ESO 03/06 - Web Story
Under Embargo till 25 January, 2006, 19:00 CET
It's Far, It's Small, It's Cool: It's an Icy Exoplanet!
Distant Planet Brings Astronomers Closer To Home
Since the discovery ten years ago of the first planet orbiting a normal star other than the Sun, 170 of these 'exoplanets' are now known, belonging to 147 planetary systems. These exoplanets come in very different masses - from below the mass of Neptune to several times the mass of the largest of the planets in our Solar System, Jupiter - and properties: some indeed are very close to their host star, completing a circle in just a little above a day, while others have very elongated orbits, the closest distance to their parent star being more than ten times smaller than the farthest.
In this sense, the exoplanets discovered until now were very different from what astronomers previously thought should exist, taking the Solar System as a role model. When they started their observing campaigns, the scientists were indeed looking for giant planets in nearly circular orbits that are rather far from their star. The new discoveries meant they had to reconsider their ideas about the formation of planets and introduce new mechanisms, such as 'orbital migration'.
The discovery announced today of a planet that seems to follow the earliest expectations is thus a relief for the astronomers as it shows that the Solar System must not be unique in its characteristics. What's more the newly found exoplanet is also most probably the smallest found orbiting a normal star, with a mass of only 5 times the mass of the Earth.
"This planet is actually the first and only planet that has been discovered so far that is in agreement with the theories for how our Solar System formed ", said Uffe Gråe Jørgensen (Niels Bohr Institute, Copenhagen, Denmark), member of the team.
Fitting the Picture
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The new planet, designated by the unglamorous identifier of OGLE-2005-BLG-390Lb, orbits a red dwarf star five times less massive than the Sun. Such red dwarfs are the most common stars of our Galaxy: in the close vicinity of the Sun for example, there are more than 10 times more red dwarfs than solar-like stars. Finding planets around such stars is thus an important step for a complete census of alien worlds.
The favoured theoretical explanation for the formation of planetary systems proposes that solid 'planetesimals' accumulate to build up planetary cores, which then accrete nebular gas - to form giant planets - if they are sufficiently massive. Around red dwarfs, this model favours the formation of Earth- to Neptune-mass planets being between 1 and 10 times the Earth-Sun distance away from their host. The newly found exoplanet fits thus perfectly into this picture.
OGLE-2005-BLG-390Lb is about 3 times further away from its host star than the Earth is from the Sun. It is thus at the same location as the main asteroid belt in the Solar System. Given the lower mass of the parent star, however, the planet takes 10 years to accomplish a full circle (asteroids do that in about 5 years).
The relatively cool and faint parent star and large orbit implies that the likely surface temperature of the planet is 220 degrees Centigrade below zero, too cold for liquid water. It is likely to have a thin atmosphere, like the Earth, but its rocky surface is probably deeply buried beneath frozen oceans. It may therefore more closely resemble a more massive version of Pluto, rather than the rocky inner planets like Earth and Venus.
Einstein's Prediction
Most of the exoplanets detected so far have been found by an indirect method - the measurement of stellar velocity variations. It is based on the gravitational pull of the orbiting planet that causes the central star to move a little back and forth; the heavier the planet, the greater is the associated change in the star's velocity. Certainly the best instrument for this kind of research is the HARPS spectrograph (High Accuracy Radial Velocity Planet Searcher), on the 3.6-m telescope at the ESO La Silla Observatory. HARPS can measure such stellar motions with an unrivalled accuracy of about 1 metre per second (m/s), cf. ESO PR 06/03 and was therefore able to find several exoplanets, including the first discovered rocky planet, having a mass of 13 times the mass of the Earth.
The newly discovered planet on the other hand is one of the few to have been discovered by the microlensing technique.
The light from a distant star is affected by the gravity of the objects it passes on its way to us. This effect was predicted by Albert Einstein early last century and observationally confirmed in 1919 when a solar eclipse allowed the study of stars close to the line of sight of the Sun. Accurate positional measurements showed that the light from those remote stars was bent by the Sun's gravitational field.
However, the light may not only be deflected, it can also be amplified. In that case, the intervening object works like a giant 'magnifying lens' that concentrates the light from the distant source.
Effects of gravitational optics in space were first observed in 1979. When produced by extended, very heavy clusters of galaxies, they may appear as large, spectacular arcs and well-separated multiple images, cf. ESO PR Photos 46d/98 and 46f/98. Less massive lenses, however, produce images with extensions that are too small to be distinguished directly.
Microlensing
Such 'microlensing' effects occur when a compact body (usually a Milky Way star moving in its galactic orbit) passes almost directly between the observer and a luminous background object (usually also a star). One then sees that the brightness of that object rises and falls as the lens passes across the line-of-sight. The observed light intensity is described by a so-called "light curve", cf. PR Photo 16a/01. Normally, the lensing object is a faint low-mass star, one of the most common objects in the Milky Way.
In most cases, these low-mass stars are too faint to be directly observed. This is especially so in crowded sky fields in which there are many much brighter stars - including the luminous giant stars that are monitored for microlensing effects. However, the gravity of a low-mass star is strong enough to produce a lensing effect if the geometrical alignment is sufficiently precise. This happens rarely, but by looking at a large number of background stars, it has been possible to detect a fair number of microlensing events during the past few years.
International collaborations like the Optical Gravitational Lensing Experiment (OGLE) and the Microlensing Observations in Astrophysics (MOA) collaboration scan the skies continuously for such microlensing events which typically last from a few weeks to some months. When a star is found to brighten in a way that looks like what is expected from microlensing, they send electronic alerts to other dedicated teams like Probing Lensing Anomalies NETwork (PLANET) and Microlensing Planet Search Project (MPS) who then intensively monitor the possible lensing events.
One of the main goals of these research programmes is to search for "dark matter". Indeed, microlensing effects are excellent tools for learning more about this mysterious component of the Universe, as they provide information about lensing objects that otherwise are too faint to be observed.
However, microlensing events may also provide very useful information about the background object (the "source"), the light of which is amplified and magnified.
"The probability that a given star undergoes a microlensing event at a given time is only about one in a million", explained Andrzej Udalski (Warsaw University Observatory, Poland), leader of the Polish-American OGLE team. "However, with more than 100 million stars being routinely monitored by OGLE each night with the Warsaw 1.3m telecope at Las Campanas Observatory (Chile), we can provide the scientific community with about 120 ongoing events and nearly 1000 events per year."
Finding a Planet with PLANET
"With this method, we let the gravity of a dim, intervening star act as a giant natural telescope for us, magnifying a more distant star, which then temporarily looks brighter ", explained team member Andrew Williams (Perth Observatory, Australia). "A small 'defect' in the brightening reveals the existence of a planet around the lens star. We don't see the planet, or even the star that it's orbiting, we just see the effect of their gravity. "
Such an intervening star causes a characteristic brightening that lasts about a month. Any planets orbiting this star can produce an additional signal, lasting days for giant planets down to hours for Earth-mass planets.
In order to be able to catch and characterize these planets, nearly-continuous round-the-clock high-precision monitoring of ongoing microlensing events is required. This is achieved by the PLANET network of 1m-class telescopes consisting of the ESO 1.54m Danish at La Silla (Chile), the Canopus Observatory 1.0m (Hobart, Tasmania, Australia), the Perth 0.6m (Bickley, Western Australia), the Boyden 1.5m (South Africa), and the SAAO 1.0m (Sutherland, South Africa). Since 2005, PLANET operates a common campaign with RoboNet, a UK operated network of 2m fully robotic telescopes currently comprising the Liverpool Telescope (Roque de Los Muchachos, La Palma, Spain) and the Faulkes Telescope North (Haleakala, Hawaii, USA).
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The OGLE serach team discovered the event OGLE-2005-BLG-390 on 11 July 2005, triggering the PLANET telescopes to start taking data. A light curve consistent with a single lens star peaking at an amplification of about 3 on 31 July 2005 was observed, until 10 August when PLANET member Pascal Fouqué, observing at the Danish 1.54m at ESO La Silla, noticed a planetary deviation. An OGLE point from the same night showed the same trend, while the last half of the planetary deviation, lasting about a day, had been covered by images from Perth Observatory. The MOA collaboration was later able to identify the source star on its images and confirmed the deviation.
Distant but Common
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The fact that the newly found exoplanet was discovered through the microlensing technique explains why it is also the farthest found. The OGLE indeed monitors the region towards the Centre of the Milky Way were many stars are present, increasing the odds of an event. The parent star lies thus close to the Centre and is therefore more than 20 000 light-years away. On the other hand, to find exoplanet with the radial velocity technique, one need the stars to be bright - and hence rather close - to be able to study them into detail.
OGLE 2005-BLG-390Lb is only the third extra-solar planet resulting so far from microlensing searches. The other two microlensing planets, detected in events OGLE 2003-BLG-235/MOA 2003-BLG-053 and OGLE 2005-BLG-071 have masses of a few times that of Jupiter. Thus Jupiter- or Saturn-like gas giant planets, which are much easier to detect than less massive rocky/icy planets, appear to be rare around red dwarfs. Since microlensing relies on the gravity of the lens star, rather than on its light, these are favoured due to their large abundance and constituted the parent star for all three detected planets. The discovery of a rocky/icy planet already as the third one detected through microlensing is a strong hint that, in contrast to massive gas giants, these objects are quite common, in qualitative agreement with the prediction from theoretical models.The microlensing technique is most probably the only method currently capable of detecting planets similar to Earth. "The search for a second Earth is the driving force behind our research and this discovery constitutes a major leap forward since it is the most Earth-like planet we know of so far ", said co-author Daniel Kubas, from ESO.
High resolution images and their captions are available on
this page.
This press release is also accompanied by Broadcast quality material.
Contacts
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Jean-Philippe Beaulieu |
Pascal Fouqué |
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Martin Dominik |
Uffe G. Jørgensen |
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Daniel Kubas |
David P. Bennett |
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Andrew Williams |
Andrzej Udalski |
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Joachim Wambsganss |
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