SRON_Spectrum_2016

indicate that a planet is moving in front of
the star.
Many exoplanets have already been discov -
ered using this transit method, for example
by the space telescope Kepler. However, the
PLATO mission will go a step further. “Kepler
can only map a very small piece of the sky.
PLATO will examine almost the entire sky so
that all bright stars can be studied at the
same time,” says Min. “Furthermore PLATO
will not just measure for a few months but
for several years. That will make it possible
to find exoplanets that take longer to orbit
around their star. Such as the earth which
takes one year to orbit the sun.”
SRON is expected to contribute to the PLATO
mission by providing test facilities. Min: “The
34 telescope cameras must all be tested at
extremely low temperatures for factors such
as stability and precision. We want to carry
out some of these tests at the institute in
Groningen.”
Eventually astronomers will collate all of the
values measured in a catalogue of thousands
of exoplanets. They will then be able to use
this to determine which exoplanets merit fur -
ther research. The next step will be to es tab -
lish the exact composition of these plan ets’
atmospheres. The ARIEL mission (Atmo spheric
Remote-Sensing Infrared Exoplanet Large
sur vey) has been proposed for that. In the
first half of 2017 the ESA will have to make a
choice between this and two other proposed
missions. If the ARIEL mission is selected then
it will be started in about 2025.
Hot Jupiters and super earths
“I think the time is ripe for a mission like
ARIEL,” says Min. “There are already a lot of
missions to identify exoplanets but few of the
planets found are being characterised. This
requires telescopes that can remain stable
over a long period of time like the James
Webb Space Telescope that will become oper -
ational in 2018. However exoplanets will be
Proposal for the ARIEL telescope (ESA).
just one of the things it observes. Hopefully
ARIEL will get the go-ahead and observe only
exoplanets.”
In the case of the ARIEL mission a telescope
onboard a space probe will carry out mea -
surements on about 500 exoplanets over a
period of 3.5 years. Those will mainly be the
There are many missions
to identify exoplanets
but few exoplanets are
being characterised
‘hot Jupiters’ and ‘super earths’: large planets
with a temperature of several hundred
degrees Celsius. “Only such planets have
atmospheres thick enough to be measured,”
says Min. If the ARIEL mission goes ahead
and it uses European detectors then engineers
from SRON will contribute by developing the
readout electronics. SRON scientists are also
eagerly looking forward to the moment that
the results from this and the PLATO mission
can be analysed. Min: “We still know little
about the cloud formation on exoplanets.
In addition, the measurements will give insight
into the formation of planets. More data
are needed to be able to statistically demonstrate
where and how planets are formed in
general.”
Extraterrestrial life
A better understanding of the formation and
evolution of planets should also lead to an
answer to the ultimate question concerning
extraterrestrial life. According to Min, how -
ever, the PLATO and ARIEL missions will get
nowhere near to answering these questions.
“We cannot characterise planets as small as
earth yet. We can characterise super earths
but only if they are located quite close to the
star. As a result of that they are too warm to
support life.”
Min is cautiously optimistic about the chance
that we are not alone. “Up until now everything
that is physically possible also occurs in
the universe. Billions of earth-like planets are
therefore expected to exist. Then we need to
ask: Is the chance of life evolving on such a
planet greater than one in a billion? We don’t
know and we are currently doing the statistics
on a sample of one. That number needs to be
bigger.”
The question about life will therefore remain
unanswered for the time being. Hopefully
future missions will be able to demonstrate
whether our solar system is commonplace or
completely unique.
YANNICK FRITSCHY
11
SRON Spectrum