The extrasolar planets

1. The extrasolar planets Detection techniques - future possibilities Angelo Angeletti Tolentino (MC), ITALY – 31 October, 2007

2. Extrasolar planets – A bit of history – Detection techniques – Present results – Our observations – Future work

3. A bit of history What is a planet? The term “planet” stems from a Greek word meaning ‘wanderer’. All celestial objects moving across the sky were dubbed ‘planets’, as opposed to the ‘fixed stars’. The list included the Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn – the only ‘planets’ visible to the naked eye. On 24 August, 2005, the International Astronomical Union (IAU) defined ‘planet’ every celestial body which: - orbits around the Sun; - is massive enough to attain a spherical shape; - has swept the region of its orbit clean of all the debris

4. A bit of history The Solar System

5. A bit of history How did the Solar System form? The Sun and planets are believed to have formed from a contracting nebula of interstellar gas; the process took place about 4.6 billion years ago According to modern theories, the primordial nebula was mainly composed of hydrogen and helium (though heavier elements and solid grains were also present) and it must have been very cold (10 K) The Orion nebula. This gas cloud hosts the cradle of a number of stars.

6. A bit of history How did the Solar System form? About 4.55 billion years ago, due to self-gravity, the density at the center of the primordial nebula grew steadily; further contraction gave birth to our Sun. The process increased at the same time the rotation velocity as well as the centrifugal force. The outer parts of the nebula flattened to a disk, while still rotating around the newly-formed star. Protoplanetary disks (proplyds) in the Orion nebula

7. A bit of history How did the Solar System form? During the final stages of the collapse a strong stellar wind must have set in, dragging the lighter elements outward – mostly hydrogen and helium. As the temperature of proto-Sun rose high enough to ignite thermo-nuclear reactions, some bodies inside the disk began to grow by collision and gravitational capture processes, sweeping their zone clean from debris. This led to the formation of the proto-planets, from which the present planets originated. The proto-Sun became a yellow main-sequence star.

8. A bit of history How did the Solar System form? The Working Group on Extrasolar Planets (WGESP) of the IAU defines as an extrasolar planet (shortened exoplanet) “…a body whose mass lies below the threshold value for the onset of deuterium thermo-nuclear fusion (which is about 13 Jupiter masses [MJ] for a typical solar composition) and at the same time is orbiting a star or a star’s remnant - no matter how evolved. The minimum mass required is Mercury‘s - not Pluto! Bodies less massive than 70 MJ (but still above the 13 MJ threshold) are to be considered ‘brown dwarfs’ – no matter how they formed. Free bodies (as those found in young stellar clusters) with masses below the 13 MJ limit are brown subdwarfs, not planets.

9. A bit of history Why search for extrasolar planets? The search for exoplanets is a most recent field in Astronomy. Being strictly related to a number of “hot” topics in other cultural areas – such as religion, philosophy and much more else – it is increasingly becoming an issue of paramount importance. In due time – perhaps earlier than we might expect! – it may even give an answer to a crucial question in the history of mankind... Do other life forms and other inhabited worlds exist?

10. A bit of history The beginnings In the past, the existence of extrasolar planets was reputed a plausible scenario. The first scientific discussions about the issue date back to 17th century. Sir Isaac Newton was the first modern scientist to give credit to the existence of exoplanets (1713). Supposedly confirmed reports of exoplanets’ ‘discoveries’ abounded in 19th century. IsaacNewton

11. A bit of history The beginnings A brand new research field opened up in 1984, when a circumstellar disk around the star β Pictoris was detected.

12. A bit of history The beginnings Several discovery reports were issued in the following years. 1989: Latham detects a 10 MJ body circling the star HD 114762. 1991: Alexander Wolszczan identifies two planets about the same mass as Earth’s, revolving around a pulsar (PSR 1257+12). 1993: Gordon Walker claims that oscillations in radial velocity of the star γ Cephei might be caused by a ≈ 2 MJ planet. However, such findings were considered too “weird” by most scientists to be taken very seriously.

13. A bit of history The beginnings On 6 October, 1995, in Florence, the discovery of a planet near the star 51 Pegasi is announced. This star is 50 light years away, and very similar to our Sun. The mass of the planet is about 160 terrestrial masses (0.5 MJ) ; it orbits very close to its star (7.5 million kilometers), in about 4 days. The discoverers: Michel Mayor and Didier Queloz, of the Geneva Observatory An artist’s impression of 51 Pegasi

14. A bit of history The beginnings The date of 6 October 1995 marks the beginning of a thorough, extensive search for extrasolar planets. At 30 October 2007, 260 exoplanets in 224 stellar systems had been discovered, located as follows: 198 single systems 18 double systems 6 triple systems 2 quadruple systems

15. Detection techniques The various methods Most exoplanets haven’t been actually seen through a telescope Direct observation of an exoplanet is an exceedingly difficult task. Its light is usually much fainter (a millionth or even less) than its parent star’s. 2M1207 b – one of the four extrasolar planets discovered through direct observation

16. Detection techniques The various methods Apart from a direct observation, several methods for detecting exoplanets have been developed. These are: – The astrometrical method – The radialvelocity method – The transit method – The gravitationalmicrolensing method – The timing method

17. Detection techniques The astrometrical method The position of the star is measured with the highest possible accuracy, with the purpose to detect a displacement - however slight - caused by a planet (both bodies orbit around the center of mass). For comparison, Jupiter – when seen from a distance of 10 light years – makes our Sun oscillate of about 1 millionth of grade, with a period of about 12 years.

18. Detection techniques The astrometrical method By this technique only very massive pianets – and very close to their star – can be detected: the so-called hot Jupiters. The mass of a hot Jupiter is the same as Jupiter’s or even more, but it revolves around the parent star at a distance less than 0.05 AU (7,5 million km), which is eight times closer than Mercury to our Sun. The typical temperature on the dayside can reach a thousand degrees Celsius. An artist’s impression of HD 209458b. The blue tail is the planet’s atmosphere, evaporating into space due to the close proximity of the parent star.

19. Detection techniques The radial velocity method The gravity of a planet close to its star induces small variations in the star’s radial velocity (i.e., the velocity along the line connecting Earth and the star). Such variations can be detected in the star’s spectrum, by measuring the shift of the spectral lines. This gives information about the planet’s mass and orbiting distance. Line shifts are very small and are proportional to the planet’s mass.

20. Detection techniques The radial velocity method This method has given the best results so far. From the collected data, and making use of Kepler’s laws, some fundamental properties can be deduced – namely, the orbital period, the distance from the parent star, plus an estimate of the planet’s mass (the last parameter depending on the orbit’s inclination as seen by the observer)

21. Detection techniques The transit method • A planet crossing the disc of its parent star (in so performing a transit), it causes a small eclipse; the star’s brightness drops then slightly. • In order to be able to detect a transit, two conditions are to be met: • Earth, planet and parent star must be sharply aligned (this seldom happens); • Observations must take place just when the alignment is achieved.

22. Detection techniques The transit method

23. Detection techniques The transit method Only hot Jupiters have been detected during transits so far - plus, they had all been previously discovered with radial velocity measurements. On 30 October, 2007 only 29 planets (out of a total reckoning of 260) had been seen transiting over the disc of their parent star.

24. Detection techniques The transit method By observing a transit the actual size of a planet can be estimated. The next step is spectral analysis. This means taking two spectra - one as the planet crosses the star’s disc, the other when it passes behind and it’s eclipsed by the star. By subtracting the spectra one can then get useful information about the planet’s atmosphere. Another artist’s impression of HD 209458b.

25. Detection techniques The gravitational microlensing method This method makes use of a passing star intercepting the light path from another star that’s much farther away. If both stars are aligned with respect to Earth, the gravity of the nearer bends the light rays coming from the farther (gravitationallens effect), enhancing its luminosity for a short time. If the passing star hosts a planet, a second luminosity peak can be observed.

26. Detection techniques The timing method A rotating pulsar (the small, ultra-dense remnant of an exploded supernova) emits radio waves at very regular intervals. The timing method consists in measuring any changes in these time intervals. Slight anomalies in the time intervals can be used to detect changes in the pulsar’s motion, which may be caused by one or more nearby planets.

27. The results At the date 30 October, 2007 260 planets are known: – by using the radial velocity method, 247 planets orbiting 213 stars have been discovered. 25 stars host a multiple system; a total of 29 planets transit over their star’s disc; – the gravitational microlensing method has revealed 4 planets revolving around 4 stars; – 4 planets orbiting 4 stars have been discovered by direct observation; – the timing method has revealed 5 planets revolving around 3 stelle (one of which hosts a triple system).

28. Present results Some exoplanets Gliese 876 b – The first planet detected around a red dwarf (Gliese 876). It orbits nearer to its star than Mercury does around the Sun. HD 209458 b – First observed transit of an exoplanet over the disc of its parent star; it also marked the first detection of an exoplanet’s atmosphere. Upsilon Andromedae – The first detection of a multiple planetary system; it is composed by three planets, all Jupiter-type giants.

29. Present results Some exoplanets HD 188753 Ab – This was the first exoplanet discovered in a multiple stellar system (three stars). HD 209458 b e HD 189733b – The first exoplanets whose spectrum was analyzed by direct observation. Gliese 581 c – This planet seems likely to harbour liquid water on its surface – a basic requirement for life. No strong clues supporting the existence of water have been found – yet, the planet is at a suitable distance from the parent star to have the right temperature interval allowing for liquid water. According to the estimates, the planet should be about 50% larger than Earth, and five times more massive.

30. The search for extrasolar planets Some exoplanets Our Solar System, compared with 55 Cancri’s planetary system In this image the inner Solar System is superimposed to the orbit of some exoplanets: HD 179949 b, HD 164427 b, e Reticuli A b, and m Arae b

31. The results The mass distribution Number of planets vs. mass Number of planets vs. mass Number of planets (95) Number of planets (164) Planetmass (MJ) Planetmass (MJ) Left: the mass distribution for smaller exoplanets (M <1 MJ). Right: the mass distribution for larger exoplanets (M >1 MJ). MJ = 1 Jupiter mass = 318 Earth masses

32. The results The distance distribution Number of planets vs. semi-major axis Number of planets vs. semi-major axis Number of planets (247) Number of planets (8) Semi-major axis Semi-major axis Left: exoplanets that are closer to their star than Jupiter is to the Sun. Right: exoplanets that are farther from their star than Jupiter is to the Sun. For comparison, Neptune’s distance from the Sun is 30 AU

33. The results Mass vs. semi-major axis Jupiter Mass of planet (MJ) Semi-major axis (AU)

34. The results Mass vs. semi-major axis Mass of planet (MJ) Semi-major axis (UA)

35. The results Considerations The results obtained so far are obviously incomplete, all methods used being strongly biased towards detection of large-size planets. The discovery of so many ‘hot Jupiters’ prompted a critical discussion and several attempts at reworking the theory of formation of planetary systems – which, in turn, relies upon classical solar nebula models.

36. Our observations The beginnings On July 2007, following a suggestion by Rodolfo Calanca (vice-editor of COELUM Astronomia Magazine as well as Planetary Reseach Team’s co-ordinator), our group joined the national project “Search the Sky!”. Focus of the project was the detection and study of extrasolar planets. Such a task was to be performed by powerful telescopes coupled to high-quality charged couple devices (CCDs) - by now common enough among Italian amateur astronomers.

37. Our observations The beginnings On 26 July, 2007, with our instruments (a home-built f/4.5 – 410mm reflector telescope, plus a SBIG ST7–ME CCD) we began observing transits of exoplanets. The task of taking images and the data reduction were performed by making use of available on-line software. Setting up the f/4.5-410mm telescope. From left to right: Angelo Angeletti, Francesco Barabucci, Fabiano Barabucci and Gianclaudio Ciampechini.

38. Our observations 26 July, 2006 – TrES 2 TrES = Trans-atlantic Exoplanet Survey

39. Our observations 31 July, 2006 – TrES 2 again

40. Our observations 5 and 12 August, 2006: HD209458 On 5 and 12 August, 2006 we have tried to image the transit of HD209458 - the first planet whose transit was imaged using amateur instruments. Alas, we failed. First failure was caused by bad weather. We are still trying to figure out what went wrong on the second attempt!

41. Our observations 17 August, 2006 – TrES 4

42. Our observations 1 September, 2006 – TrES 2 again

43. Our observations 14 September, 2006 – WASP 1 WASP = Wide Angle Search for Planets

44. Our observations 15 September, 2007 – TrES 1

45. Our observations The present Our trial-and-error approach has eventually provided a suitable step-by-step observation sequence, resulting in high-precision imaging of transiting exoplanets. The next step in our schedule: devising and implementing a new observational method enabling the discovery of a new exoplanet by the transit method. (It may be worthwhile to remark that no amateur astronomer has discovered a new extrasolar planet yet)

46. Future work All future work is devoted to a single aim: discovery of Earth-like planets lying within the habitable zone of their planetary system. The image to the right displays theoretical limits of the next generation instruments – either Earth- or space-based – in detecting exoplanets until the year 2015 (lines in colour) (P.R. Lawson, S.C. Unwin e C.A. Beichman, 2004)

47. Future work The habitable zone The habitable zone of a planetary system is the region where a rocky planet might harbour liquid water under stable conditions

48. Future work Astrometry The ESO (European Southern Observatory) is planning a ground- based search for giant planets orbiting some hundred nearby stars; the project is scheduled to start by 2008. It will make use of the PRIMA device, which will be installed upon the great 120-m VLTI (Very Large Telescope Interferometer), which is located in the Chilean Andes.

49. Future work Astrometry • Two space-based projects are completing the preliminary phase: • SIM (Space Interferometry Mission), by NASA, a 20-m interferometer placed upon a beam, is composed of two 40-cm telescopes. Its launch is scheduled for the year 2009. • SIM will search for exoplanets around 1500 stars (among the closest to Sun); the device is sensitive enough to detect exoplanets of some terrestrial masses at a distance of less than 15 light years dal Sole. • - GAIA, by ESA, a device measuring the reciprocal positions of the stars (brighter than the magnitude 20) and their changes with time. • GAIA will be able to detect any change in the position of 1,5 billion stars. Its accuracy is high enough to detect Jupiter-sized exoplanets around 20000 stars. Launch is on schedule for the year 2012.

50. Future work Transits Hundred of small- and middle-sized telescopes (up to one metre) are now active throughout Europe, working hard to detect ‘hot Jupiters’ by taking advantage of transits. As for space-based research, the French space agency (CNES) - in partnership with other European countries - launched this year CoRoT, a 30-cm telescope whose task (among others) is the search for planetary transits over 60000 stars. CoRoT is sensitive enough to successfully detect exoplanets as massive as twice the Earth.