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TABLE OF CONTENTS WHAT ARE EXTRASOLAR PLANETS?........................................................................................................3 WHY SEARCH FOR EXTRASOLAR PLANETS?..........................................................................................3 HOW DO ASTRONOMERS SEARCH FOR AND DETECT PLANETS?....................................................4 How to find exoplanets, and what we can learn about them............................................................................4 Basic Observational Techniques..................................................................................................................................4 Astrometry...................................................................................................................................................................5 Radial Velocity or Doppler Method...................................................................................................................5 Transit Method...........................................................................................................................................................5 Gravitational Microlensing ...................................................................................................................................6 Pulsar Timing..............................................................................................................................................................6 Eclipsing Binary.........................................................................................................................................................6 Circumstellar Disks ..................................................................................................................................................7 Coronagraphy .............................................................................................................................................................7 Space Based Technology..................................................................................................................................................7 Convection Rotation and Planetary Transit (COROT) ..............................................................................7 Microvariability and Oscillations of Stars (MOST) .....................................................................................8 Kepler Mission............................................................................................................................................................8 Space Based Technology – Future Missions...........................................................................................................9 PEGASE..........................................................................................................................................................................9 Space Interferometry Mission .............................................................................................................................9 New Worlds Mission................................................................................................................................................9 Terrestrial Planet Finder (TPF) ....................................................................................................................... 10 Darwin Mission ....................................................................................................................................................... 10 Gaia Mission ............................................................................................................................................................. 10 Planetary Transits and Oscillations of Stars (PLATO) ........................................................................... 10 Transiting Exoplanet Survey Satellite (TESS) ........................................................................................... 10 EXTRASOLAR PLANETARY SYSTEMS .................................................................................................... 11 Systems ................................................................................................................................................................................. 11 Properties ............................................................................................................................................................................ 11 Orbital characteristics.......................................................................................................................................... 11 Mass and Size........................................................................................................................................................... 12 Characteristics......................................................................................................................................................... 12 Atmospheres ............................................................................................................................................................ 12 How do discovered systems relate to our understanding based on our knowledge of our own system .................................................................................................................................................................................... 12 NEAR AND LONG TERM PROSPECTS FOR FINDING TERRESTRIAL­LIKE PLANETS................ 13 BIBLIOGRAPHY............................................................................................................................................. 14 www.astrochix.com 2
WHAT ARE EXTRASOLAR PLANETS?
1.
An extrasolar (or exoplanet) planet is a planet outside of our own solar system. As
of 31 May 2010, astronomers have made confirmed detections of 455 such planets
(ExtraSolarWeb1). Most planets discovered so far are gas giants, however noting that
the more massive planets have been easier to detect with current technology it’s
thought that this represents a sampling bias which in the future may mean that the
smaller planets discovered might be found to outnumber gas giants. It is now believed
that a substantial number of stars have planets, meaning that millions of them must
exist in our own galaxy, let alone those that exist outside of our galaxy.
WHY SEARCH FOR EXTRASOLAR PLANETS?
2.
For centuries, human beings have wondered ‘Are we alone?’ They have
speculated that other worlds must exist and that some would harbor other forms of life.
In our lifetime, advances in science and technology have enabled us to be able ponder
an answer to the question.
3.
Dominican Monk Giordano Bruno wrote, in 1584 there were countless suns and
countless earths all rotating around their suns. Although he was not the first to imagine
that other worlds existed his words are remembered. At the beginning of the 20th
century astronomer Edwin Hubble realised that the small fuzzy nebulae in his telescope
were in fact neighboring galaxies. Along with this discovery came the realization that
other galaxies had hundreds of millions of stars, and those stars may very well have
their own orbiting planets.
4.
In the mid 1980s researchers from the University of Arizona observed a disk of
dust around the star Beta Pictoris, the first evidence of a young planetary system in the
making. Despite a number of suggested observed planets, no claims were verified until
the 1990s. This delay was largely due to the inability of technology to observe the
planets, due to their small size against their host star and the enormous distance
involved.
5.
The first confirmed discovery was in 1994, when Dr. Alexander Wolszczan
reported that he had proof of extrasolar planetary systems. However, Wolszczan had
discovered two or three planet-sized objects orbiting a pulsar, rather than a normal star.
Observing regular variations in the pulsar’s rapidly pulsed radio signal, indicating the
planets’ complex gravitational effects on the dead star, made this discovery possible.
The origins of the pulsar planets are still under debate, but any planets orbiting a pulsar
could not support any sort of life, as we know it.
6.
The first discovery of a planet orbiting a star similar to the sun came in 1995, with
the announcement of a rapidly orbiting planet, located very close to the star 51 Pegasi.
The planet was at least half the mass of Jupiter, and was observed indirectly, using the
radial velocity method. Shortly afterwards two more planets were discovered. The end
of the 20th century dozens realised the discovery of many more planets.
7.
Since the first discoveries, there has been a dramatic increase in the discovery of
exoplanets. The sudden increase in discoveries is attributed to the following issues:
(NasaWeb2)
•
Significant improvements in spectrometers, instruments that separate starlight into
its component colors for analysis.
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Better electronic sensors that record the incoming starlight collected by telescope
optics.
The development of computer software that can reliably discern fluctuations in
starlight and the motion induced by the gravitational pull of unseen companions.
The maturation of these technologies has led to intensified searches and data
gathering.
•
•
•
HOW DO ASTRONOMERS SEARCH FOR AND DETECT PLANETS?
How to find exoplanets, and what we can learn about them
8.
Pointing a large telescope at stars and making observations might appear that the
most obvious way to find an exoplanet. However, this direct method of observation is
not generally possible for a number of reasons; stars are too far from us, they are bright
and obscure any planets that might be orbiting the star, the angular separation between
star and planet is very small and lastly the optical limit of telescopes (diffraction limit) is
such that the bright stellar images obscure any planetary images.
9.
Only the very largest ground based telescopes are able to observe exoplanets, but
to have any hope of observing a planet, it must be well separated from the star. As
direct methods are generally ineffective, other observational techniques must be used.
Of the 453 exoplanets discovered so far, the vast majority of planets were observed by
the radial velocity method and other indirect methods. The chart below compares the
success of the detection methods used to date.
Chart 0 Exoplanets discovered to March 2010 (data from www.exoplanet.eu and www.nasa.gov)
10. This chart shows that the number of planets discovered has increased each year,
and that the radial velocity and transit methods are clearly the most successful.
Basic Observational Techniques
11. Although detection can be difficult, there are a variety of technologies and
strategies that can make observation of exoplanets easier. Each has it’s own method of
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detection and strengths and weaknesses.
summarised below.
The main observational techniques are
Astrometry
12. Astrometry is the measurement of stellar positions on the celestial sphere. This
method consists of precisely measuring a star's position in the sky, and then making
observations of the movement of the star over time. If the star has an orbiting planet or
planets, then the gravitational influence of the objects will cause the star to move in a
tiny circular or elliptical orbit around the common center of mass. Finding Earth-mass
planets by astrometry requires sub-microarcsecond precision. Because the motion of
the star is so small, this method has not yet been very productive at detecting
exoplanets. ‘In the near term, astrometric accuracy from ground-based telescopes is
expected to improve to better than 100 µas on large telescopes as the various sources
of wavefront error are understood.’ (Lunine 2009) At that level of precision, groundbased astrometry will concern itself chiefly with finding giant planets and determining
their masses and orbits.
Radial Velocity or Doppler Method
13. As a star moves in its orbit around a system's center of mass, its velocity changes.
Variations in the star's radial velocity can be deduced from displacements in the star's
spectral lines due to the Doppler effect. If the motion is towards the observer the
received wavelengths are shorter than those emitted by the source, and longer if the
motion is away from the observer. ‘Whereas for the astrometric method the orientation
of the star’s orbit is immaterial, in the radial velocity method the orientation has to be
other than face-on, otherwise there is no variation in the radial velocity’. (Jones 2008)
Extremely small radial-velocity variations can be detected, down to roughly 1 m/s. The
Anglo-Australian Planet Search is a long-term program that searches for giant planets
‘around more than 240 nearby solar type stars’ (AngloAusWeb1) and to date has
discovered more than 20 exoplanets using the Doppler method. This has been by far
the most productive method of discovering exoplanets, with ‘90% of all the known
planets outside out solar system have been found by that technique.’ (Lunine 2009)
Transit Method
14. The transit method measures the faint dip in
brightness of a star when a planet transits the star. As the
exoplanet transits in front of its parent star, then the
observed brightness of the star drops by a small amount,
as shown in Figure 1. ‘In the past few years, the transit
method of planet detection has emerged as one of the
prevailing techniques to search for exoplanets. The tiny
amount of stellar light blocked by a planet when it
crosses (transits) in front of its host star provides
information about the size of the planet.’ (Pin-Gao 2010).
Figure 1: Transit Method
15. The amount by which the star dims depends on its size and on the size of the
planet. ‘When an exoplanet’s orbit is presented to us sufficiently close to edge-on, then
the planet will pass between us and its star.’ (Jones, 2008) A local example of this
phenomenon was the transit of Venus across the face of the Sun on 08 June 2004. This
has been the second most productive method of detection, though confirmation from
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another method is usually considered necessary.
16. Dips in apparent brightness can arise from events other than a planetary transit,
such as a grazing transit by a fainter companion star, however these events can be
identified from the shape of the light curve. Hans Deeg notes that ‘with transits we can
learn much more about the planets than with any other method to find planets…it’s the
only method currently where we can measure the size of the planet’s fairly reliably.’
(Matson 2010)
Gravitational Microlensing
17. Gravitational Microlensing happens when the gravitational field of a star acts like
a lens and magnifies the light of a background star. When the alignment is exact you
might think that the background star would be hidden from view, however the
gravitational field of the foreground star bends towards us the light from the
background star that passes near it. ‘So far there are 13 secure detections of planets by
microlensing. Eight have been published: five Jovian, two somewhat smaller than
Neptune, and one 3M⊕ planet.’ (Lunine 2009) This method has the advantage of being
very sensitive to planets at large angular separations from the parent stars. Another
advantage is that the intensity of the planetary deviation does not depend on the planet
mass as strongly as effects in other techniques do. This makes microlensing well suited
to finding low-mass planets. One major disadvantage is that the event can’t be
repeated, as the alignment is unlikely to occur again. Also the planets tend to be very
distant so the other methods are unable to confirm the observations.
Pulsar Timing
18. A pulsar emits radio waves extremely regularly as it rotates. Slight anomalies in
the timing of its observed radio pulses can be used to track changes in the pulsar’s
motion caused by the presence of planets. The presence of a planet orbiting a star
affects the timing of the regular signals emitted by the star itself. This phenomenon can
be used to detect planets around a pulsar. This method is very sensitive and is capable
of detecting of a very small mass. In 1992 Wolszczan and Frail used this method to
discover the first exoplanet around the pulsar PSR 1257+12 (Wolszczan 1992).
Unfortunately pulsars are pretty rare, so this method is not going produce a large
number of exoplanet discoveries. Also, life it’s unlikely that life could not survive on
planets orbiting pulsars since high-energy radiation there is intense.
Eclipsing Binary
19. When a double star (binary) system is aligned such that the stars pass in front of
each other in their orbits, the system is called an eclipsing binary star system. If a planet
has a large orbit that carries it around both members of an eclipsing double star system,
then the planet can be detected through small variations in the timing of the stars'
eclipses of each other. CM Draconis eclipse minimum times have been monitored with
high precision between 1994 and 1999 (Deeg 2000); periodic deviations of minimum
times from a linear ephemeris may indicate the presence of an orbiting third body.
Recent work by Morales et al (2008) leads them to believe that there may be a third
body in the system and that further measurements will provide ‘a better basis for
investigating the possible presence of a third body in the system.’
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Circumstellar Disks
20. Disks of space dust surround many stars, and this dust can be detected because it
absorbs ordinary starlight and re-emits it as infrared radiation. Features in dust disks
may suggest the presence of planets. Scientists believe that dust is generated by
collisions of small objects, including comets and/or asteroids. Radiation pressure from
stars will push dust particles out into stellar space; therefore any detection of dust
around a star indicates the possibility of recent collisions and other objects. A 2005
study of the Eridani Debris Disk confirmed that the ‘structure within the dust ring
suggests perturbations by a planet orbiting at tens of AU’ (Greaves et al 2005). Further
observations will shed light on the final stages of planet formation, and provide more
Figure 2: Image of Fomalhaut b
information about the existence of possible planets.
Coronagraphy
21. A coronagraph is an object which can be attached
to a telescope, which is designed to block out the direct
light from a star so that nearby objects, which otherwise
would be hidden in the star's bright glare, can be
observed. In the past coronagraphs have been developed
to view the corona of the Sun, but a new version of
similar instruments are being used to find extrasolar
planets around nearby stars. Coronagraphs can be attached to either ground based or
space based telescopes. While stellar and solar coronagraphs are similar in concept,
they are quite different in design. This is so that observations can be made of
exoplanets which are much more distant than our own sun.
22. In 2004 the high-resolution camera on the Hubble space telescope produced the
first-ever resolved visible-light image of the region around Fomalhaut, at Figure 2. The
image showed a ring of protoplanetary debris approximately 21.5 billion miles across.
Authors of the study noted that ‘Hubble observations were incredibly demanding.
Fomalhaut b is 1 billion times fainter than the star’ (NasaWeb6).
23. A stellar coronagraph concept is currently being studied to fly on the Terrestrial
Planet Finder mission. On ground-based telescopes, a stellar coronagraph can be
combined with adaptive optics to search for planets around nearby stars.
Space Based Technology
24. Many extrasolar planet candidates have been found using ground-based
telescopes. However, many of the methods can work more effectively with space-based
telescopes that avoid atmospheric haze and turbulence. COROT and Kepler are two
active space missions dedicated to extrasolar planet search. Hubble and MOST have
found or confirmed a few planets, however neither of those observatories are dedicated
to the search for exoplanets. There are also several planned or proposed space missions
geared towards exoplanet observation. Below are summaries of COROT, MOST and
Kepler, three of the most successful space based missions currently in operation.
Convection Rotation and Planetary Transit (COROT)
25. Convection Rotation and planetary Transit was launched in 2006 and is dedicated
to extrasolar planet detection. COROT is the first mission capable of detecting rocky
planets several times larger than Earth, around nearby stars. It consists of a 30www.astrochix.com 7
centimetre telescope; which it will use to monitor the changes in a star’s brightness that
happens when a planet crosses the front of the sun, the transit method. ‘From the
ground, the only planets detected around other stars have been giant gaseous worlds
(Jupiter-like planets), over 10 times the diameter of the Earth. Not affected by the
distorting effects of the atmosphere, COROT will be the first spacecraft capable of
finding worlds made of rocks.’ (ESAWeb1)
26. COROT detected its first extrasolar planet, COROT 1b in May 2007, and has gone
on to discover 9 exoplanets. The mission was originally scheduled to end 2.5 years
from launch but was extended to 2013. Below is a table listing the exoplanets
discovered by COROT to date.
Table 1: Exoplanets discovered by COROT
Planet
Constellation
Mass (MJ)
Radius (RJ)
Year
COROT 1b
Monoceros
1.03
1.49
2007
COROT 2b
Serpens
3.31
1.465
2007
COROT 3b
Aquila
21.6
1.01
2008
COROT 4b
Monoceros
0.72
1.19
2008
COROT 5b
Monoceros
0.459
1.28
2008
COROT 6b
Monoceros
3.3
1.16
2009
COROT 7b
Monoceros
0.0151
0.150
2009
COROT 8b
Unknown
COROT 9b
Serpens
2010
0.84
1.05
2010
Data gathered from http://smsc.cnes.fr/COROT/index.htm Microvariability and Oscillations of Stars (MOST)
27. Launched in June 2003, the MOST mission is the Canadian Space Agency's first
space telescope. MOST is a small space telescope, about the size of a suitcase, and
orbits the earth at about 800 kilometers. MOST is the first spacecraft dedicated to the
study of asteroseismology. MOST was one of the first satellites used to observe
exoplanets. Most ‘spends up to seven weeks at a time observing a star and watching
the fluctuations in its brightness.’ (NasaWeb7) MOST uses the transit method, to
measure the drop in brightness as a planet transits its star.
Kepler Mission
28. The Kepler Mission launched in March 2009, is designed to discover exoplanets
using the transit method. Kepler uses a photometer to monitor the brightness of over
145,000 main sequence stars in a fixed field of view. Kepler is designed to look for
planets 30 to 600 times less massive than the standard of exoplanet currently
discovered, closer to the order of Earth's mass using the transit method.
29. The first significant results were announced on 4 January 2010. ‘The first six
weeks of observations recorded by the space faring telescope, combined with followup studies from the ground, have revealed five previously unknown extrasolar
planets—one body roughly the size of Neptune and four low-density versions of Jupiter.
All reside within roasting distance of their parent stars’ (Cowen 2010).
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Table 2: Exoplanets discovered by Kepler
Planet
Constellation
Mass (MJ)
Radius (RJ)
Year
Kepler 1
Kepler 2
Planets Kepler 1-3 discovered by ground based telescopes, but are now within Kepler’s field of
view.
Kepler 3
Kepler 4b
Draco
.077
.357
2010
Kepler 5b
Cygnus
2.114
1.431
2010
Kepler 6
Cygnus
.669
1.323
2010
Kepler 7
Lyra
.433
1.478
2010
Kepler 8
Lyra
.603
1.419
2010
Data gathered from http://kepler.nasa.gov/Mission/discoveries/
Space Based Technology – Future Missions
PEGASE
30. The proposal for this mission is to build a double-aperture interferometer
composed of three free-flying satellites. ‘Its main scientific goals are the spectroscopic
study of hot giant extra-solar planets, brown dwarfs and circumstellar disks in the nearinfrared domain (1.5 to 6 µm), with a possible extension towards the visible
wavelengths’ (Absil et al 2005). One of the main goals of the mission is the study of
Hot Jupiters and brown dwarfs. The mission is scheduled to launch in 2012.
Space Interferometry Mission
31. One of the main goals of the Space Interferometry Mission (SIM) is the hunt for
earth-sized planets orbiting the habitable zone of nearly by stars. Another goal of the
SIM is to construct a map of the Milky Way Galaxy. The spacecraft will use optical
interferometry to accomplish these and other scientific goals. The SIM will perform its
search for nearby, Earth-like planets by looking for the ‘wobble’ in the parent’s motion
as the planet orbits. Measurements of over ‘1300 distant stars’ (NasaWeb4) will be
taken, and will be up to 100 times better than previous measurements taken. It’s
anticipated that the launch will take place in 2015/16.
New Worlds Mission
32. The New Worlds Mission aims to discover terrestrial exoplanets, and will be the
first space telescope to use a starshade or occulter, to find exoplanets. Once a planet is
detected the system would map the planetary systems and separate the light from the
parent star to take high quality images.
33. The basic building premise of the mission is a starshade and a collector that
function together as a single pinhole camera. ‘The starshade serves as the pinhole for
the camera, though the entire shade will be a kilometer (0.6 miles) or more in diameter,
with a hole about 10 meters (10 yards) across punched in the center.’
(ExtraSolarWeb5). The collector holds a telescope, and is separated from the starshade
by at least 200,000km. A series of images of the planetary system would all measure
the planetary orbits, and the brightness and broadband colors of the planets giving us
information about the basic nature of each planet. The project was given extra funding
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in 2008 for further study, but to date there is no planned launch date.
Terrestrial Planet Finder (TPF)
34. The Terrestrial Planet Finder comprises two complementary observatories that will
study all aspects of planets outside our solar system: including formation, development,
size, number and habitability. ‘TPF will take the form of two separate and
complementary observatories: a coronagraph operating at visible wavelengths and a
large-baseline interferometer operating in the infrared.’ (NasaWeb5)
The TPF
observatories will measure the size, temperature, and placement of planets as small as
the Earth in the habitable zones of other solar systems. TPF's spectroscopy instruments
will provide information on atmospheric conditions, and allow scientists to determine
whether a planet does, or could someday support life. As of June 2008 there has been
no funding allocated, and the mission remains without a launch date.
Darwin Mission
35. Darwin was a proposed European Space Agency (ESA) mission, which would
have involved a constellation of four to nine craft designed to directly detect
exoplanets. An early design proposed telescopes, each three to four meters in diameter,
flying in formation as an astronomical interferometer. The proposal has not progressed
since 2007, and appears unlikely to receive further funding. It’s considered that the
design was similar to that of the Terrestrial Planet Finder with a similar design and
scientific aims.
Gaia Mission
36. The Gaia mission will examine up to a thousand million stars in our Galaxy, and
monitor each of the stars about ‘70 times during a five-year period, precisely charting
their positions, distances, movements, and changes in brightness’ (EsaWeb3). ‘Gaia’s
main contribution to exoplanet science will be its unbiased census of planetary systems
orbiting hundreds of thousands nearby (d < 200 pc), relatively bright (V ≤ 13) stars
across all spectral types, screened with constant astrometric sensitivity’ (Lattanzi 2010).
37. Mission planners anticipate that every day Gaia will discover ‘on average, 10 stars
possessing planets, 10 stars exploding in other galaxies, 30 ‘failed stars’ known as
brown dwarfs, and numerous distant quasars, which are powered by giant black holes’
(EsaWeb3). It will do this by watching for tiny movements in the star’s position caused
by the minute gravitational pull of the planet on the star. At this stage the mission is
funded and launch is planned for 2012.
Planetary Transits and Oscillations of Stars (PLATO)
38. PLATO is a proposed space observatory that will use a group of photometers to
discover exoplanets. ‘The mission will have a nominal lifetime of six years, divided into
three phases. The first two phases will be used for long-duration observations, with
each observation focusing on a part of the sky that is expected to contain a high density
of cool dwarf stars’ (EsaWeb4) The last phase will include observation of interesting
targets for several months each. The spacecraft is planned for launch in 2017.
Transiting Exoplanet Survey Satellite (TESS)
39. TESS is a proposed space telescope developed by MIT. The spacecraft would be
fitted with up to 9 large angle telescopes and CCD detectors. The mission would
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conduct a two-year long all sky program for exoplanets transiting nearby bright stars.
The survey would focus on G and K spectral type stars brighter than 12 magnitudes. At
this stage there has been no funding for this project and no launch date although it’s
been suggested launch could occur in 2017.
EXTRASOLAR PLANETARY SYSTEMS
Systems
40. To date there have been 48 multiple extrasolar planetary systems discovered
(ExtraSolarWeb1). Astronomers have discovered planets orbiting a substantial number
of stars examined. However, the total fraction of stars with planets is uncertain. The
fraction of stars with smaller or more distant planets remains difficult to estimate.
Extrapolation does suggest that small planets are more common than giant planets.
41. It is now thought that a substantial number of stars have planetary systems,
meaning that billions of them must exist in our galaxy alone. The total number of
exoplanets must be very large, as our own galaxy has at least 100 billion stars, it may
contain as many as hundreds of billions of planets. There are also planets that orbit
brown dwarfs and free-floating planets that don’t orbit a host star.
42. Most of the exoplanets discovered so far are gas giants, and thought to resemble
Jupiter. This is due to a sampling bias, in that planets of a larger mass have been easier
to detect with current technology. However, recently several relatively lightweight
exoplanets, only a few times more massive than Earth, have been detected and it’s been
suggested that these will eventually be found to outnumber giant planets.
Properties
Orbital characteristics
Figure 3: Exoplanet Semi-Major Axis compared to
Planet Eccentricity.
43. Most known exoplanets are discovered
using indirect methods, and therefore the type
of
information
about
the
physical
characteristics of the planet available is
limited. Research so far indicates that many
exoplanets have orbits with very small semimajor axes, and are much closer to their
parent planet than any planet is in our own
system. ‘About 30% of exoplanets have an
orbital separation a less than 0.1 AU’ (Baraffe
2010). It’s suspected that is due to observational selection as the radial-velocity
method is most sensitive to planets with small orbits.
44. It also appears that planets on large orbits may be more common than ones on
small orbits. Cumming et al (2008) ‘estimate that 17–19% of stars have a gas giant
planet within 20 AU… extrapolating to low masses gives 11% of stars with an Earth
mass planet or larger within 1 AU.’
45. Most exoplanets with short orbital periods have near-circular orbits of very low
eccentricity; by contrast, most known exoplanets with longer orbital periods have quite
eccentric orbits, see Figure 3. The prevalence of elliptical orbits is a major puzzle, since
current theories of planetary formation strongly suggest planets should form with
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circular orbits.
46. ‘A number of planetary transit surveys have revealed that most of these exoplanets
belong to the special category of 'hot Jupiters', so named because, in contrast to Jupiter,
these gas-giant planets orbit their stars just a few stellar radii away from them, and so
are scorched by the stars' intense radiation’ (Pin-Gao 2010).
47. By examining astrometric and radial velocity measurements, it has been found
that, exoplanets appear to not orbit their star in the same orbital plane, and have varied
inclinations. In addition several hot Jupiters have been found with retrograde orbits
that raises questions about the formation of planetary systems.
Mass and Size
48. The vast majority of exoplanets discovered to date have high masses, some
considerable more massive than Jupiter. Although this is in large part due to the fact
that current detection methods are more likely to discover massive planets. This makes
statistical analysis difficult, but it appears that lower-mass planets are actually more
common than higher-mass ones, at least within a broad mass range that includes all
giant planets. The number of exoplanets discovered span a wide range of masses from a
few Earth masses to a few tens of Jupiter masses. ‘The realm of terrestrial exoplanets
starts to open its doors with the lightest known exoplanet, GJ 581e, detected by radial
velocity and having a mass M. sin i = 1.9 M⊕’ (Baraffe 2010).
Characteristics
49. Most known exoplanets orbit stars similar to our Sun, spectral categories, F, G or
K, and appear to be mainly composed of hydrogen and helium. Another compelling
property of exoplanetary systems is the ‘correlation between planet-host star metallicity
and frequency of planets. The probability of finding giant planets is a strong function of
the parent star metallicity, indicating that an environment enriched in heavy material
favors planet formation’ (Baraffe 2010). Stars of higher metallicity are much more likely
to have planets, and the planets they have tend to be more massive than those of lowermetallicity stars.
Atmospheres
50. Spectroscopic measurements can be used to study a transiting planet's
atmospheric composition. Elements including water vapor, sodium vapor, methane,
and carbon dioxide have been detected in the atmospheres of various exoplanets. This
technique might disclose atmospheric characteristics that suggest the presence of life
on an exoplanet, but to date no discovery has been made. ‘Most exoplanets yet
detected through radial velocity and photometric transits are giant planets,
characterised by the presence of a gaseous envelope mostly made of hydrogen and
helium’ (Baraffe 2010)
How do discovered systems relate to our understanding based on our knowledge of
our own system
51. Initially astronomers made an assumption that all other planetary systems must be
like our own. Then as we started to make observations of exoplanets it was discovered
that most of the planets were in fact large gas giants, with small circular orbits and it
was then assumed that this was the norm. Then more recently it was realised that
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observational bias was indicating that planetary systems had larger planets, and that in
fact it seemed more likely that there were a large number of terrestrial rocky planets
that first thought. Now we know that that exoplanets have a wide variety of eccentric
orbits, much more so that our own solar system. Each exoplanet discovered brings us
closer to understanding our own solar system, and determining if we are within the
standard planetary model or if we are an exception to the standard.
NEAR AND LONG TERM PROSPECTS FOR FINDING TERRESTRIAL-LIKE PLANETS
52. As of April 2010 Gliese 581 d appears to be the best candidate for a terrestrial
exoplanet orbiting within a habitable zone around its host star. The ‘super-Earth’
Gl5181 c is ‘clearly outside the habitable zone, since it is too close to the star’ (von
Bloh 2007). In contrast, Gl581 d is a tidally locked habitable super-Earth near the outer
edge of the habitable zone. ‘Despite the adverse conditions on this planet, at least some
Primitive forms of life may be able to exist on its surface.’ (von Bloh 2007) Taking this
into account it appears that Gl581 d is an interesting target for the planned TPF/Darwin
missions, which will search for signs of life on exoplanets.
53. Beckwith (2008) notes that ‘If all stars have planetary systems (but only 15% have
massive planets), and all planetary systems have at least one Earth-like planet within the
habitable zone, and this planet always evolves life to dominate its atmospheric
chemistry, then nearly 100% of the suitable stars will have planets indicative of life.’
54. The coming decades will bring a large number of discoveries and new knowledge
in the field of exoplanets. From the observational standpoint, a multitude of missions
based on different techniques will provide an enormous amount of data. Current and
future missions, like JWST, COROT and Kepler, will significantly increase the number
of known planets and transitioning systems.
55. A large number of additional planets will also emerge in the next years from
ground-based wide-field surveys. Increasing the number of detections will allow a more
comprehensive study of exoplanet physical properties and will either confirm or
contradict current observed trends. ‘The discovery of the triple planetary-mass system
orbiting HR 8799 has demonstrated the potential of ground-based direct imaging
projects. The development of a new generation of adaptive-optics (AO) systems, such
as VLT-SPHERE or the Gemini Planet Imager (GPI) augurs well for direct imaging of
planets orbiting solar type stars, enabling direct detection of hundreds of warm Jovian
planets in the next decade’ (Baraffe 2010).
56. The holy grail of astrobiology, the detection of life outside of Earth takes one step
closer to conclusion with the possibility of identifying habitable planets, with the
detection of water vapor and signs of chemical disequilibrium in their atmospheres, is
given by projects such as DARWIN, TPF and JWST. This could be one of the most
significant and stimulating achievements of Science, as it may tell us that we are not
alone in the universe.
www.astrochix.com 13
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