Exoplanets Press Kit Exoplanets 1 Contents Preface 3 Early discoveries 5 Techniques for detection 7 Direct detection 7 Imaging 7 Indirect detection 7 Radial velocity tracking 8 Astrometry 10 Pulsar timing 10 Transits 10 Gravitational microlensing 11 What can we learn from exoplanets? 13 What are exoplanets like? 14 Life outside the Solar System 16 Exoplanet research at eso 17 ESo’s current exoplanet instruments 18 Exoplanet research in the future at ESO 19 2 Exoplanets Cover: Artist’s impres- sion of the exoplanets HD 189733b | ESA, NASA, G. Tinetti (Uni- versity College London, UK & ESA) and M. Korn- messer (ESo) Left: Artist’s impression of an exoplanets orbiting its star | ESA, NASA, M. Kornmesser (ESo) and STScI Preface Since planets were first discovered out- This guide provides an overview of the side the Solar System in 1992 (orbiting a history of exoplanets and of the current pulsar) and in 1995 (orbiting a “normal” state of knowledge in this captivating star), the study of planets orbiting other field. It reveals the various methods that stars, known as exoplanets, or extrasolar astronomers use to find new exoplanets planets, has become one of the most and the information that can be inferred. dynamic research fields in astronomy. The last section summarises the impres- our knowledge of exoplanets has grown sive findings of exoplanet research at immensely, from our understanding of ESo and the current and near-future their formation and evolution to the devel- technologies available in the quest for opment of different methods to detect new worlds. them. Exoplanets 3 4 Exoplanets Left: ESo 3.6-m tele- scope, La Silla observa- tory | ESo Early discoveries “There are an infinite number of In 1995, the Geneva-based astronomers worlds, some like this world, others Michel Mayor and Didier Queloz de- unlike it.” tected the first exoplanet around a “nor- mal” (main sequence) star, 51 Pegasi. Epicurus — letter to Herodotus The planet, named 51 Pegasi b, has (~ 300 BC) around half the mass of Jupiter and whiz- zes around its parent star in just over four Earth days, lying almost eight times A planet is an object orbiting a star that is closer to it than Mercury is to the Sun. massive enough both to have achieved an almost spherical shape and to have Since 1995, this area of astronomy has cleared the rotating disc of dense gas, become a very dynamic research field known as the protoplanetary disc, that and astronomers have found over 450 surrounds a newly formed star. Planets exoplanets (as of May 2010), using a host differ in this from dwarf planets (such as of techniques. Pluto), which do not have enough mass to clear the protoplanetary disc area. The first detection of an exoplanet occurred in 1992 when the astrophysi- cists Aleksander Wolszczan and Dale Frail discovered three exoplanets. They were found in an unexpected environ- ment, orbiting the pulsar PSR1257+12. Exoplanets 5 Artist’s impression of the planetary system around HD 69830 | ESo 6 Exoplanets Techniques for detection Searching for exoplanets is like looking Direct detection The adaptive optics instrument, NACo, for the proverbial needle in a haystack. on ESo’s Very Large Telescope (VLT) has Planets emit little or no light of their own, Imaging obtained the first image of an exoplanet. while their host stars shine brightly. See- The European Extremely Large Telescope ing the light from a distant planet is like The hardest way to detect an exoplanet is (E-ELT) planned for 2018, will search for spotting a dim candle in front of a raging to try to image it directly. This is because new planets using direct imaging, thanks forest fire. of the extreme contrast between the light to its very sharp vision. emitted by the parent star and by the Nowadays six investigative tools are used companion planet. To expose the planet, to spot hidden exoplanets. the starlight must be dimmed or masked Indirect detection in some way so as to enable observers to Direct detection see into the shadow. one method is to The majority of all exoplanets discovered – Imaging use infrared radiation, rather than visible so far have been detected using indirect light. The visible light output of a Jupiter- methods — identifying their existence by Indirect detection like planet is one billionth of that of its their effect on their host star. – Radial velocity tracking host star, while in the infrared the con- – Astrometry trast is just a factor of a few thousandths. The presence of a planet affects its host – Pulsar timing This is particularly true when the planet is star in several ways. The weak gravity of – Transits still very young and thus contracting, the planet pulls the star in a small circular – Gravitational microlensing thereby emitting heat. Another method is orbit, introducing a minute wobble that to physically block out the starlight, using can be detected using radial velocity a coronograph that masks the bright tracking or astrometry (see pages 8 –10). central core of the star, leaving only the Alternatively, as the planet moves be- corona, the outer plasma region of the tween the star and the observer, the star’s atmosphere, visible and so allowing measured luminosity of the star will any nearby planets to shine through. change. These tiny variations are impor- tant for astronomers, as it makes the Direct imaging is the only way to assess indirect observation of exoplanets possi- some important physical parameters, ble. such as the amount of water on the sur- face and the properties of any possible biosphere. Possibly the first image of an exoplanet (red spot), obtained with NACo at the VLT. The planet orbits a brown dwarf (blue spot in mid- Beta Pictoris as seen in dle) | ESo infrared light | ESo debris disc Beta Pictoris location of the star 0.5� 2003 2009 Size of Saturn’s orbit Planet Beta Pictoris b around the Sun Exoplanets 7 The planetary system around Gliese 581 (artist’s impression) | ESo Radial velocity tracking for example in the change of pitch of an The radial velocity method has proven to ambulance siren as it drives past on the be the most successful in finding new An astronomer can determine much street. planets. At present, the most successful about a distant star by recording its low-mass exoplanets hunter is HARPS spectrum. As the star moves in the small The periodic changes in the star’s radial (High Accuracy Radial Velocity for Plane- orbit resulting from the pull of the exo- velocity depend on the planet’s mass and tary Searcher), which is mounted on planet, it will move towards the Earth and the inclination of its orbit to our line of ESo’s 3.6-metre telescope at La Silla, then away as it completes an orbit. The sight. These tiny changes or “wobbles” Chile. velocity of the star along the line of sight can be measured by a distant observer. of an observer on Earth is its radial veloc- Astronomers use high precision spec- ity. Changes in the radial velocity of the trographs to study Doppler-shifted spec- star cause the lines in the star’s spectrum tra, looking for small regular variations in to shift towards redder wavelengths the radial velocity of a star. As the inclina- when the star is moving away from us tion of the planetary orbit is unknown, the and towards bluer wavelengths when the measurement of this regular variation planet is approaching us (see image). gives a minimum value for the mass of This is the Doppler effect, and it is notice- the planet. able with sound waves in everyday life, 8 Exoplanets The radial velocity method | ESo Exoplanets 9 1.01 1 0.99 0.98 x OGLE-TR-113 Flu 0.97 ve 1.01 Relati 1 0.99 0.98 y 0.97 OGLE-TR-132 nsit te In –0.1 –0.05 0 0.05 0.1 Time Phase 1. The measured drop in 2. Brightness variations of brightness of the star two stars with transiting when the planet passes exoplanets | ESo in front of it | ESo 3 10 Aug 11 Aug 2005 2005 Observer Source star 2.5 2 Planet Magnification 1.5 1 Observer Lens Star Source star 3. Light curve of oGLE- 4. Gravitational lensing 2005-BLG-390 | ESo caused by the pres- ence of a star and an exoplanet | ESo Astrometry Pulsar timing Transits The astrometry method is similar to radial The presence of a planet orbiting a star When a planet passes between the Earth velocity tracking and is used to detect affects the timing of the regular signals and its host star, this is known as a tran- exoplanets by measuring the small regu- emitted by the star itself. This phenome- sit. The planet blocks some of the star- lar perturbation in the position of a star non can be used to detect planets light during the transit and creates a peri- due to its unseen companion. The star around a pulsar. Pulsars emit radio waves odic dip in the brightness of the star. This moves in a tiny circular orbit on the sky regularly as they rotate, creating a peri- effect can be measured using photome- with a radius that depends on the mass odically pulsed beam, like a lighthouse. If try, which measures the amount of light of the planet and its distance from the an orbiting planet perturbs the motion coming from celestial objects. star, but not on the inclination. No planets of the star, then the timing of the beam is have been discovered so far using this also affected, and this is how the first We can learn much about the composi- method.