The search for exoplanets
C.Baumgartner
There is a young and rapidly growing field in astrophysics: The search for extrasolar planets or as they
are also called “exoplanets”. There are multi-million dollar projects funded and astronomers worldwide spend hours and hours looking at the sky and collected data from stars in order to find new
planets outside our Solar System. But what makes these gigantic compounds of rock and gases that
interesting, which are floating in space so far away, that we can’t even observe them directly? Why
are scientists so eager to constantly look for new planets and what kind of methods are used to find
them? Here I discuss the short history of the search for exoplanets starting a few decades ago and the
reasons behind it.(𝟏) There will be a brief introduction to popular techniques for finding new planets
like the “Radial Velocity”-, “Pulsar Timing”- and the “Transit Photometry” method. (𝟐) Finally we talk
about important projects that supported this enterprise. (𝟏) Until today those projects have found
over 4600 candidates for exoplanets, which include 1700 confirmed planets and those numbers are
constantly growing day by day.(𝟑) Studying exoplanets will yield us precious information about
planetary and Solar System development and maybe one day we will find a habitable planet even with
extraterrestrial life.
Since human mankind existed on earth, there
was always the fascination for the dark night sky
and its glimmering stars. That led inevitably to
the research of this subject and nowadays we
estimate that there are something like 1011 to
1012 stars in our galaxy and even 1022 to 1024
stars in our universe.(4) A mindboggling
number, that isn’t easier to imagine, if you think
about the fact, that there are more stars in our
universe than there are grains of sand on all
beaches on earth. Now if someone thinks about
our Solar System and how it developed, the
question has to arise, is our Solar System an
exception or the common state in our universe?
If there are other planets it would be also
interesting to know, if there are other life-forms
living on them. Therefore astronomers were
always eager to find planets outside our Solar
System, the so called extrasolar planets or
exoplanets. But aside from curiosity there are
two other important reasons to look for
exoplanets. First of all the sun, our life-giving
energy source, has a day of expiry. She’s now
about 4.5 billion years old and has therefore
lived half of her predicted lifetime. Secondly
human history showed, that humans aren’t
treating their planet well. There are acts of war
(e.g. atom bombs on Hiroshima and Nagasaki)
and catastrophic pollution (e.g. oil tanker
accidents, Chernobyl and Fukushima disaster)
that made whole landscapes uninhabitable.
With those reasons nobody knows exactly when
our planet Earth will become uninhabitable, but
there is a need to find another habitable planet,
if human mankind wants to survive. Out of all
those reasons scientists tried to find exoplanets,
but for a long time it was impossible to observe
planets outside our Solar System, because of the
lack of suitable devices. But with technology
improving at an extremely fast pace in the past
decades, it was possible to develop methods to
search for exoplanets. At the beginning this field
of study didn’t receive much attention due to
the fact, that many researcher didn’t think it
would be possible to find exoplanets with
available means at that time. But after the first
few confirmed extra-solar planets were found,
the field started to flourish and many multimillion dollar projects were funded to find
more. In this paper I discuss the history of the
search for exoplanets and explain some of the
methods used to find them. Finally I present
some of the projects that were funded.
Since the 19th century many scientists claimed
to have found new planets outside our solar
system. First reports occurred in 1855 by
Captain W.S. Jacob from the East Indian
Observatory in Madras. He thought, that he had
found signs of an exoplanet around the doublestar 70 Ophiuchi. But like many others to follow,
it could not be proven. In 1963 Peter von de
Kamp, director of the Sproul-observatory at the
Swarthmore College in Philadelphia published
papers about a new found planet around the so
called Barnards-Star, but after further research
it was assumed that there was a failure in
measurements. Starting in the 1980s Gordon
Walker of the University of British Columbia
observed 29 stars similar to our sun at the
Canada France Hawaii Telescope (CFHT) for
about 12 years. Due to only 6 nights per year
observation-time it was no surprise, that they
weren’t able to find any signs of extrasolar
planets. It’s important to note that in 1988 they
found signs of an exoplanet in the double-star
system Gamma Cephei. Because of many
sceptics and their own uncertainty, they
withdraw their claim on having found the first
exoplanet. Other researches confirmed their
discovery in 2003. Later on another problem
became apparent. Theorists proclaimed, that
our solar-system with high density solid planets
on the inner orbits and the low density gas
giants further away from the sun should be the
common state in our universe. That’s also a
reason, why at first astronomers looked for
similar behavior of other solar-systems and
therefore weren’t especially successful.
Things started to change in 1992. Aleksander
Wolszczan and Dale Frail from the Pennsylvania
State University found the first confirmed
exoplanet around pulsar PSR B1257+12 in the
constellation Virgo. Not quite what they were
looking for, because a planet next to a pulsar,
which constantly emits deadly radiation doesn’t
support life. But it was the first success of
finding a planet outside our solar system. In the
following year other planets around pulsars
were discovered.
The discovery of exoplanets around ‘living’ stars
started in 1990. The Swiss researcher Michel
Mayor from the University of Geneva worked on
a high-precision spectrometry system called
ÈLODIE. After its completion it was installed at
the Observatory de Haute Provence. He
examined about 142 stars, and on the 5th of July
in 1995 he was able to find the first exoplanet
around the star 51 Pegasi. It has half of the mass
of Jupiter and circuits his star in 4.2 days. From
then on the discovery of many more planets
followed, but none of them was earth-sized and
orbiting his star in its habitable zone.
On April 17th 2014 NASA’s Kepler-telescope
discovered the first Earth-size planet Kepler186f in a habitable zone, which is also
accompanied by four other planets. It orbits its
star (a red dwarf) once every 130 days and
receives one-third of the energy that the earth
gets from the sun. Not much is known about its
mass or composition, but it is thought that is
very likely to be rocky.
Until today over 4600 candidates exoplanets are
found, which include 1700 confirmed planets
and those numbers are constantly growing day
by day.(3)
After all that history the question arises, how
astronomers look for new exoplanets and which
methods they are using. Because exoplanets are
extremely faint, all methods are indirect. That
means, that not the planets themselves are
observed but their influence on the light
emitted by the star it orbits. To improve
research in that field, they needed to develop
special techniques and auxiliary means that
improved the researchers’ perception. There
was also a demand for a device that helps
collecting and evaluating large amounts of data.
The first special techniques were developed in
the 1950 but the necessary auxiliary devices
weren’t available until the 1990s. Also the
computer played a special role in collecting and
evaluating data.
Concerning devices, a giant step in improving
precision was achieved, when “Adaptive Optics”
were invented. They were used to reduce
atmospheric wave-front distortions of the
observed light. This happens with the help of
sensors measuring atmospheric turbulences and
then sending correction-signals to deformable
mirrors in telecopes. Although the idea was
invented by Horace W. Babcock in 1953 it took
until the 1990s that “Adaptive Optics” found
their way into civil observatories.
Another important invention were CCDs (Charge
Couple Devices) in 1969. Those image sensors
were able to convert incoming photons into
electron charges, which could be used to
directly save the observed image and to get
more accurate data.
One of the first methods, the so called ‘Radial
Velocity’-method, was invented by Otto von
Struve in the 1950s.(5) According to theory
planets orbiting a star have gravitational
influence on the stars movement. By observing
the emitted light of the star and taking the
Doppler shift into consideration the amplitude K
of its radial velocity variations can be measured:
1
𝐾=
2 𝜋 𝐺 3 𝑀𝑝 sin 𝑖
1
(𝑃 )
2
2
√1−𝑒
𝑜𝑟𝑏
(𝑀𝑠 +𝑀𝑝 )3
(Eq.1)
where 𝑃𝑜𝑟𝑏 is the orbital period, 𝑖 the angle
between the normal to the orbital plane and the
line of sight, and e is the orbit’s eccentricity. This
is the most successful version developed until
today. But a downside of this technique is that it
is most sensitive to massive planets and planets
in short-period orbits. Precise measurements
also require a large amount of spectral lines,
which are not provided by hot stars (spectral
class A, B, O).
The first confirmed detections of extrasolar
planets were provided by ‘Pulsar Timing’.(6) The
first pulsar PSR B1919+21 was observed on
November 28 in 1967, and with this observation
the idea to use their periodic emitted radio
waves in order to detect exoplanets evolved.
Those emissions are in most cases extremely
periodic but may vary in measurements, if the
distance between pulsar and telescope changes
in a non-linear fashion. Such variations are
caused by the Earth’s motion around the Sun
and Earth’s rotation. The influence of this
motion can be calculated and subtracted from
the data. If there are still periodic variations in
the reduced data, that might be an indication
for exoplanets around this pulsar.
Another popular technique is ‘Transit
Photometry’. If a planet lies on approximately
the same orbital plane like the earth does to the
sun, the transit of that planet can be observed
from Earth. This periodic transits cause
variations in the star’s brightness. If we are
neglecting variations of brightness across the
stellar disk, the fractional decrease in luminosity
L is given by:
𝛥𝐿
𝐿
𝑟𝑃 2
= ( )
𝑟
𝑆
(Eq.2)
One big advantage is, that planets found with
the ‘Transit Photometry’ method are observable
via ‘Radial Velocity’ as well. With mass and
radius a density can be calculated. Another
important fact is, if the planet has some kind of
atmosphere, it absorbs parts of the light
spectrum of its star. Therefore conclusions on
the composition of the exoplanets atmosphere
can be drawn.
The technique of ‘Transit Photometry’ is also
the predominant method used in different
space projects. The first mission designed to
search exclusively for exoplanets by using this
method was the CoRoT-Telescope launched on
December 27 in 2006 as a cooperation of the
European (ESA) and the French (CNES) space
associations. CoRoT is an abbreviation for
‘Convection Rotation and planetary transits’. It
observed 12 000 stars simultaneously and
searched in the constellations Monoceros
(unicorn), Aquila (eagle), Scutum (shield) and
Serpens (snake). In spring 2007 it found its first
exoplanet, named CoRoT – 1b. It is 1500 lightyears away, has 1.3 times the mass of Jupiter
and orbits its sun in 1.5 days. The telescope was
supposed to work until March 2013, but due to
a computer failure the program had to be shut
down in November 2012. During its lifetime
CoRoT discovered 15 planets, which was less
than expected.
Another space program was launched by the
NASA on March 7, 2009. It is called the KeplerTelescope.(7) It also searches exclusively for
extrasolar planets with the transit method. It’s
monitoring about 100.000 stars in the
constellation Cygnus (swan). Special about
Kepler is, that it isn’t in an orbit around earth
but in a heliocentric one to get rid of earth’s
influence on observations. Until today Kepler
has found about 410 planets and on April 27th
2014 it found the first Earth-like planet in a
habitable zone.(8)
As a conclusion the search for exoplanets is a
young field of astronomy that still has plenty to
learn. Before the discovery of extrasolar planets,
models of planetary growth suggested that
most single solar-type stars possess planetary
systems that are grossly similar to our Solar
System. But after many observations nature
proofed us wrong and since exoplanets are so
far away and appear very faintly next to their
stars, there will always be the need to improve
our technological devices in order to be able to
spot more of them. With the discovery of every
new planet we learn more about the
development of solar systems throughout the
universe and maybe someday we find a planet
other than our planet Earth that supports live.
Sources:
(1) S.Piper, Exoplaneten: Die Suche nach einer
neuen Erde (Springer-Verlag, Berlin Heidelberg,
2014)
(2) J.J.Lissauer, I.Pater, Fundamental Planetary
Science (Cambridge University Press, New York,
2013)
(3)
NASA
Jet
Propulsion
Laboratory,
http://planetquest.jpl.nasa.gov/news/158
(29.04.2014)
(4) J.Bennet et al, Astronomie: Die kosmische
Perspektive (Pearson Studium, Munich, 2010)
(5) O.Struve, The Observatory 72, pp. 199-200
(1952)
(6) J.M.Cordes, ASP Conference Series 36, pp.4260 (1993)
(7) D.G.Koch et al, The Astrophysical Journal
Letters 713, L79 (2010)
(8) E.V.Quintana et al., Science 344, pp.277-280
(2014)
Da aktuelle Daten hierzu schwer zu finden sind, habe ich bei (3) als Quelle eine Internetseite verwendet,
da ich annehme, dass die NASA als Quelle vertrauenswürdig genug erscheint (auch wenn man im
Normalfall auf Internetquellen verzichten sollte). Ich habe im Folgenden zu findende Kopien der
entsprechenden Seiten gemacht und das Datum notiert, an dem ich die Informationen dort entnommen
habe, damit man im Notfall die Quelle nachverfolgen könnte.
(3) Nasa Jet Propulsion Laboratory, http://planetquest.jpl.nasa.gov/news/158 (29.04.2014)
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