MICROLENSING: FIRST EXOPLANET CONFIRMED WITH HST AND KECK-II
Observations taken with the Keck-II telescope and the Hubble Space Telescope by a collaboration including two researchers from the IAP, confirmed the existence of a Uranus-like planet detected via microlensing. This result led to two publications in The Astrophysical Journal, on July 30th, 2015.
The Keck telescope in Hawaii (Mauna Kea, “W.M. Keck Observatory”) and the Hubble Space Telescope (HST, NASA) independently confirmed the existence of a planet orbiting 600 million kilometers away from its star (3.4 astronomical units), i.e. slightly closer than Jupiter is from the Sun. It is estimated to be a 13.2 Earth mass Uranus-like planet orbiting a K5 main sequence star of 0.65 Sun mass. This object is probably an example of a failed-Jupiter, forming a 10 Earth mass core, made of ice and rocks, but in which formation was too slow to efficiently capture a significant mass of hydrogen and helium. This results in a planet twenty times less massive than Jupiter. These objects are predicted to be more common than Jupiter-like planets, especially around stars less massive than the Sun.
The planet, named OGLE-2005-BLG-169Lb, was discovered using the gravitational microlensing technique, and its system is located in the galactic disk, 4,000 parsec (13,000 light years) away from us. It lies in the Sagittarius constellation. First observed in 2005, it is the first time that a planetary microlensing signature is confirmed well after the event.
The microlensing technique probes cold planets with orbits of several astronomical units, near their birth site. Indeed, the core accretion theory of planetary formation predicts that massive planets form beyond the snow line (distance to the star at which water ice grains can form) and may later come closer to the star (migration). A microlensing phenomenon is rare and occurs when a star in the foreground (lens) gets aligned with a background star (source) with a precision of the order of a thousandth of a second of arc (milliarcsecond or mas). The source is generally located in the bulge of the Milky Way, whereas the lens is in the galactic disk, often mid-way between our solar system and the galactic center. The gravitational field of the lens deviates the light coming from the source and concentrates it towards the observer, implying a magnification of the source. If the lens has planetary companions, they can cause additional deviations whose shape and duration reveal the presence of these small bodies. Thus, although astronomers observe the variations of the source flux, it is around the lens that they search for planetary companions.
OGLE-2005-BLG-169Lb was discovered in 2005 by three collaborations: the Optical Gravitational Lensing Experiment (OGLE), the Microlensing Follow-Up Network (MicroFUN), and the Microlensing Observations in Astrophysics (MOA), in the context of the annual microlensing campaign to search for extrasolar planets towards the galactic bulge. This microlensing programme consists of a survey towards the galactic center seven months a year (from March to October), from the southern hemisphere (in Chile, New Zealand, and South Africa). The observations are done in 85 fields of 1.4 square degrees each, covering a total area of ~120 square degrees of the sky. The rate of observations depends on the position of the field, being higher in the fields that are closer to the galactic center, where the star density is higher: it ranges from one image every 15 minutes to one per night. 330 millions of stars are probed continuously, in order to follow several hundreds of microlensing phenomena at the same time. Indeed, the probability for such a phenomenon to occur is one out of one million of stars (more information at http://ogle.astrouw.edu.pl/sky/ogle4-BLG/).
The analysis led in 2005, when OGLE-2005-BLG-169Lb was discovered, did not permit to precisely determine the mass of the host star of the planet, hence limiting the planet mass determination as well. Nevertheless, the modeled planetary perturbation caused by the planet (that is detected in the light curve of the microlensing event) predicts the speed at which the lensing planetary system (star and planet) moves across the background source. This value has been confirmed recently by measuring the angular separation between the source and the lens, using high resolution imaging. Originally, part of the planetary anomaly of OGLE-2005-BLG-169 was not covered by the data and the light curve could be fitted by two models, with different planet-to-star mass ratios and different proper motions. The follow-up observations with the Keck-II telescope made it possible to discriminate between these models.
Actually, the observations with Keck-II led by Virginie Batista and Jean-Philippe Beaulieu (IAP), as well as the observations with HST led by David Bennett (Goddard, Washington DC, USA), confirmed for the first time a planetary detection by microlensing, and allowed them to better identify the planet. First of all, the high resolution images enable to isolate the source-lens couple from any other neighbouring star. Although the observations with HST were done 6.5 years after the microlensing event, the source and the lens were too close to each other to be resolved, but the elongated profile of their combined flux permitted to fit their individual flux profiles. combined to with information provided by the light curve during the microlensing event, the luminosity of the lens yielded the planet mass and the distance to its star, as well as the distance between this system and the earth. The two stars were observed in different filters with the Wide Field Camera 3 of Hubble (WCF3), providing then several independent confirmations of the mass and the distance of the system.
Additional observations performed with the Near Infrared Camera 2 (NIRC2) of the Keck-II telescope, more than 8 years after the microlensing event, gave a precise measurement of the relative proper motion between the source and the lens. The figure below shows the significant improvement in resolution between a 4-m telescope (VISTA http://www.eso.org/public/france/teles-instr/surveytelescopes/vista/, at the European South Observatory (http://www.eso.org/) and the 10-m Keck-II telescope with adaptive optics. It is the first time that the source and the lens are spatially resolved for a planetary microlensing event. Indeed, it generally requires about ten years for them to become clearly separated (by a minimum of ~50-70 mas), because at these distances (several kiloparsecs away), their proper motions, that is their motions in the Milky Way, appear very slow once projected onto the sky. For OGLE-2005-BLG-169, the lens-source relative velocity is of 7.44 mas per year.
Since the first planetary detections ten years ago, we enter in an era of posterior analyses of microlensing systems and reduction of the published uncertainties on their characteristics, thanks to the measurement of the stars relative proper motion. If in the past, the method of planetary detection by microlensing had been disappointing because of large uncertainties on the masses and distances of the detected systems, this technique has been drastically improved during the last decade and reaches a five to ten times higher precision: the uncertainty on the planetary masses is now lower than 10%. Indeed, if before the planet-to-star mass ratio was precisely known, as well as their projected separation in Einstein units (a unit whose conversion into astronomical units depend on the distance of the lensing star), the lack of information on the host star (the lens) was introducing uncertainties on the physical characteristics of the system (absolute mass of the star and the planet and their separating distance in AU).
Today, several tools have been developed to address this problem. Researchers studying microlensing first set up effective means to measure some parallax effects, by increasing the number of ground telescopes and by combining ground observations with observations from space (Spitzer and Kepler-2). This allows them to observe the phenomenon from different points of view, and then to estimate the distances of the lens and the source thanks to the perspective effect. In addition, with complementary high resolution imaging, as presented here, a systematic characterization of the host star will be performed (mass, luminosity, proper motion relative to the background source) for each detected planet during the microlensing observational campaigns by OGLE, MOA, KMTNet*, RoboNet**, microFUN and PLANET***. These improvements are also essential for the preparation of the forthcoming space missions that include microlensing programmes for planets detections, such as the American mission WFIRST (to be launched in 2026), and the European mission EUCLID (to be launched in 2020).
This work was supported by Region Ile-de-France with the “Domaine d’Intérêt Majeur Astrophysique et Conditions d’Apparition de la Vie” (DIM ACAV), the Programme National de Planétologie (PNP), the programme “Promouvoir l’excellence de la recherche à Sorbonne Universités” (PERSU), and NASA.
* Korean Microlensing Telescope Network.
** Robotic Network, Las Cumbres Observatory.
*** Probing Lensing Anomalies Network, Paris IAP.
The leftmost image was obtained with the VISTA 4m telescope of ESO. The sequence of images shows the resolution improvement when the adaptive optics system of the KECK-II telescope is running. The rightmost image shows the resolved source and lens: they are only 60 milliarcsecs apart. This is the first planetary microlens for which such a high precision has been achieved.
Institut d’astrophysique de Paris, Paris, France
batista at iap point fr
Institut d’astrophysique de Paris, Paris, France
beaulieu at iap point fr
University of Notre Dame
bennett at nd point edu