OONN THETHE TRACKTRACK OFOF VERVERYY LOWLOW--MASSMASS PLANETSPLANETS WITHWITH HARPSHARPS

IN ONLY ONE AND A HALF OF OPERATION HARPS HAS DISCOVERED EIGHT NEW EXTRA-SOLAR PLANET CANDIDATES. MANY MORE DISCOVERIES ARE EXPECTED WITH THE INCREASE OF THE DURATION OF THE SURVEY. WHAT MAKES HARPS UNIQUE COMPARED WITH OTHER INSTRUMENTS IS ITS UNPRECEDENTED PRECISION AND ITS ABILITY TO DISCOVER PLANETS WITH VERY LOW . IN FACT, ALL THE PLANETS DISCOVERED WITH HARPS LIE IN THE LOW-MASS TAIL OF THE PLANETARY COMPANION MASS DISTRIBUTION. THIS DISTRIBUTION, WHICH IS KNOWN TO BE HIGHLY SENSITIVE IN ITS LOW MASS END TO THE DETECTION THRESHOLD, CAN BE EXPLORED WITH HARPS IN A COMPREHENSIVE WAY. THE EXPLORATION OF THE VERY LOW-MASS END OF GIANT PLANETS MAY BRING NEW CONSTRAINTS ON PLANET FORMATION AND EVOLUTION SCENARIOS.

FRANCESCO PEPE1, MICHELICHEL MAYOR1, DIDIERIDIER QUELOZ1, WILLILLY BENZ2, JEAN-LOUP BERTAUX3,FRANÇOIS BOUCHY4, CHRISTOPHE LOVIS1, CHRISTOPH MORDASINI2, NUNO SANTOS5, JEAN-PIERREIERRE SIVIVAN4, STÉPHANE UDRY1

1OBSERVATOIRE DE GENÈVE, SWITZERLAND; 2PHYSIKALISCHES INSTITUT, UNIVERSITÄT BERN, SWITZERLAND; 3SERVICE D’AÉRONOMIE DU CNRS, FRANCE; 4LABORATOIRE D’ASTROPHYSIQUE DE MARSEILLE, FRANCE; 5CENTRO DE ASTRONOMIA E ASTROFÍSICA DA UNIVERSIDADE DE LISBOA, OBSERVATÓRIO ASTRONÓMICO DE LISBOA, PORTUGAL

ARPS, THE High-Accura- Intense sequences carried out during astero- m2sini = 14 M ⊕ with an of cy Radial-velocity Planet seismology observations also made it clear P = 9.5 days. This discovery demonstrated Searcher on the 3.6-m ESO that HARPS’capability for planet search was that oscillation noise could be averaged out telescope, became opera- not limited by its performance but rather by sufficiently to unveil a small tional in October 2003. Since the itself. Indeed, stellar p-mode oscilla- signature of a Neptune-mass planet compan- Hthen,H the HARPS Consortium has been car- tions on a short-time scale and stellar jitter on ion. rying out a comprehensive planet-search pro- a long-time scale introduce extra radial-ve- gramme on the assigned Guarantee Time Ob- locity noise that cannot be neglected at the 10 servations (GTO). The first results, which accuracy level of HARPS. For instance, even ]

a very “quiet” G or K dwarf shows oscilla- – were obtained during the first GTO period 1 5 and published in early 2004 (Pepe et al., tion modes of several 10 cms–1 which might 2004), announced the discovery of a Saturn- add up to radial-velocity amplitudes as large 0 as several ms–1. Moreover, any exposure with mass planet with an orbital period of seven -5 R days around the star HD 330075. Besides integration time shorter than the oscillation V +

period of the star might fall arbitrarily on any 9 -10

unveiling a new extra-solar planet, these mea- 3 4 phase of the pulsation cycle leading to ad- 4 surements clearly demonstrated the detection [ 012345678 m efficiency and the amazing radial-velocity ditional radial-velocity noise. This phenom- s BJD – 2 453 160 precision of this instrument (see The Messen- enon may seriously compromise our ability ger Nr. 114, 2003, page 20, for a full descrip- of detecting very low-mass planets around Figure 1: Asteroseismology observations of µ Ara. solar-type by radial-velocity measure- Although the dispersion of the radial velocity tion): First, asteroseismology observations caused by stellar oscillations can be 10 ms–1, one measured a short-time precision (during the ments. easily sees the “low-frequency” variation induced night) of about 20 cms–1. Indeed the stellar In June 2004 a European group of astrono- by the planetary companion on the daily radial- p-mode oscillations were clearly and direct- mers (see ESO Press Release 22/04) proved velocity average. ly identified in the time series of observa- that p-mode radial velocity variability was tions of a few stars. Second, the low residu- not an issue for detecting planets. During Since the discovery of the very low-mass als around the orbital solution of the discov- asteroseismology measurement campaigns companion of µ Ara, other very low-mass ered planet showed that the radial-velocity on the star µ Ara they measured its p-oscil- candidates have been discovered (Butler et accuracy over several months was better than lation modes, but in addition, they also ob- al. 2004, McArthur et al. 2004) . These plan- 2 ms–1. This achievement is even more com- served an unexpected coherent night-to-night etary companions start to populate the lower pelling considering that the instrument suf- variation of very small semi-amplitude of end of the secondary-mass distribution, a fered several modifications during the first 4.1 ms–1 that was later confirmed to be region so far affected by detection incom- months of its life. the signature of a planetary companion of pleteness (see Figure 2). 22 The Messenger 120 sequent evolution over the system’s entire moving as the evolves (see for exam- lifetime has to be considered. In what follows ple Sasselov & Lecar 2000). Detailed mod- we venture into some speculations regarding els of planetary formation including these possible evolutionary scenarios leading to effects have yet to be developed. µ Ara type objects. In the direct collapse scenario planets form MORE NEPTUNE-MASS PLANETS TO COME through gravitational collapse of patches of The semi-amplitude of the radial-velocity the protoplanetary disc (Boss 2002). High- wobble of µ Ara-like objects is hardly larger resolution simulations of this process show than typical stellar p-mode oscillations. The that planets tend to form on elliptical orbits discovery of Neptune-mass planets may only with semi-major axis of several astronomical be feasible by applying an adequate observa-

units and between 1 and 7 MJup. Even tional strategy that includes an increase of the if such a planet would subsequently migrate integration time beyond the typical period of inwards to 0.1 AU, evaporation will not be stellar oscillations, i.e. more than 5 minutes, able to reduce its mass to the one inferred for in order to average them out. In practice, the µ Ara. After all, several planets of the order integration time is fixed to 15 minutes inde- Figure 2: Mass distribution of companions to solar- of a Jupiter mass are observed to exist at these pendently from the star magnitude, provided type stars from 1 Mᓃ down to the mass of Neptune –5 distances in orbit around similar stars. that the detector is not saturated. An example (< 5 10 Mᓃ). The right-hand slope represents the tail of the stellar companion distribution. The left- In the core accretion scenario (Pollack et of the effect of this strategy is presented in hand peak shows the planetary companions. In al 1996; Alibert et al. 2005), forming planets Figure 3, which shows the low residuals of between, the brown-dwarf “desert” appears clear- in this mass range is not trivial either. The 0.9 ms–1 obtained on the radial-velocity curve ly. The very high-precision HARPS GTO survey is devoted to explore the shape of that mass function main reason is that in the standard scenario of the planet-harbouring star HD 102117 from1MJup (0.001 Mᓃ) down to the mass of once the core mass has reached a critical (Lovis et al. 2005). Uranus and Neptune. Note that all newly detected value, the accretion of the gaseous envelope HARPS planets clearly lie in this low-mass range. proceeds in a runaway fashion. Since this crit- HARPS is one of the most sensitive and efficient instruments to explore this domain of the mass ical mass is of order 10–15 M⊕, planets are distribution. It will provide many new inputs for the expected to form either less or significantly formation and evolution theories of . more massive than this value. Ida & Lin (2004) have argued that this mass range should be severely depleted (a planetary des- ert). If correct, the relatively numerous small- mass objects discovered so far start to pose a The discovery of these very low-mass plan- real problem. While mass loss from initially ets close to the detection threshold of radial- more massive objects could possibly account velocity surveys for a limited sample of stars for the planets very close to their star (Baraffe suggests that this kind of object may be rather et al. 2004), it is not clear whether µAra locat- frequent. But already the simple existence of ed at a distance of close to 0.1 AU could ac- such planets may produce headaches for tually result from the evaporation of a more the theoretician. Indeed, statistical consider- massive object. ations predict that a planet with a mass If this were not, or only partially, the case, between 1 and 0.1 MSat and with semi-major the existence of the planetary desert or at least axis of 0.1 to 1 AU must be rare (Ida and Lin, its depth must be questioned. In Ida & Lin 2004). At least for the moment, the recent (2004), the growth rate of a planet is given by Figure 3: Measured radial velocity of HD 102117 in discoveries contradict these predictions. In the -Helmholtz time which becomes phase with the orbital period of the planet. The fitted orbital solution is shown as well. The residu- any case, the continuous detection of planets increasingly shorter with increasing mass. On als of the data points to this solution are only with increasingly lower mass will set new the other hand, in Alibert et al. (2005), the gas 0.9 ms–1 rms. This value includes photon noise and constraints to possible planetary-system for- accretion rate is actually limited by the disc remaining “stellar noise”. mation and evolution models. once the planet has opened a gap (Veras & Armitage 2004). Monte Carlo models are CONSTRAINING FORMATION AND being computed in order to check whether EVOLUTION SCENARIOS this more realistic approach changes the We conduct this strategy on a set of 200 Low-mass giant planets, i.e. planets with a extent and/or the depth of the planetary des- selected stars of the HARPS GTO survey. For mass in the range 10–100 M⊕ are of particu- ert. From an observational point of view a this very high-precision survey an accuracy lar importance in order to constrain forma- larger sample especially of objects for which better than 1 ms–1 is sought for each individ- tion and evolution models. Indeed, and per- evaporation clearly cannot play a role, i.e. far ual measurement. So far the results obtained haps contrary to intuition, the presence of a enough away from the star, would be of para- demonstrate that this strategy is successful. large number of planets within this mass mount importance to constrain these issues. The histogram of the radial-velocity disper- range would not be expected by any of the Finally, given their close location to their sion peaks at 2 ms–1 and decreases rapidly current theoretical formation models. How- star, these small-mass planets are likely to towards higher values. More than 80 % of ever, because all the currently known low- have migrated to their current position from these stars show dispersion smaller than 6 mass planets are located close to their star, further out in the nebula. The chemical com- ms–1, and more than 30% have dispersions one cannot exclude that these objects were position of these planets will depend upon the below 2 ms–1. It must be noted that the dis- much more massive in the past and lost a sig- extent of their migration and the thermal his- persion value includes photon noise, stellar nificant amount of their mass through evap- tory of the nebula and hence the composition oscillations and jitter, and, in particular, it is oration during their lifetime (see for example of the planetesimals along the accretion path “polluted” by known extrasolar planets Baraffe et al. 2004). The situation is therefore of the planet. The situation is made more com- (µ Ara, HD 102117, etc.) or still undetected a complex one in which formation and sub- plicated by the fact that the ice-line itself is planetary companions. © ESO – June 2005 23 Figure 4: Left: Histogram of the radial-velocity scatter for all targets belonging to the very high- precision HARPS planet survey. Right: Cumula- tive distribution of the radial-velocity scatter. The distribution peaks at 2 m/s and is mainly domi- nated by stellar oscillations and jitter. We must point out here that µ Ara (corrected for the drift due to µ Ara b) and HD 102117, are part of this distri- bution, and that the orbital motions of their planets alone induce a radial-velocity scatter of 2.5 ms–1 and 6 ms–1 rms, respectively!

WHERE IS THE LIMIT? The CNES-ESA satellite COROT to be exciting aspect is its capability to explore the The threshold of the lowest-mass planet launched in late 2006 will conduct a transit domain of Neptune-mass planets with short detectable by radial-velocity survey keeps search survey in selected fields. It may detect period where one may expect some overlap decreasing. Today with the current achieved Neptune-sized transiting planet candidates between rocky planets and giant planets. precision of about 1 ms–1 a Neptune mass with orbits similar to µ Ara c. The radial- planet can be discovered. Nobody has yet ex- velocity follow-ups of the COROT planetary THE FUTURE IS STILL BRIGHT plored in detail the domain below the 1 ms–1 candidates is within reach of the capabilities FOR RADIAL VELOCITIES level. The measurements on µ Ara have de- of HARPS. It may deliver the precise mass HARPS has demonstrated that radial-veloci- monstrated that it is possible to beat the stel- of these objects. When this information is ty measurements with an accuracy better than lar pulsation noise by investing sufficient combined with the transit observation par- 1 ms–1 can be achieved. This performance observing time. One open issue however re- ameters one may obtain the mass-radius rela- makes possible an ambitious and exciting mains unsolved: the behaviour of the stars on tion of planets in the domain of Neptune planet search programme. The HARPS GTO a longer time scale, where stellar jitter and masses. This combined approach is current- Planet Search Programme has led so far to the spots may impact the final achievable accu- ly carried out for OGLE planetary candidates discovery of eight new low-mass planets. racy. In this case, an accurate pre-selection of of about the mass and the size of Jupiter and We can easily speculate that many more are the stars may help focusing on “good” candi- we refer the reader to the article of Pont et al. likely to be detected in the future to further dates and optimizing the observation time. In in this issue of The Messenger (page 19) for populate the very low-mass end of the giant addition, bisector analysis and follow-up of more details. In the COROT context the most planet mass distribution. New inputs on the

activity indicators such as log R’HK, as well theory of planetary formation are expected as photometric measurements would allow from these discoveries. Follow-up of COROT the identification of potential error sources. shallow transit candidates may allow us to Nevertheless, the discovery of an extra- push the detection limit to even lower mass. solar planet by means of the Doppler tech- In parallel to these developments, the pos- nique requires that the radial-velocity signal sibility to increase even further the radial- induced by the planet is significantly higher velocity measurement precision is being in- than the dispersion, or requires alternatively vestigated in the frame of the CODEX project a large number of data points. This is partic- (COsmic Differential EXpansion), a visible ularly important to rule out artefacts, given high-resolution spectrograph to be coupled the relatively high number of free parameters with ESO’s concept of an Extremely Large in the orbital solution and more specifically Telescope, the 100-m diameter OWL now for multi-planet systems. A large number of under study. The main scientific driver of measurements could overcome this problem, CODEX is the direct measurement of the uni- but would demand an enormous investment verse’s expansion acceleration-deceleration of observing time. (Monnet G. and D’Odorico S., 2004). A proj- The transit survey may provide another ect group headed by ESO and involving interesting route towards the characterisation several European institutes is investigat- of the very low-mass planets. If one consid- ing the feasibility of such an instrument to ers a transit signal with known orbital period reach a radial-velocity measurement preci- it is obvious that measuring its mass is less sion of 1 cms–1 over an extended time span demanding both on the number and the accu- (> 10 years). The combination of the unique Figure 5: The mass-radius diagram from stars to racy of the radial-velocity measurements. For planets. The low-mass end of the diagram can collecting power of OWLwith the most accu- example a 2 M⊕-planet on a 4- orbit would be investigated by the combination of precise tran- rate radial-velocity measurement techniques produce a radial-velocity amplitude of about sit search and radial-velocity measurements. developed for the extra-solar planet search –1 With COROT and HARPS the Neptune-mass may make the realization of such an instru- 80 cms . Given the present precision of domain will already be accessible. The inserted –1 HARPS which is estimated to about 60 cms diagram on the top left shows a simulation of ment possible. In addition to the determina- it may be possible to detect and measure the the radial-velocity variation caused by a two-Earth- tion of the cosmological model with a dy- amplitude of the radial-velocity wobble with mass planet on a 4-day orbit around a solar-type namical measurement at high redshift, this star. With the HARPS precision of about 60 cms–1 just a few radial-velocity measurements if we only 50 measurements are needed to determine measurement precision will open new possi- knew the period of the system. the planet mass with an accuracy of 10 %. bilities also in the domain of extra-solar plan- 24 The Messenger 120 ets. 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CCONFIRMAONFIRMATIONTION OFOF THETHE FFIRSTIRST IIMAGEMAGE OFOF ANAN EEXTRAXTRA-S-SOLAROLAR PPLANETLANET

Last , the team reported a faint red object Anne-Marie Lagrange, another member of 2MASSWJ1207334-393254 in the close vicinity of a young brown dwarf the team from the Grenoble Observatory in (see ESO PR 23/04). The red object, now France, looks towards the future: “Our dis- called , is more than 100 times covery represents a first step towards one of fainter than the brown dwarf, 2M1207A. The the most important goals of modern astro- spectrum of 2M1207b contains a strong sig- physics: to characterise the physical structure nature of water molecules, confirming that it and chemical composition of giant and, even- must be cold. Based on the infrared colours tually, terrestrial-like planets.” and the spectral data, evolutionary model cal- culations led to the conclusion that 2M1207b (Based on ESO Press Release 12/05) is a 5-Jupiter-mass planet. Its mass can also

778 mas be estimated from a different method, which 55 AU at 70 pc N focuses on the strength of its gravitational field; this technique suggests that the mass E might be even less than 5 Jupiter masses. At the time of its discovery in April 2004, (centre) and its Planetary Companion it was impossible to prove that the faint source (red). The photo is based on three near-infrared is not an unrelated background object. In Feb- exposures (in the H, K and Lo wavebands) with NACO. ESO PR Photo 14a/05. ruary and March of this year, new images were obtained of the young brown dwarf and its giant planet companion with NACO on the VLT. They show with high confidence that the two objects are moving together and hence are gravitationally bound. A team of astronomers1 has confirmed the The new observations therefore show that discovery of a giant planet, approximately this really is a planet, the first planet that has five times the mass of Jupiter, that is gravita- ever been imaged outside of our solar system. tionally bound to a young brown dwarf, put- The separation between the planet and the ting an end to a year-long discussion on the brown dwarf is 55 times the separation of the nature of this object. Earth and . “Given the rather unusual properties of the 2M1207 system, the giant planet most prob- Relative position of 2M1207b with respect to 1 The team consists of Gaël Chauvin and 2M1207A at three different epochs (April 2004, Christophe Dumas (ESO), Anne-Marie Lagrange ably did not form like the planets in our solar February and March 2005). The top panel and Jean-Luc Beuzit (LAOG, Grenoble, France), system,” says Gaël Chauvin, the leader of the shows the separation between the two objects in Benjamin Zuckerman and Inseok Song (UCLA, team. “Instead it must have formed the same milli-arcseconds, while the lower one represents Los Angeles, USA), David Mouillet (LAOMP, the relative angle. The blue line shows the pre- Tarbes, France) and Patrick Lowrance (IPAC, way our Sun formed, by a one-step gravita- dicted change in position if the faint red source Pasadena, USA). tional collapse of a cloud of gas and dust.” were a background object. © ESO – June 2005 25