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Once the planet’s orbital period and duration of the
transit event have been determined, the orbital radius can be
calculated. The transit data also helps to determine the inclina-
tion of the orbital plane, by recording the transit duration and
times at which the planet ‘enters’ and ‘exits’ the stellar disc,
known as ingress and egress times. If the planet does not pass
across the centre of the disc, the transit time will be shorter
when compared to a planet that does pass through the centre
of the disc. Thus, the inclination of the orbit can be calculated
using the shape of the transit curve. The mass of the planet can
be estimated using the transit data and measurements of the
host star’s radial velocity, made using other telescopes, as it
moves in response to the gravitational pull of the orbiting
planets. Once estimates of the mass and the radius of the
exoplanet are available, the average density and surface gravity
can be derived. And this is where it gets interesting – the
density is a key parameter for enabling conclusions to be drawn
about the composition of the planet, such as whether it is
gaseous or rocky. The researchers are focusing on rocky planets
because their physical properties might make them suitable for
the existence of life as we know it. Indicators of life as it arose
on Earth could be, for example, an atmosphere, liquid water,
tectonic activity or a magnetic field.
To date, the above techniques have been used to deter-
mine the parameters of 292 extrasolar planets. Of these, 69
have been classified as ‘super-Earths’, as their mass does not
exceed 10 times that of Earth. Some of these might be rocky
planets with liquid water on their surface, providing conditions
suitable for life, as we know it. A prerequisite for the existence
of water is that the planet lies in the habitable zone, also
referred to as the ‘Goldilocks zone’. Such planets are neither
too near to nor too far from their parent star – outside this zone,
water will either evaporate or freeze. An appropriate atmos-
phere is also favourable.
Looking for the right candidate
Few super-Earths located within their planetary systems’
habitable zone have been found to date. The system around
Gliese 581 might be such a candidate. Gliese 581 is about 20
light years away, and is orbited by at least four planets, three of
which are known to be super-Earths. One of these, Gliese 581d,
could be located within the system’s Goldilocks zone. If its
atmosphere is sufficiently dense and contains high enough
concentrations of greenhouse gases, a planet meeting some of
the conditions for life, as we know it, will have been found. Due
to the inclination of its orbit, it is difficult to study its atmos-
phere and it is not possible to obtain a transit light curve. So,
unfortunately, the researchers have yet to determine whether it
could harbour life.
This means that the search for extrasolar planets remains
exciting and it must be hoped that the rate of new discoveries
continues to increase rapidly. The CoRoT mission was intended
to start its second extension in March 2013. The French space
agency CNES and its partners, including DLR, decided this in
October 2012. However, on 2 November 2012, CoRoT suffered
a computer failure that is preventing it from retrieving data from
its telescope. Attempts to work around this problem are contin-
uing, but if CoRoT is no longer able to return data from orbit,
the satellite will still have left behind plenty of work for the
team at the Institute of Planetary Research. The planet we are
looking for may already be among the thousands of candidates
– a rocky planet located in its system’s Goldilocks zone.
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More information:
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Transit light curve of the star CoRoT 3a. The drop in the brightness
of the star CoRoT 3a, produced by the transit of the planet CoRoT
3b, is clearly visible.
This diagram shows all the discovered exoplanets with a mass less
than 10 Earth masses (as of September 2012). The horizontal axis
shows the distance to the parent star and the vertical axis stands for
the mass of the parent star. The planets of our Solar System have
been incorporated for comparison. The yellow band indicates the
range of the habitable zone for the various spectral classes.
The exoplanet Corot-9b has an orbital period of 95 days. Unlike the
previously discovered planets, the distance to its star is relatively
large, and the temperatures are moderate.
Image: Instituto de Astrofísica de Canarias
In early 2009 researchers discovered the first rocky planet out-
side our Solar System with the European CoRoT space tele-
scope. CoRoT-7b was the first super-Earth with a known radius.
Image: CNES
The first 15 exoplanets found by CoRoT, and later confirmed.
Name
Mass
[M-Jupiter]
Radius
[R-Jupiter]
Period
[Days]
CoRoT-1b
1.03
1.49
1.50
CoRoT-2b
3.31
1.47
1.74
CoRoT-3b
21.66
1.01
4.26
CoRoT-4b
0.72
1.19
9.20
CoRoT-5b
0.467
1.39
4.04
CoRoT-6b
2.96
1.17
8.89
CoRoT-7b
0.023
0.15
0.85
CoRoT-7c
<0.0264
-
3.70
CoRoT-7d
0.052
-
9.02
CoRoT-8b
0.22
0.57
6.21
CoRoT-9b
0.84
1.05
95.27
CoRoT-10b 2.75
0.97
13.24
CoRoT-11b 2.33
1.43
2.99
CoRoT-12b 0.917
1.44
2.83
CoRoT-13b 1.308
0.885
4.03
CoRoT-14b 7.6
1.09
1.51
CoRoT-15b 60
0.8
3.0
CoRoT-16b 0.535
1.17
5.35
CoRoT-17b 2.45
1.02
3.77
CoRoT-18b 3.47
1.31
1.90
CoRoT-19b 1.11
1.45
3.90
CoRoT-20b 4.24
0.84
9.24
CoRoT-21b 2.53
1.30
2.72
CoRoT-22b <0.15
0.52
9.76
CoRoT-23b 2.8
1.05
3.63
CoRoT-24b <0.1
0.33
5.11
CoRoT-24c
0.13
0.44
11.76
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Planetary Research
Evidence of the second planet, CoRoT-7c, was provided by the transit
measurements of CoRoT-7b. The existence of the second planet was
confirmed by radial velocity measurements. However, scientists have
been unable to determine the radius from the data acquired. CoRoT-7c
is not transiting its parent star.
One Earth mass = 0.00314 Jupiter masses
One Jupiter mass = 317.83 Earth masses
Extrasolar planets
detected by CoRoT
(January 2013)
