Download Exoplanets: The search for planets beyond our

Exoplanets: The search for planets
beyond our solar system
The Institute of Physics and the Royal Astronomical Society held
a joint seminar in London on 15 June 2011 to discuss progress,
recent results and future projects in the discovery and study of
planets orbiting stars other than our Sun.
Cover image: HD 188753 planetary system. Artwork of a view from a moon of the
gas giant planet that orbits the primary star of the triple star system HD 188753.
The primary star (A) is not seen, but the other two stars of HD 188753 (which form a
binary star system called B) are seen on the horizon at lower left. The existence of this
gas giant planet was inferred from gravitational wobbles of the primary star (A). The
discovery was announced in July 2005 by Maciej Konacki of Caltech, USA. The planet
orbits its star in 3.35 days, and has a mass at least 1.14 times that of Jupiter. Several
gas giant planets have now been discovered orbiting nearby stars. Credit: Detlev Van
Ravenswaay/Science Photo Library.
Exoplanets: The search for planets beyond our solar system
Introduction
The existence of worlds other than our own has always excited
popular interest. This curiosity has grown over the past two
decades since the first discovery of planets outside our solar
system. Using ground-based telescopes and space missions,
more than 700 extrasolar planets – or “exoplanets” – have now
been identified, and thousands more candidates are awaiting
confirmation. Advances in technology, combined with ingenious
detection methods, have not only speeded up the rate at which
exoplanets are being found, but have also enabled scientists to infer
many of their characteristics, including atmospheric composition,
size, mass and temperature. In this way, they are starting to
construct a picture of how planets form and what the galactic
planetary population looks like. Further scientific and technological
progress should give us a clearer idea of how common Earth-like
planets are, and whether they could harbour life.
This seminar explored the various methods used to detect
exoplanets, describing some of the current and proposed searches.
The speakers at the seminar highlighted some of the most significant
discoveries and technical advances, and speculated on future
prospects for exoplanet science. Prof. Dame Jocelyn Bell Burnell,
president of the Institute of Physics, who is well known for her role
in the discovery of rotating neutron stars, or pulsars, chaired the
seminar. Prof. Hugh Jones of the University of Hertfordshire gave
an overview of exoplanet discovery and described one of the main
techniques employed, the radial velocity method. Dr Suzanne Aigrain
of the University of Oxford explained the role of the other main
technique used so far in exoplanet hunting – the transit method
– and summarised how scientists are slowly building up a view of
planetary demographics across our galaxy. Dr Giovanna Tinetti from
University College London discussed how the transit method could
be employed to analyse and study the atmospheres of exoplanets,
thus establishing a new field of galactic planetology and paving the
way to identify planets that might be hosting life.
E x o p l a n e t s : Th e
se arch for pl ane t s be yond our sol ar sys tem
D e c e m b e r 2 0 11
i
Exoplanets: The search for planets beyond our solar system
The age of discovery of new worlds
Prof. Jones explained that the current era of
exoplanet discovery would be seen in the future
as a significant time for humanity. Hundreds of
extrasolar planets have been discovered in the
past two decades, and there is already evidence
for planets in other galaxies. “The discovery of
an abundance of worlds around other stars is
gradually revealing our place in the galaxy and in
the universe,” he said.
Jones described how the history of early extrasolar
planet study had been chequered. In the 16th
century, Italian monk and philosopher Giordano
Bruno speculated that there were an infinite
number of stars with planets encircling them, and
he was later burnt at the stake for his heresies.
It was not until the later decades of the 20th
century that astronomers started to look for the
tiny signature signals in the observational data
of various stars that might indicate the presence
of an orbiting planet. The first exoplanet claims
turned out to be false, however, as a result of
data being misinterpreted. The field of exoplanethunting was also somewhat linked with SETI (the
Search for Extra Terrestrial Intelligence) and so was
not taken very seriously until recently. “The subject
was a bit of a career-wrecker,” admitted Jones.
The first confirmed extrasolar planets were found
in 1992, orbiting an unlikely stellar candidate – a
pulsar, which is the dense core of an exploded
star. It is still not clear how the planets could
have survived the explosion and this is a matter
of theoretical intrigue, Jones pointed out.
Shortly after, in 1995, Michel Mayor and Dider
Queloz (University of Geneva) discovered the first
exoplanet around a sun-like star, 51 Pegasi b. This
might have been another false start, but crucially,
within days, the discovery was confirmed by
another, now world-leading, planet-hunting team
led by Geoffrey Marcy (University of California
at Berkeley) and Paul Butler (Carnegie Institution
of Washington). The planet has a mass of about
half that of the gas giant Jupiter, but it orbits at
a searingly hot 8 million km away from the star
in only 4.2 days; 51 Pegasi b was the first of the
so-called “hot Jupiters”.
E x o p l a n e t s : Th e
The first planet discovered around a solar-type star, 51 Pegasi, reveals itself
in the periodic variations in the host star’s radial velocity.
The Doppler wobble
The method that the teams used is based on
measuring small periodic changes in a star’s radial
velocity (RV) caused by the gravitational effect of
an orbiting planet. The RV is the speed at which
the star moves in the direction of the line of
sight of an observer, and is revealed by the shift
in wavelength of spectral absorption lines in the
star’s light due to the Doppler effect. The lines are
shifted back and forth as the RV changes. For this
reason, the technique is sometimes referred to as
the “Doppler wobble” method. The signals are
extremely small and require extensive processing
to extract the data, which is why the existence
of extrasolar planets took some time to confirm.
Nevertheless, by analysing the RV curves, the
number of planets, their relative masses to that
of the parent star and their types of orbit can be
inferred. It is not surprising that the first planets
discovered had a relatively large mass and orbited
at a close distance, since such bodies would
produce the largest signal (i.e. the largest RV
change).
se arch for pl ane t s be yond our sol ar sys tem
D e c e m b e r 2 0 11
1
Exoplanets: The search for planets beyond our solar system
habitable zone
mass of star relative to sun
2
1
Mars
Earth
Venus
0.5
0
0.1
1
10
40
radius of orbit relative to Earth’s
The habitable zones in different star systems.
The Deep Impact probe used in the EPOXI mission.
The first RV measurements were made with
relatively small telescopes using extremely
sensitive spectrographs. Prof. Jones has been
leading a UK planet-hunting team using the 3.9 m
Anglo-Australian Telescope in Australia. “We
had a calibration system that was held together
with duct tape until 2004, when the field became
credible enough to get proper funding,” he said.
The technology is now developing rapidly, with
dedicated instruments being constructed and
deployed.
Where are we now?
Today, many different types of star with different
masses, and at different stages of evolution,
are being investigated. A number of multiple
planetary systems, some potentially not so
different from our own solar system, have been
discovered. Exoplanets range in size and mass
– from those like Jupiter, through those similar
in mass to Neptune, to so-called “super-Earths”,
which are just a few times larger than the Earth.
Even free-floating planets that are not connected
to a star have been identified, particularly in
star-forming regions.
One of the main aims of current planet searches
is, of course, to discover Earth-like planets that
exist in a star’s “habitable zone” – the region
around a star where liquid water, and therefore
life, can potentially exist. The smaller the mass of
a star, the easier it is to detect a smaller orbiting
planet – and the colder the star is, the closer the
habitable zone is to the star. It is not surprising,
therefore, that small, cool M-dwarf stars have
been regarded as suitable early targets for finding
other habitable Earths.
2
E x o p l a n e t s : Th e
Current planet searches are gradually moving
towards identifying Earth-like planets around
solar-type stars. Using the RV method, Jones’s
team recently announced the discovery of a
planet in a nearby Sun-like system. The planet
had a mass of 10 to 12 Earth masses and a period
of about 350 Earth days. The telltale RV signal is
about 1.6 m/s. However, to find true “Earths” in a
habitable zone would require extracting a signal
of only 0.1 m/s. “Using a combination of data from
different telescopes, and with improvements
in our understanding of stellar activity, we can
expect a rich variety of multiple planetary systems
with Earth-mass planets to be announced soon,”
said Jones. “Potentially, Earth-like planets are
very common, however, their observation remains
beyond our current capabilities,” he added.
The search for exoplanets is now a major global
effort, involving the world’s largest observatories
such as the Hubble and Spitzer space telescopes
and the ground-based Very Large Telescope (VLT)
of the European Southern Observatory in Chile.
Dedicated space missions – Convection Rotation
and planetary Transits (CoRoT) and Kepler –
have been launched, which have found the first
super-Earths using another detection method –
the transit method (see below). The NASA space
probe, Deep Impact, launched in 2005 to study
a comet, is now being deployed in a second
mission, called Extrasolar Planet Observation and
Deep Impact Extended Investigation (EPOXI),
to investigate exoplanets. A new generation of
spectrographs on future observatories, such as the
European Extremely Large Telescope (E-ELT), could
reach a sensitivity for RV measurements of just a
few centimetres a second.
se arch for pl ane t s be yond our sol ar sys tem
D e c e m b e r 2 0 11
Exoplanets: The search for planets beyond our solar system
Discovering new Earths
with the transit method
In her talk, Dr Aigrain focused on achievements
using the transit method, which involves looking
for the small dip in the light from a star as an
orbiting planet passes in front. This dip in flux
may be between 0.01 and 1%, depending on the
relative sizes of the star and the planet. The planet
must orbit in the plane along the line of sight,
which means that many stars must be monitored
to find systems with planets in a suitable orbital
inclination. However, the transit method enables
the size of the planet to be measured. When
combined with the mass measurement obtained
from the RV method, it gives an estimate of the
mean density of the planet, from which the mean
composition can be inferred.
1
2
3
star
planet
brightness
1
2
3
time
Schematic diagram of the transit method. Credit: H Deeg.
The first transit observations were made in 1999
by US teams using a very small telescope called
STARE with an aperture of only 5 cm, which was
based on the Canary Islands. The first exoplanet
detected in this way, HD 209458b, had already
been found by the RV method. It prompted the
launch of several projects to scan the skies for
more transiting exoplanets, using small-aperture,
wide field-of-view ground-based telescopes.
However, it was not until 2004 that they made
their first discoveries – many of the difficulties
associated with identifying transits had initially
been underestimated. At about the same time,
ultra-precise observations of transiting planets
using the Hubble and Spitzer space telescopes led
to the first detection of exoplanet atmospheres,
giving astronomers access to characteristics such
as their temperature and composition (see below).
These results marked the beginning of exoplanet
science as a major new field of research.
While the majority of exoplanet discoveries
have been made with the RV method, transit
measurements can determine the orbital period,
size and distance from the host star, and when
combined with the RV method provide the
absolute planetary masses. Periodic changes in
light flux can be due to other phenomena such as
the presence of a binary stellar companion that
might partially eclipse the host star, or even star
spots, so transit observations are usually followed
up by RV measurements. Large ground-based
telescopes are used to check for eclipsing binaries.
The SuperWASP-North instrument.
Credit: www.superwasp.org.
One of the most successful transit programmes
is the UK-led Wide Angle Search for Planets
(SuperWASP), which uses two telescopes: one
in the Canary Islands (Isaac Newton Group of
Transits can be observed with quite modest
telescopes, so amateur astronomers and
students can make contributions. The exoplanet
HD 80606 b, which orbits a star 200 light
E x o p l a n e t s : Th e
Telescopes) and one in South Africa (South African
Astronomical Observatory). Each one uses eight
inexpensive telephoto lenses. “They were bought
on eBay,” said Aigrain. “What really matters is the
sensitivity of the cameras, which are very large
CCDs. These allow SuperWASP to survey nearly
half of the sky, monitoring millions of stars,” she
added. So far, 48 planets have been discovered
with SuperWASP.
se arch for pl ane t s be yond our sol ar sys tem
D e c e m b e r 2 0 11
3
Exoplanets: The search for planets beyond our solar system
1.01
1.0002
1.00
normalised flux
1.0000
0.99
0.98
2860
2880
2900
HJD
2920
2940
2960
0.9998
0.9996
2980
Detecting small planets with CoRoT: the raw light curve (above
left, black/blue) must first be filtered to remove the effect of star
spots (left, red/green). The transits become visible only when the
light curve is “folded” on the planet’s orbital period (right).
0.0002
0.0000
–0.0002
–0.10
Simulation of the temperature variations on HD 80606 b. This planet has a
wildly eccentric orbit swinging from a distance within 5 million km from its
star out to 132 million km away. The resulting huge temperature changes
would produce some extreme weather. Credit: NASA/JPL/J Langton.
years from Earth and is four times the mass of
Jupiter, was discovered by astronomers and
students at University College London using
the University of London Observatory at Mill
Hill. Transit observations from the roof of the
physics department at the University of Oxford
are now part of the undergraduate curriculum,
commented Aigrain.
Dedicated space missions
to search for transiting
planets
Ground-based telescopes are mostly limited to
identifying Jupiter-sized planets orbiting very
4
E x o p l a n e t s : Th e
–0.05
0.00
0.05
0.10
close to their star. To find smaller planets requires
continuous observations from space. Aigrain
is part of a team observing with the CoRoT
satellite, which is led by the French National Space
Agency. Its aperture is only the size of an LP disc,
but it is able to observe more than 10 000 stars
continuously for several months at a time. This
allows CoRoT to pick out smaller planets, including
some with a more rocky (Earth-like) composition.
In June 2011, the team announced the discovery
of 10 new exoplanets. “In some cases the transits
are very obvious when the flux from the star is
measured as a function of time, even though the
dips are a small fraction of a percent,” said Aigrain.
In one case, combining the transit observations
with RV measurements made with the ESO
High Accuracy Radial Velocity Planet Searcher
spectrograph revealed CoRoT 23b, a dense hot
Jupiter with an extremely eccentric orbit.
Other transits can be less obvious because of
additional large variations due to star spots.
Nevertheless, using computer filtering and
processing techniques, very small periodic
variations previously hidden in the raw data can
be picked out, so that it is possible to extract a dip
as little as three parts in 10 000, says Aigrain. This
was how one of the first super-Earths, CoRoT 7 b,
was discovered in 2009. Its radius is only twice
that of Earth. One of the planets discovered in
2011 appears to orbit a very young star. Aigrain
says that by measuring the radii of such planets
accurately, it is possible to learn more about how
planets evolve.
Another much larger dedicated telescope, Kepler,
was launched in 2009 by NASA. Kepler stares
at a given star field for four years in order to
se arch for pl ane t s be yond our sol ar sys tem
D e c e m b e r 2 0 11
Exoplanets: The search for planets beyond our solar system
size relative to Earth
20
b
10
c
Jupiter
4
Uranus
1
Earth
1
4
10
orbital period (days)
40
d
100
Above left: the demographics of planet candidates from the first four months of Kepler observations (from Borucki et al. 2011).
Above right: the HR 8799 system imaged directly using the Keck Telescope (from Marois et al. 2008).
Two possible spacecraft concepts for the PLATO
mission, the next-generation transit survey
currently under study at the European Space
Agency. Credit: ESA.
A graphic of the Kepler satellite. Credit: NASA.
extract as much data as possible. The first year of
observations has already yielded more than 1200
planetary candidates, many of which are likely to
be confirmed in the near future. A large number of
these are smaller than Neptune, and it should not
be long before true Earth analogues are found.
The data are also slowly building up a picture of
planetary demographics. “We already know that
5% of stars have Jupiter-sized planets, 17% have
‘Neptunes’ and at least 7% have super-Earths,”
said Aigrain. Although low-mass exoplanets are
much harder to find, they are expected to exist
in much more abundance. A few more years’
observations should reveal many more lower-mass
planets, particularly with a new European mission
called PLATO due to launch at the end of the
decade. It will look for transits across bright
stars in an area covering 40% of the sky, and
will enable the complete characterisation of
exoplanets and their host stars, including mass,
size, age and physicochemical properties.
E x o p l a n e t s : Th e
Exoplanet research is providing crucial evidence
for understanding how stars and planets form and
evolve. For example, free-floating planets, some
of which appear to exist as multiple systems, may
simply represent the lowest-mass objects that
condense from a star-forming nebula, or they may
have been ejected from a young stellar system.
Other methods
Very large planets can even be seen directly. In
2004, observations with the VLT produced an
image of a planetary companion to a brown dwarf
(a body not massive enough to become a proper
star). Then, in 2008, a multiple planetary system,
HR 8799, was directly seen with the Keck and
Gemini Observatories. “When I saw the picture,
it blew me away,” said Aigrain. The observations
relied on increasing the optical contrast, whereby
the star’s overwhelmingly bright light is blocked
out by an instrument called a coronagraph so that
the weaker, reflected light from the planets can
be seen.
se arch for pl ane t s be yond our sol ar sys tem
D e c e m b e r 2 0 11
5
Exoplanets: The search for planets beyond our solar system
star changes position on
the sky (astrometry)
star moves back and forth along
line of sight (radial velocity)
Diagram comparing the astrometry and radial
velocity methods (adapted from Wikipedia).
astrometry, particularly those that are far away
from their parent star.
Artist’s impression of HD 209458 b showing how this hot Jupiter is losing
its atmosphere as it orbits close to its star. Credit: ESA/ Alfred Vidal-Madjar
(Institut d’Astrophysique de Paris, CNRS, France and NASA).
Another method that has already produced
results depends on the phenomenon of
“microlensing”. When one star passes behind
another in the line of sight, the intervening
star acts as a “gravitational lens”, increasing
the brightness of the background star by tens
of times. The presence of an orbiting planet
around the lensing star affects the pattern of
brightening in a measurable way. This method
can uniquely observe very distant star systems
and is particularly sensitive to low-mass planets
that are far out from their stars. Unfortunately,
microlensing is a one-off event, so it is mainly
useful for building up the statistics of planetary
discovery, and as a check on whether those
planets found in our local neighbourhood are
typical of the galaxy as a whole.
A dynamical method that has not produced any
results yet, but will be significant in future years,
involves measuring directly the position of the
star on the sky – astrometry – and then its motion
in that plane. By combining astrometry and RV
measurements (which measures motion in the
line of sight, i.e. at right angles to the astrometry
measurement) it will be possible to obtain an
accurate measure of both the mass and the orbit.
The ESA’s mission Gaia, to be launched in 2013,
will aim to map our entire galaxy. It will measure
the positions of stars and identify exoplanets via
6
E x o p l a n e t s : Th e
Atmospheres of extrasolar
worlds
Extraordinarily, astronomers are already able to
pick out and analyse the spectra of exoplanets,
using both space- and ground-based telescopes.
One method is to measure the radius of the planet
at different wavelengths as it passes in front of
the star. Since different wavelengths of stellar
light penetrate the planetary atmosphere to
different degrees, it is possible to obtain a profile
of the composition, temperature and dynamics
across the limb of the planet. The star’s light will
be absorbed at wavelengths that are characteristic
of the particular chemical species present and the
resulting transmission spectrum will reveal these
signature absorptions. Further information is
given by measuring the star’s spectrum when the
planet had gone behind it (the secondary transit),
so that the planetary contribution to the total
light emitted can then be subtracted out. This
gives an emission spectrum for the planet largely
in the infrared, or a spectrum of reflected light
in the visible, providing information about the
planet’s bulk composition.
“Investigating planetary atmospheres is a relatively
recent development,” said Dr Tinetti. She described
some of the findings to date. In 2001, the Hubble
space telescope revealed the presence of sodium
in the atmosphere of a hot Jupiter, HD 209458 b,
by looking at the primary transit. This planet is
60% of the size of Jupiter and is so close to its star
– only 7 million km away – that it remains tidally
se arch for pl ane t s be yond our sol ar sys tem
D e c e m b e r 2 0 11
Exoplanets: The search for planets beyond our solar system
locked in orbit such that one side always faces the
parent star. On the day side, temperatures are
thought to reach more than 1500 °C. Indeed, later
observations showed that the upper atmosphere is
literally escaping away, sweeping up several ionic
species in its tail. Remarkably, the spectrograph on
the VLT was able to show that carbon monoxide
was streaming across from the day side of the
planet to the night side.
The atmosphere of another hot Jupiter,
HD 189733 b, has also been well studied. Tinetti’s
own team found evidence for water vapour in its
atmosphere. This was followed by indications of
carbon dioxide and methane. Similar results have
also been found for HD 209458 b. Tinetti points
out that due to the small amount of available
data, there are still large uncertainties in the
abundances of the molecular species currently
detected in exoplanets. These uncertainties grow
when observing smaller planets. For example,
observations with the VLT of the atmosphere of
the transiting exoplanet, GJ1214 b, which is about
six Earth masses and has a temperature of around
200 °C, revealed little in terms of atmospheric
composition.
“I would really like to see a mission launched
that is dedicated to characterising planetary
atmospheres,” said Tinetti. The Exoplanet
Characterisation Observatory (EChO), which is
a candidate in the European Space Agency’s
“Cosmic Vision 2015–2025” plan, is just such a
mission. It is designed to be a 1.2 m telescope that
will carry out detailed spectroscopy of a wide
range of representative exoplanets, from gas
giants to super-Earths. By making spectroscopic
observations with very long exposures, it will be
able to analyse the composition and temperature
structure of planetary atmospheres, detecting up
to 30 different kinds of molecules.
The search for life
The aim will be to understand better how
planetary systems form and evolve – and will
truly establish a new science of planetology on
a galactic scale. Exoplanet research will uncover
how common solar systems like our own are.
Eventually, the aim is to identify true Earth-like
planets in the habitable zone of solar-system
analogues. Analysing their planetary atmospheres
E x o p l a n e t s : Th e
0.010
relative brightness
0.008
0.006
0.004
0.002
0.000
8
10
12
wavelength (microns)
14
Infrared spectrum of HD 189733 b taken with the Spitzer space telescope.
Credit: NASA/Caltech/H Knutso.
Gliese 581 is a red dwarf that could have up to six planets, some of which
orbit close to its habitable zone. The orbits of planets in the Gliese 581
system are shown compared with those of our own solar system.
Credit: National Science Foundation/Zina Deretsky.
is the first and crucial step to finding life outside
the Earth. Observations of both water and a
non-primordial, oxygen-containing atmosphere
with a composition that is not in thermodynamic
equilibrium would certainly make a strong case.
With the new generation of very large,
ground-based telescopes – the 40 m E-ELT, the
Atacama Large Millimetre Array and the Square
Kilometre Array to detect radio signals – and new
space missions, we are set to enter a golden era of
exoplanet exploration.
se arch for pl ane t s be yond our sol ar sys tem
D e c e m b e r 2 0 11
7
Exoplanets: The search for planets beyond our solar system
Box 1: Methods used to discover exoplanets
A number of sophisticated methods for
identifying planets have been developed.
These techniques are used on a wide range of
ground-based and space-based instruments, and
are being combined to provide more detailed and
accurate data.
Radial velocity (RV)
As a planet orbits a star, the star also moves in
its own small orbit around the system’s centre of
mass, which also depends on any planets present.
Variations in the star’s radial velocity – the speed
at which it moves towards or away from us on
Earth – can be detected from displacements in the
star’s spectral lines due to the Doppler effect.
the infrared can reveal dusty planetary systems in
the making.
Timing of eclipsing binaries
If a planet orbits both partners in an eclipsing
double star system, then the planet can be
detected through small variations in the timing of
the stars’ eclipses of each other.
Pulsar timing
A pulsar emits a beam of radio waves with
clockwork regularity. If any planets are present,
they will cause slight anomalies in the timing of
the pulses.
Astrometry
Planetary transits
If a planet crosses in front of its parent star’s disc,
then the observed brightness of the star drops
slightly. This is the second most successful method
for detecting exoplanets, although effects due
to other phenomena such as star spots and the
presence of an eclipsing stellar companion have
to be filtered out. Transits are also used to study
planetary atmospheres.
Microlensing
Microlensing occurs when the gravitational field
of a star acts like a lens, magnifying the light of
a distant background star so that it brightens.
A planet orbiting the lensing star causes the
brightening to vary over time. This method is
especially sensitive to planets orbiting far from
their parent stars.
Astrometry involves measuring a star’s position
in the sky precisely and monitoring it over time.
The motion of a star due to the gravitational
influence of a planet may be observable. No
exoplanets have yet been discovered using this
method.
Nulling interferometry
Light waves received from a star by several
telescopes can be combined constructively
to generate a strongly reinforced signal – a
technique called interferometry. However, the
light waves can also be combined so that they
cancel each other out, thus eliminating the star’s
light. The light from any orbiting planet can
then be observed. This technique is still being
developed.
Polarimetry
Direct imaging
Observations with powerful telescopes in the
visible and infrared part of the spectrum are
able to capture directly a few very large planets
that are far from their host star. A disc-shaped
coronagraph is used to block out the star’s bright
light so that any planets can be seen. Imaging in
8
E x o p l a n e t s : Th e
When starlight is reflected by a planet, it
becomes polarised (the light waves oscillate in
a specific direction) due to interactions with the
atoms and molecules in the atmosphere. Highly
accurate polarimeters are being developed to
detect this polarised light.
se arch for pl ane t s be yond our sol ar sys tem
D e c e m b e r 2 0 11
Exoplanets: The search for planets beyond our solar system
Box 2: Exoplanets detected so far
Planets detected as of 1.12.2011: 688
RV measurements: 650 (532 planetary
systems, 78 multiple planetary systems)
●● By
microlensing observations: 13 (12 planetary
systems, 1 multiple planetary systems)
●● By
transit observations: 186 (173 planetary
systems, 16 multiple planetary systems)
●● By
direct imaging: 26 (26 planetary systems,
1 multiple planetary system)
●● By
E x o p l a n e t s : Th e
●● By
se arch for pl ane t s be yond our sol ar sys tem
the timing method: 18 (8 planetary systems,
3 multiple planetary systems)
D e c e m b e r 2 0 11
9
Exoplanets: The search for planets beyond our solar system
About the societies
The Institute of Physics is a leading scientific society promoting physics
and bringing physicists together for the benefit of all. It has a worldwide
membership of around 40 000 comprising physicists from all sectors, as well
as those with an interest in physics. It works to advance physics research,
application and education, and engages with policy-makers and the public
to develop awareness and understanding of physics. Its publishing company,
IOP Publishing, is a world leader in professional scientific communications.
The Royal Astronomical Society, founded in 1820, encourages and promotes the
study of astronomy, solar-system science, geophysics and closely related branches
of science. With more than 3500 members, the RAS organises scientific meetings,
publishes international research and review journals, recognises outstanding
achievements by the award of medals and prizes, maintains an extensive library,
supports education through grants and outreach activities, and represents UK
astronomy nationally and internationally.
10
E x o p l a n e t s : Th e
se arch for pl ane t s be yond our sol ar sys tem
D e c e m b e r 2 0 11
Exoplanets:
The search for planets
beyond our solar
system
­­­
For further information, or a large-print version, please contact:
Sophie Robinson
Institute of Physics
76 Portland Place
London W1B 1NT UK
Tel +44 (0) 20 7470 4887
Fax +44 (0) 20 7470 4848
E-mail [email protected]
www.iop.org
www.iopblog.org
Registered charity number 293851
Scottish charity register number SC040092
© Institute of Physics