in theoretical
physics focuses on cosmology,
neutron stars and particles
beyond the standard modelViews Columnist