exoplanets: the search for planets beyond our solar · pdf filehugh jones of the university of...

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.

Exoplanets: The search for planets

beyond our solar system

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

iE x o p l a n E t s : th E s E a r c h f o r p l a n E t s b E y o n d o u r s o l a r s y s t E m d E c E m b E r 2 011

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.

Introduction


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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 exoplanet-hunting 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”.

The Doppler wobbleThe 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).

The age of discovery of new worlds

The first planet discovered around a solar-type star, 51 Pegasi, reveals itself in the periodic variations in the host star’s radial velocity.

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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.

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.


The habitable zones in different star systems. The Deep Impact probe used in the EPOXI mission.

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Discovering new Earths with the transit methodIn 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.

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.

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

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.

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


Schematic diagram of the transit method. Credit: H Deeg.

The SuperWASP-North instrument. Credit: www.superwasp.org.

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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 planetsGround-based telescopes are mostly limited to identifying Jupiter-sized planets orbiting very

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


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0.0000Detecting 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).

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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.


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.

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 methodsVery 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.


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A graphic of the Kepler satellite. Credit: NASA.

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.

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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

astrometry, particularly those that are far away from their parent star.

Atmospheres of extrasolar worldsExtraordinarily, 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


Diagram comparing the astrometry and radial velocity methods (adapted from Wikipedia).

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).

star changes position on the sky (astrometry)

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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 lifeThe 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

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.


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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.



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.

Planetary transitsIf 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.

MicrolensingMicrolensing 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.

Direct imagingObservations 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

the infrared can reveal dusty planetary systems in the making.

Timing of eclipsing binariesIf 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 timingA 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.

AstrometryAstrometry 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 interferometryLight 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.

PolarimetryWhen 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.

Box 1: Methods used to discover exoplanets



Planets detected as of 1.12.2011: 688

●● By RV measurements: 650 (532 planetary systems, 78 multiple planetary systems)

●● By transit observations: 186 (173 planetary systems, 16 multiple planetary systems)

●● By microlensing observations: 13 (12 planetary systems, 1 multiple planetary systems)

●● By direct imaging: 26 (26 planetary systems, 1 multiple planetary system)

●● By the timing method: 18 (8 planetary systems, 3 multiple planetary systems)

Box 2: Exoplanets detected so far


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.


Exoplanets:The search for planets

beyond our solar system

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exoplanets: the search for planets beyond our solar · pdf filehugh jones of the university of...

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