Category: Extrasolar Planets

A research paper by an international team of astronomers [2] provides sound arguments in favour, but the definitive answer is now awaiting further observations.

On several occasions during the past years, astronomical images revealed faint objects, seen near much brighter stars. Some of these have been thought to be those of orbiting exoplanets, but after further study, none of them could stand up to the real test. Some turned out to be faint stellar companions, others were entirely unrelated background stars. This one may well be different.

In April of this year, the team of European and American astronomers detected a faint and very red point of light very near (at 0.8 arcsec angular distance) a brown-dwarf object, designated 2MASSWJ1207334-393254. Also known as “2M1207”, this is a “failed star”, i.e. a body too small for major nuclear fusion processes to have ignited in its interior and now producing energy by contraction. It is a member of the TW Hydrae stellar association located at a distance of about 230 light-years. The discovery was made with the adaptive-optics supported NACO facility [3] at the 8.2-m VLT Yepun telescope at the ESO Paranal Observatory (Chile).

The feeble object is more than 100 times fainter than 2M1207 and its near-infrared spectrum was obtained with great efforts in June 2004 by NACO, at the technical limit of the powerful facility. This spectrum shows the signatures of water molecules and confirms that the object must be comparatively small and light.

None of the available observations contradict that it may be an exoplanet in orbit around 2M1207. Taking into account the infrared colours and the spectral data, evolutionary model calculations point to a 5 jupiter-mass planet in orbit around 2M1207. Still, they do not yet allow a clear-cut decision about the real nature of this intriguing object. Thus, the astronomers refer to it as a “Giant Planet Candidate Companion (GPCC)” [4].

Observations will now be made to ascertain whether the motion in the sky of GPCC is compatible with that of a planet orbiting 2M1207. This should become evident within 1-2 years at the most.

Just a speck of light
Since 1998, a team of European and American astronomers [2] is studying the environment of young, nearby “stellar associations”, i.e., large conglomerates of mostly young stars and the dust and gas clouds from which they were recently formed.

The stars in these associations are ideal targets for the direct imaging of sub-stellar companions (planets or brown dwarf objects). The leader of the team, ESO astronomer Gael Chauvin notes that “whatever their nature, sub-stellar objects are much hotter and brighter when young – tens of millions of years – and therefore can be more easily detected than older objects of similar mass”.

The team especially focused on the study of the TW Hydrae Association. It is located in the direction of the constellation Hydra (The Water-Snake) deep down in the southern sky, at a distance of about 230 light-years. For this, they used the NACO facility [3] at the 8.2-m VLT Yepun telescope, one of the four giant telescopes at the ESO Paranal Observatory in northern Chile. The instrument’s adaptive optics (AO) overcome the distortion induced by atmospheric turbulence, producing extremely sharp near-infrared images. The infrared wavefront sensor was an essential component of the AO system for the success of these observations. This unique instrument senses the deformation of the near-infrared image, i.e. in a wavelength region where objects like 2M1207 (see below) are much brighter than in the visible range.

The TW Hydrae Association contains a star with an orbiting brown dwarf companion, approximately 20 times the mass of Jupiter, and four stars surrounded by dusty proto-planetary disks. Brown dwarf objects are “failed stars”, i.e. bodies too small for nuclear processes to have ignited in their interior and now producing energy by contraction. They emit almost no visible light. Like the Sun and the giant planets in the solar system, they are composed mainly of hydrogen gas, perhaps with swirling cloud belts.

On a series of exposures made through different optical filters, the astronomers discovered a tiny red speck of light, only 0.8 arcsec from the TW Hydrae Association brown-dwarf object 2MASSWJ1207334-393254, or just “2M1207”, cf. PR Photo 26a/04. The feeble image is more than 100 times fainter than that of 2M1207. “If these images had been obtained without adaptive optics, that object would not have been seen,” says Gael Chauvin.

Christophe Dumas, another member of the team, is enthusiastic: “The thrill of seeing this faint source of light in real-time on the instrument display was unbelievable. Although it is surely much bigger than a terrestrial-size object, it is a strange feeling that it may indeed be the first planetary system beyond our own ever imaged.”

Exoplanet or Brown Dwarf?
What is the nature of this faint object [4]? Could it be an exoplanet in orbit around that young brown dwarf object at a projected distance of about 8,250 million km (about twice the distance between the Sun and Neptune)?

“If the candidate companion of 2M1207 is really a planet, this would be the first time that a gravitationally bound exoplanet has been imaged around a star or a brown dwarf” says Benjamin Zuckerman of UCLA, a member of the team and also of NASA’s Astrobiology Institute.

Using high-angular-resolution spectroscopy with the NACO facility, the team has confirmed the substellar status of this object – now referred to as the “Giant Planet Candidate Companion (GPCC)” – by identifying broad water-band absorptions in its atmosphere, cf. PR Photo 26b/04.

The spectrum of a young and hot planet – as the GPCC may well be – will have strong similarities with an older and more massive object such as a brown dwarf. However, when it cools down after a few tens of millions of years, such an object will show the spectral signatures of a giant gaseous planet like those in our own solar system.

Although the spectrum of GPCC is quite “noisy” because of its faintness, the team was able to assign to it a spectral characterization that excludes a possible contamination by extra-galactic objects or late-type cool stars with abnormal infrared excess, located beyond the brown dwarf.

After a very careful study of all options, the team found that, although this is statistically very improbable, the possibility that this object could be an older and more massive, foreground or background, cool brown dwarf cannot be completely excluded. The related detailed analysis is available in the resulting research paper that has been accepted for publication in the European journal Astronomy & Astrophysics (see below).

Implications

The brown dwarf 2M1207 has approximately 25 times the mass of Jupiter and is thus about 42 times lighter than the Sun. As a member of the TW Hydrae Association, it is about eight million years old.

Because our solar system is 4,600 million years old, there is no way to directly measure how the Earth and other planets formed during the first tens of millions of years following the formation of the Sun. But, if astronomers can study the vicinity of young stars which are now only tens of millions of years old, then by witnessing a variety of planetary systems that are now forming, they will be able to understand much more accurately our own distant origins.

Anne-Marie Lagrange, a member of the team from the Grenoble Observatory (France), looks towards the future: “Our discovery represents a first step towards opening a whole new field in astrophysics: the imaging and spectroscopic study of planetary systems. Such studies will enable astronomers to characterize the physical structure and chemical composition of giant and, eventually, terrestrial-like planets.”

Follow-up observations
Taking into account the infrared colours and the spectral data available for GPCC, evolutionary model calculations point to a 5 jupiter-mass planet, about 55 times more distant from 2M1207 than the Earth is from the Sun (55 AU). The surface temperature appears to be about 10 times hotter than Jupiter, about 1000 ?C; this is easily explained by the amount of energy that must be liberated during the current rate of contraction of this young object (indeed, the much older giant planet Jupiter is still producing energy in its interior).

The astronomers will now continue their research to confirm or deny whether they have in fact discovered an exoplanet. Over the next few years, they expect to establish beyond doubt whether the object is indeed a planet in orbit around the brown dwarf 2M1207 by watching how the two objects move through space and to learn whether or not they move together. They will also measure the brightness of the GPCC at multiple wavelengths and more spectral observations may be attempted.

There is no doubt that future programmes to image exoplanets around nearby stars, either from the ground with extremely large telescopes equipped with specially designed adaptive optics, or from space with special planet-finder telescopes, will greatly profit from current technological achievements.

More information
The results presented in this ESO Press Release are based on a research paper (“A Giant Planet Candidate near a Young Brown Dwarf” by G. Chauvin et al.) that has been accepted for publication and will soon appear in the leading research journal “Astronomy and Astrophysics”. A preprint is available here.

Notes
[1]: This press release is issued simultaneously by ESO and CNRS (in French) .

[2]: The team consists of Gael Chauvin and Christophe Dumas (ESO-Chile), Anne-Marie Lagrange and Jean-Luc Beuzit (LAOG, Grenoble, France), Benjamin Zuckerman and Inseok Song (UCLA, Los Angeles, USA), David Mouillet (LAOMP, Tarbes, France) and Patrick Lowrance (IPAC, Pasadena, USA). The American members of the team acknowledge funding in part by NASA’s Astrobiology Institute.

[3]: The NACO facility (from NAOS/Nasmyth Adaptive Optics System and CONICA/Near-Infrared Imager and Spectrograph) at the 8.2-m VLT Yepun telescope on Paranal offers the capability to produce diffraction-limited near-infrared images of astronomical objects. It senses the radiation in this wavelength region with the N90C10 dichroic; 90 percent of the flux is transmitted to the wavefront sensor and 10 percent to the near-infrared camera CONICA. This mode is particularly useful for sharp imaging of red and very-low-mass stellar or substellar objects. The adaptive optics corrector (NAOS) was built, under an ESO contract, by Office National d’Etudes et de Recherches A?rospatiales (ONERA), Laboratoire d’Astrophysique de Grenoble (LAOG) and the LESIA and GEPI laboratories of the Observatoire de Paris in France, in collaboration with ESO. The CONICA camera was built, under an ESO contract, by the Max-Planck-Institut f?r Astronomie (MPIA) (Heidelberg) and the Max-Planck Institut f?r extraterrestrische Physik (MPE) (Garching) in Germany, in collaboration with ESO.

[4]: What is the difference between a small brown dwarf and an exoplanet ? The border line between the two is still being investigated but it appears that a brown dwarf object is formed in the same way as stars, i.e. by contraction in an interstellar cloud while planets are formed within stable circumstellar disks via collision/accretion of planetesimals or disk instabilities. This implies that brown dwarfs are formed faster (less than 1 million years) than planets (~10 million years). Another way of separating the two kinds of objects is by mass (as this is also done between brown dwarfs and stars): (giant) planets are lighter than about 13 jupiter-masses (the critical mass needed to ignite deuterium fusion), brown dwarfs are heavier. Unfortunately, the first definition cannot be used in practice, e.g., when detecting a faint companion as in the present case, since the observations do not provide information about the way the object was formed. On the contrary, the above mass criterion is useful in the sense that spectroscopy and astrometry of a faint object, together with the appropriate evolutionary models, may reveal the mass and hence the nature of the object.

Original Source: ESO News Release

Astronomers announced today the first discovery of a new class of planets beyond our solar system about 10 to 20 times the size of Earth – far smaller than any previously detected. The planets make up a new class of Neptune-sized extrasolar planets.

In addition, one of the new planets joins three others around the nearby star 55 Cancri to form the first known four-planet system.

The discoveries consist of two new planets. They were discovered by the world renowned planet-hunting team of Drs. Paul Butler and Geoffrey Marcy of the Carnegie Institute of Washington and University of California, Berkeley, respectively; and Barbara McArthur of the University of Texas, Austin. Both findings were peer- reviewed and accepted for future publication in the Astrophysical Journal. NASA and the National Science Foundation funded the research.

“NASA, along with our partner NSF, is extremely proud of this significant planetary discovery,” said Al Diaz, Associate Administrator of NASA’s Science Mission Directorate. “The outcome of the tremendous work of the project scientists is a shining example of the value of space exploration.”

“These Neptune-sized planets prove that Jupiter-sized, gas giants aren’t the only planets out there,” Marcy said. Butler added, “We are beginning to see smaller and smaller planets. Earth-like planets are the next destination.”

Future NASA planet-hunting missions, including Kepler, the Space Interferometry Mission and the Terrestrial Planet Finder, will seek such Earth-like planets. Nearly 140 extrasolar planets have been discovered.

Both of the new planets stick very close to their parent stars, whipping around them in a matter of days. The first planet, discovered by Marcy and Butler, circles a small star called Gliese 436 about every two-and-one-half days at just a small fraction of the distance between Earth and the Sun, or 4.1 million kilometers (2.6 million miles). This planet is only the second known to orbit an M dwarf, a type of low-mass star four-tenths the size of our own sun. Gliese 436 is located in our galactic backyard, 30 light-years away in the constellation Leo.

The second planet, found by McArthur, speeds around 55 Cancri in just under three days, also at a fraction of the distance between Earth and the sun, at approximately 5.6 million kilometers (3.5 million miles). Three larger planets also revolve around the star every 15, 44 and 4,520 days, respectively. Marcy and Butler discovered the outermost of these in 2002. It is still the only known Jupiter-like gas giant to reside as far away from its star as our own Jupiter. The 55 Cancri is about 5 billion years old, a bit lighter than the sun, and is located 41 light-years away in the constellation Cancer. “55 Cancri is a premier laboratory for the study of planetary system formation and evolution,” McArthur said.

Because the new planets are smaller than Jupiter, it is possible they are made of rock, or rock and ice, rather than gas. According to the scientists, the planets may have, like Earth, formed through gradual accumulation of rocky bodies. “A planet of Neptune’s size may not have enough mass to hold onto gas, but at this point we don’t know,” Butler said.

Both discoveries were made using the “radial velocity” technique, in which a planet’s gravitational tug is detected by the wobble it produces in the parent star. Butler, Marcy and collaborators, including Dr. Deborah Fischer of San Francisco State University and Dr. Steven Vogt of the University of California, Santa Cruz, discovered their “Neptune” after careful observation of 950 nearby stars with the W.M. Keck Observatory in Mauna Kea, Hawaii. They were able to spot such a relatively small planet, because the star it tugs on is small and more susceptible to wobbling.

McArthur and collaborators Drs. Michael Endl, William Cochran and Fritz Benedict of the University of Texas discovered their “Neptune” after obtaining over 100 observations of 55 Cancri from the Hobby- Eberly Telescope at McDonald Observatory in West Texas. Combining this data with past data from Marcy, Fischer and Butler from the Lick Observatory in California, and archival data from NASA’s Hubble Space Telescope, the team was able to model the orbit of 55 Cancri’s outer planet. This, in turn, allowed them to clearly see the orbits of the other three inner planets, including the new Neptune-sized one.

Original Source: NASA News Release

A European team of astronomers [1] has discovered the lightest known planet orbiting a star other than the sun (an “exoplanet”).

The new exoplanet orbits the bright star mu Arae located in the southern Altar constellation. It is the second planet discovered around this star and completes a full revolution in 9.5 days.

With a mass of only 14 times the mass of the Earth, the new planet lies at the threshold of the largest possible rocky planets, making it a possible super Earth-like object. Uranus, the smallest of the giant planets of the Solar System has a similar mass. However Uranus and the new exoplanet differ so much by their distance from the host star that their formation and structure are likely to be very different.

This discovery was made possible by the unprecedented accuracy of the HARPS spectrograph on ESO’s 3.6-m telescope at La Silla, which allows radial velocities to be measured with a precision better than 1 m/s. It is another clear demonstration of the European leadership in the field of exoplanet research.

A unique planet hunting machine
Since the first detection in 1995 of a planet around the star 51 Peg by Michel Mayor and Didier Queloz from the Geneva Observatory (Switzerland), astronomers have learned that our Solar System is not unique, as more than 120 giant planets orbiting other stars were discovered mostly by radial-velocity surveys (cf. ESO PR 13/00, ESO PR 07/01, and ESO PR 03/03).

This fundamental observational method is based on the detection of variations in the velocity of the central star, due to the changing direction of the gravitational pull from an (unseen) exoplanet as it orbits the star. The evaluation of the measured velocity variations allows to deduce the planet’s orbit, in particular the period and the distance from the star, as well as a minimum mass [2].

The continued quest for exoplanets requires better and better instrumentation. In this context, ESO undoubtedly took the leadership with the new HARPS spectrograph (High Accuracy Radial Velocity Planet Searcher) of the 3.6-m telescope at the ESO La Silla Observatory (see ESO PR 06/03). Offered in October 2003 to the research community in the ESO member countries, this unique instrument is optimized to detect planets in orbit around other stars (“exoplanets”) by means of accurate (radial) velocity measurements with an unequalled precision of 1 metre per second.

HARPS was built by a European Consortium [3] in collaboration with ESO. Already from the beginning of its operation, it has demonstrated its very high efficiency. By comparison with CORALIE, another well known planet-hunting optimized spectrograph installed on the Swiss-Euler 1.2-m telescope at La Silla (cf ESO PR 18/98, 12/99, 13/00), the typical observation times have been reduced by a factor one hundred and the accuracy of the measurements has been increased by a factor ten.

These improvements have opened new perspectives in the search for extra-solar planets and have set new standards in terms of instrumental precision.

The planetary system around mu Arae
The star mu Arae is about 50 light years away. This solar-like star is located in the southern constellation Ara (the Altar) and is bright enough (5th magnitude) to be observed with the unaided eye.

Mu Arae was already known to harbour a Jupiter-sized planet with a 650 days orbital period. Previous observations also hinted at the presence of another companion (a planet or a star) much further away.

The new measurements obtained by the astronomers on this object, combined with data from other teams confirm this picture. But as Fran?ois Bouchy, member of the team, states: “Not only did the new HARPS measurements confirm what we previously believed to know about this star but they also showed that an additional planet on short orbit was present. And this new planet appears to be the smallest yet discovered around a star other than the sun. This makes mu Arae a very exciting planetary system.”

During 8 nights in June 2004, mu Arae was repeatedly observed and its radial velocity measured by HARPS to obtain information on the interior of the star. This so-called astero-seismology technique (see ESO PR 15/01) studies the small acoustic waves which make the surface of the star periodically pulsate in and out. By knowing the internal structure of the star, the astronomers aimed at understanding the origin of the unusual amount of heavy elements observed in its stellar atmosphere. This unusual chemical composition could provide unique information to the planet formation history.

Says Nuno Santos, another member of the team: “To our surprise, the analysis of the new measurements revealed a radial velocity variation with a period of 9.5 days on top of the acoustic oscillation signal!”

This discovery has been made possible thanks to the large number of measurements obtained during the astero-seimology campaign.

From this date, the star, that was also part of the HARPS consortium survey programme, was regularly monitored with a careful observation strategy to reduce the “seismic noise” of the star.

These new data confirmed both the amplitude and the periodicity of the radial velocity variations found during the 8 nights in June. The astronomers were left with only one convincing explanation to this periodic signal: a second planet orbits mu Arae and accomplishes a full revolution in 9.5 days.

But this was not the only surprise: from the radial velocity amplitude, that is the size of the wobble induced by the gravitational pull of the planet on the star, the astronomers derived a mass for the planet of only 14 times the mass of the Earth! This is about the mass of Uranus, the smallest of the giant planets in the solar system.

The newly found exoplanet therefore sets a new record in the smallest planet discovered around a solar type star.

At the boundary
The mass of this planet places it at the boundary between the very large earth-like (rocky) planets and giant planets.

As current planetary formation models are still far from being able to account for all the amazing diversity observed amongst the extrasolar planets discovered, astronomers can only speculate on the true nature of the present object. In the current paradigm of giant planet formation, a core is formed first through the accretion of solid “planetesimals”. Once this core reaches a critical mass, gas accumulates in a “runaway” fashion and the mass of the planet increases rapidly. In the present case, this later phase is unlikely to have happened for otherwise the planet would have become much more massive. Furthermore, recent models having shown that migration shortens the formation time, it is unlikely that the present object has migrated over large distances and remained of such small mass.

This object is therefore likely to be a planet with a rocky (not an icy) core surrounded by a small (of the order of a tenth of the total mass) gaseous envelope and would therefore qualify as a “super-Earth”.

Further Prospects
The HARPS consortium, led by Michel Mayor (Geneva Observatory, Switzerland), has been granted 100 observing nights per year during a 5-year period at the ESO 3.6-m telescope to perform one of the most ambitious systematic searches for exoplanets so far implemented worldwide. To this aim, the consortium repeatedly measures velocities of hundreds of stars that may harbour planetary systems.

The detection of this new light planet after less than 1 year of operation demonstrates the outstanding potential of HARPS for detecting rocky planets on short orbits. Further analysis shows that performances achieved with HARPS make possible the detection of big “telluric” planets with only a few times the mass of the Earth. Such a capability is a major improvement compared to past planet surveys. Detection of such rocky objects strengthens the interest of future transit detections from space with missions like COROT, Eddington and KEPLER that shall be able to measure their radius.

More information
The research described in this Press release has been submitted for publication to the leading astrophysical journal “Astronomy and Astrophysics”. A preprint is available as a postscript file at http://www.oal.ul.pt/~nuno/.

Notes
[1]: The team is composed of Nuno Santos (Centro de Astronomia e Astrofisica da Universidade de Lisboa, Portugal), Fran?ois Bouchy and Jean-Pierre Sivan (Laboratoire d’astrophysique de Marseille, France), Michel Mayor, Francesco Pepe, Didier Queloz, St?phane Udry, and Christophe Lovis (Observatoire de l’Universit? de Gen?ve, Switzerland), Sylvie Vauclair, Michael Bazot (Toulouse, France), Gaspare Lo Curto and Dominique Naef (ESO), Xavier Delfosse (LAOG, Grenoble, France), Willy Benz and Christoph Mordasini (Physikalisches Institut der Universit?t Bern, Switzerland), and Jean-Louis Bertaux (Service d’A?ronomie de Verri?re-le-Buisson, Paris, France).

[2] A fundamental limitation of the radial-velocity method is the unknown of the inclination of the planetary orbit that only allows the determination of a lower mass limit for the planet. However, statistical considerations indicate that in most cases, the true mass will not be much higher than this value. The mass units for the exoplanets used in this text are 1 Jupiter mass = 22 Uranus masses = 318 Earth masses; 1 Uranus mass = 14.5 Earth masses.

[3] HARPS has been designed and built by an international consortium of research institutes, led by the Observatoire de Gen?ve (Switzerland) and including Observatoire de Haute-Provence (France), Physikalisches Institut der Universit?t Bern (Switzerland), the Service d’Aeronomie (CNRS, France), as well as ESO La Silla and ESO Garching.

Original Source: ESO News Release

Fifteen years ago, the largest telescopes in the world had yet to locate a planet orbiting another star. Today telescopes no larger than those available in department stores are proving capable of spotting previously unknown worlds. A newfound planet detected by a small, 4-inch-diameter telescope demonstrates that we are at the cusp of a new age of planet discovery. Soon, new worlds may be located at an accelerating pace, bringing the detection of the first Earth-sized world one step closer.

“This discovery demonstrates that even humble telescopes can make huge contributions to planet searches,” says Guillermo Torres of the Harvard-Smithsonian Center for Astrophysics (CfA), a co-author on the study.

This research study will be posted online at http://arxiv.org/abs/astro-ph/0408421 and will appear in an upcoming issue of The Astrophysical Journal Letters.

This is the very first extrasolar planet discovery made by a dedicated survey of many thousands of relatively bright stars in large regions of the sky. It was made using the Trans-Atlantic Exoplanet Survey (TrES), a network of small, relatively inexpensive telescopes designed to look specifically for planets orbiting bright stars. A team of scientists co-led by David Charbonneau (CfA/Caltech), Timothy Brown of the National Center for Atmospheric Research (NCAR) and Edward Dunham of Lowell Observatory developed the TrES network. Initial support for the TrES network came from NASA’s Jet Propulsion Laboratory and the California Institute of Technology.

“It took several Ph.D. scientists working full-time to develop the data analysis methods for this search program, but the equipment itself uses simple, off-the-shelf components,” says Charbonneau.

Although the small telescopes of the TrES network made the initial discovery, follow-up observations at other facilities were required. Observations at the W.M. Keck Observatory which, for the University of California, Caltech, and NASA, operates the world’s two largest telescopes in Hawaii, were particularly crucial in confirming the planet’s existence.

Planet Shadows
The newfound planet is a Jupiter-sized gas giant orbiting a star located about 500 light-years from the Earth in the constellation Lyra. This world circles its star every 3.03 days at a distance of only 4 million miles, much closer and faster than the planet Mercury in our solar system.

Astronomers used an innovative technique to discover this new world. It was found by the “transit method,” which looks for a dip in a star’s brightness when a planet crosses directly in front of the star and casts a shadow. A Jupiter-sized planet blocks only about 1/100th of the light from a Sun-like star, but that is enough to make it detectable.

To be successful, transit searches must examine many stars because we only see a transit if a planetary system is located nearly edge-on to our line of sight. A number of different transit searches currently are underway. Most examine limited areas of the sky and focus on fainter stars because they are more common, thereby increasing the chances of finding a transiting system. However the TrES network concentrates on searching brighter stars in larger swaths of the sky because planets orbiting bright stars are easier to study directly.

“All that we have to work with is the light that comes from the star,” says Brown. “It’s much harder to learn anything when the stars are faint.”

“It’s almost paradoxical that small telescopes are more efficient than the largest ones if you use the transit method, since we live in a time when astronomers already are planning 100-meter-diameter telescopes,” says lead author Roi Alonso of the Astrophysical Institute of the Canaries (IAC), who discovered the new planet.

Most known extrasolar planets were found using the “Doppler method,” which detects a planet’s gravitational effect on its star spectroscopically by breaking the star’s light into its component colors. However, the information that can be gleaned about a planet using the Doppler method is limited. For example, only a lower limit to the mass can be determined because the angle at which we view the system is unknown. A high-mass brown dwarf whose orbit is highly inclined to our line of sight produces the same signal as a low-mass planet that is nearly edge-on.

“When astronomers find a transiting planet, we know that its orbit is essentially edge-on, so we can calculate its exact mass. From the amount of light it blocks, we learn its physical size. In one instance, we’ve even been able to detect and study a giant planet’s atmosphere,” says Charbonneau.

Sorting Suspects
The TrES survey examined approximately 12,000 stars in 36 square degrees of the sky (an area half the size of the bowl of the Big Dipper). Roi Alonso, a graduate student of Brown’s, identified 16 possible candidates for planet transits. “The TrES survey gave us our initial line-up of suspects. Then, we had to make a lot of follow-up observations to eliminate the imposters,” says co-author Alessandro Sozzetti (University of Pittsburgh/CfA).

After compiling the list of candidates in late April, the researchers used telescopes at CfA’s Whipple Observatory in Arizona and Oak Ridge Observatory in Massachusetts to obtain additional photometric (brightness) observations, as well as spectroscopic observations that eliminated eclipsing binary stars.

In a matter of two month’s time, the team had zeroed in on the most promising candidate. High-resolution spectroscopic observations by Torres and Sozzetti using time provided by NASA on the 10-meter-diameter Keck I telescope in Hawaii clinched the case.

“Without this follow-up work the photometric surveys can’t tell which of their candidates are actually planets. The proof of the pudding is an orbit for the parent star, and we got that using the Doppler method. That’s why the Keck observations of this star were so important in proving that we had found a true planetary system,” says co-author David Latham (CfA).

Remarkably Normal
The planet, called TrES-1, is much like Jupiter in mass and size (diameter). It is likely to be a gas giant composed primarily of hydrogen and helium, the most common elements in the Universe. But unlike Jupiter, it orbits very close to its star, giving it a temperature of around 1500 degrees F.

Astronomers are particularly interested in TrES-1 because its structure agrees so well with theory, in contrast to the first discovered transiting planet, HD 209458b. The latter world contains about the same mass as TrES-1, yet is around 30% larger in size. Even its proximity to its star and the accompanying heat don’t explain such a large size.

“Finding TrES-1 and seeing how normal it is makes us suspect that HD 209458b is an `oddball’ planet,” says Charbonneau.

TrES-1 orbits its star every 72 hours, placing it among a group of similar planets known as “hot Jupiters.” Such worlds likely formed much further away from their stars and then migrated inward, sweeping away any other planets in the process. The many planetary systems found to contain hot Jupiters indicate that our solar system may be unusual for its relatively quiet history.

Both the close orbit of TrES-1 and its migration history make it unlikely to possess any moons or rings. Nevertheless, astronomers will continue to examine this system closely because precise photometric observations may detect moons or rings if they exist. In addition, detailed spectroscopic observations may give clues to the presence and composition of the planet’s atmosphere.

The paper describing these results is authored by: Roi Alonso (IAC); Timothy M. Brown (NCAR); Guillermo Torres and David W. Latham (CfA); Alessandro Sozzetti (University of Pittsburgh/CfA); Georgi Mandushev (Lowell), Juan A. Belmonte (IAC); David Charbonneau (CfA/Caltech); Hans J. Deeg (IAC); Edward W. Dunham (Lowell); Francis T. O’Donovan (Caltech); and Robert Stefanik (CfA).

This joint announcement is being issued simultaneously by CfA, IAC, NCAR, the University of Pittsburgh, and Lowell Observatory.

The W.M. Keck Observatory is operated by the California Association for Research in Astronomy, a scientific partnership of the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

Original Source: Harvard CfA News Release

Until a decade ago, astronomers weren’t even sure there were any planets outside the Solar System. You’d be hard-pressed to find anyone who believed we had the only planets in the entire Universe, but we still didn’t have any direct evidence they existed. That all changed in October 5, 1995 when Michel Mayor and Didier Queloz announced they had discovered a planet half the mass of Jupiter orbiting furiously around a star called 51 Pegasi. The discoveries came fast; at last count, there are 122 confirmed extrasolar planets.

But these extrasolar systems generally look nothing like our own Solar System. Many contain massive planets which orbit extremely close to their parent star; no chance for life there. Planets roughly the size and orbit of Jupiter have been uncovered, but it’s impossible for the current technology to see anything the size of our own Earth.

Fortunately, there’s a series of ground and space-based observatories in the works that should be capable of detecting Earth-sized planets around other stars. NASA and the ESA are working towards the goal of being able to directly photograph these planets and measure the composition of their atmospheres. Find large amounts of oxygen, and you’ve found life.

Corot – 2006
The European Space Agency will be the first off the mark in the hunt for rocky planets with the launch of Corot in 2006. It’ll carefully monitor the brightness of stars, watching for a slight dimming that happens in regular intervals. These dimmings are called “transits”, and happen when a planet passes in between the Earth and a distant star. The concept of a “transit” should be fresh in your mind – Venus performed one recently on June 8, 2004. Corot will be sensitive enough to detect rocky planets as small as 10 times the size of the Earth.

A follow on mission, Eddington, was originally scheduled for launch in 2007, would have been able to spot planets half the size of the Earth. But it was recently canceled, unfortunately.

Kepler – 2007
The first space observatory designed to find Earth-sized planets in orbit around other stars will be Kepler, named after the German astronomer who devised the laws of planetary motion. It’s scheduled to launch in 2007, and will also use the transit method to detect planets.

Kepler has an extremely sensitive photometer hooked up to its one-metre telescope. It’ll monitor the brightness of hundreds of thousands of stars in a chunk of sky about the same size as your outstretched hand, and watch for that telltale periodic “dimming”.

Over the course of its four year mission, Kepler should discover plenty of objects orbiting other stars, and its photometer is just sensitive enough that it should notice an Earth-sized planet as it crosses in front of a star for a few hours.

Space Interferometry Mission – 2009
Next up will be the Space Interferometry Mission, due for launch in 2009. Once in space, the SIM will take up a position in orbit that trails the Earth as it goes around the Sun, slowly drifting further and further away – this’ll give it a good, stable view of the heavens, without having the Earth around to block the view.

The observatory is designed to measure the distance to stars with incredible precision. It’s so precise, that it should be able to spot a star being moved through the gravitational interaction with its planets. For example, if you looked at the position of our own Sun from a distant point, it would look like it’s wobbling around thanks to the gravity of Jupiter, Saturn, and even the Earth. SIM will be able to detect a star’s interactions with planets down to the size of a few times the mass of the Earth. That’s precise.

Terrestrial Planet Finder – 2012-2015
Unlike the previous missions, which will detect Earth-sized planets indirectly, the Terrestrial Planet Finder (TPF) will “see” them. It’s scheduled for launch in 2012 and will nullify the light from distant stars by a factor of 100,000 times, revealing their planets. The final design is still in the works, but it could end up being a group of spacecraft flying in close formation, merging their light together to form a much larger virtual space telescope.

The TPF will pick up where SIM leaves off, surveying the habitable zone of stars 50 light years away from the Earth. Not only will it be able to see Earth-sized planets in these zones, it’ll be able to analyze the composition of their atmospheres. Here’s the key: the TPF will be able to spot the presence of oxygen, water vapour, methane and carbon dioxide in Earth-sized planets in the habitable zone of other stars. If could find the fingerprint for life in the atmospheres of these planets.

Find life on other planets, and you can assume that it’s probably common throughout our Milky Way galaxy, and maybe even the entire Universe.

Darwin – 2014
Shortly after the TPF gets to work, the European Space Agency is planning to launch Darwin; a flotilla of 8 spacecraft working together to find Earth-sized planets and search for the chemical signatures of life. Darwin will be the most powerful space-based observatory, providing images 10-times more detailed than even the James Webb Space Telescope (due for launch 2009).

Stars are billions of times brighter than the planets that orbit them, so Darwin will solve this problem by observing in the infrared spectrum, where this difference is much smaller. It’ll also be capable of canceling out starlight to reveal the much dimmer planets.

Darwin is similar enough to the Terrestrial Planet Finder, that the two agencies are considering combining their designs into a single mission funded by both groups.

Maybe we aren’t alone after all.
In just a decade, and less than 20 years after the discovery of the first planets orbiting other stars, astronomers should be able to supply us with an answer to one of the most fundamental questions humans have asked themselves… are we alone? If the Terrestrial Planet Finder hasn’t turned up evidence of life yet, then the answer will still be, “not yet”. But there’s a chance that in 10 years, you’ll be reading news that that life has been discovered orbiting another star.

But that won’t be the end of it. The scientists will press on, with new equipment, observatories and techniques to search even deeper into space. And the philosophers and theologians will get to work considering our place in a very crowded Universe.

McDonald Observatory astronomers Bill Cochran, Michael Endl, and Barbara McArthur have exploited the Hobby-Eberly Telescope’s (HET’s) capabilities to rapidly find and confirm, with great precision, the giant telescope’s first planet outside our solar system. The event serves as proof-of-concept that HET, combined with its High Resolution Spectrograph instrument, is on track to become a major player in the hunt for other worlds. The research has been accepted for publication in an upcoming edition of Astrophysical Journal Letters.

With a mass 2.84 times that of Jupiter, the newly discovered planet orbits the star HD 37605 every 54.23 days. HD 37605 is a little smaller and little cooler than the Sun. The star, which is of a type called “K0” or “K-zero,” is rich in heavy chemical elements compared to the Sun.

Of the approximately 120 extrasolar planets found to date, this new planet has the third most eccentric orbit bringing it in close in to its parent star like a “hot Jupiter,” and swinging it back out. The planet’s average distance from its star is 0.26 Astronomical Units (AU). One AU is the Earth-Sun distance.

The team used the “radial velocity” technique, a common planet-search method, to find the planet. By measuring changes in the star’s velocity toward and away from Earth –its wobble– they deduced that HD 37605 is orbiting the center of mass of a star-planet system.

“In 100 days of observations –less than two full orbits– we were able to get a very good solution for this planet’s orbit,” Cochran said. The quick results were due to HET’s “queue scheduling” system. Astronomers do not travel to the observatory to operate the telescope themselves. Rather, a telescope operator at McDonald Observatory has a list of all HET research projects and selects the ones best suited to any given night’s weather conditions and Moon phase. This way, many targets for different research projects can be observed each night, and any particular target can be observed dozens of nights in a row. According to Cochran, “queue scheduling is the ideal way to do planet searching. If the HET had a normal scheduling system, it would have taken us a year or two to confirm this planet.”

Endl added that “with the queue scheduling mode, we can put every candidate star BACK into the queue at a high priority to secure follow-up telescope observations immediately.”

Cochran added that the high precision of the team’s radial velocity measurements “proves that the HET and the High Resolution Spectrograph have met their design specs.” He explained that the total error (called “root-mean-square deviation”) in the team’s velocity measurements was 3 meters per second — state of the art for planet searching. Many of the team’s measurements had even lower errors. The High Resolution Spectrograph that made this research possible was built by Phillip MacQueen, Robert Tull, and John Good of The University of Texas at Austin.

The Hobby-Eberly Telescope is a joint project of The University of Texas at Austin, The Pennsylvania State University (Penn State), Stanford University, Ludwig-Maximilians-Universitat Muenchen, and Georg-August- Universitat Goettingen.

This planet detection research is supported by the National Aeronautics and Space Administration.”

Original Source: University of Texas at Austin News Release

Image credit: David A. Hardy
Astronomers studying the Tau Ceti system have discovered that it contains ten times as much material in the form of asteroids and comets as our own solar system.

Tau Ceti, only 12 light years away, is the nearest sun-like star and is easily visible without a telescope. It is the first star to be found to have a disk of dust and comets around it similar in size and shape to the disk of comets and asteroids that orbits the Sun.

The astronomers’ discovery, being published in Monthly Notices of the Royal Astronomical Society, suggests that even though Tau Ceti is the nearest Sun-like star, any planets that may orbit it would not support life as we know it due to the inevitable large number of devastating collisions. It also suggests that the tranquil space environment around the Earth may be more unusual than previously realized.

Though the star Tau Ceti is similar to the Sun, any planets it has are unlikely to be havens for life, say a team of UK astronomers. Using submillimeter images of the disk of material surrounding Tau Ceti, they found that it must contain more than ten times as many comets and asteroids than there are in the Solar System.

With so many more space rocks hurtling around the star, devastating collisions of the sort that could lead to the destruction of life would be much more likely in the Tau Ceti system than in our own planetary system.

Publication of the result in Monthly Notices of the Royal Astronomical Society coincides with an exhibit ‘Hunting for Planets in Stardust’ at the Royal Society Summer Exhibition by the same science team from the UK Astronomy Technology Centre in Edinburgh and the University of St. Andrews.

The similarity between Tau Ceti and our own sun ends with their comparable sizes and luminosities, explains Jane Greaves, Royal Astronomical Society Norman Lockyer Fellow and lead scientist: ‘Tau Ceti has more than ten times the number of comets and asteroids that there are in our Solar System. We don’t yet know whether there are any planets orbiting Tau Ceti, but if there are, it is likely that they will experience constant bombardment from asteroids of the kind that is believed to have wiped out the dinosaurs. It is likely that with so many large impacts life would not have the opportunity to evolve.’

The discovery means that scientists are going to have to rethink where they look for civilizations outside our Solar System.

Jane Greaves continues, ‘We will have to look for stars which are even more like the Sun, in other words, ones which have only a small number of comets and asteroids. It may be that hostile systems like Tau Ceti are just as common as suitable ones like the Sun.’

The reason for the larger number of comets is not fully understood explains Mark Wyatt, another member of the team: ‘It could be that the Sun passed relatively close to another star at some point in its history and that the close encounter stripped most of the comets and asteroids from around the Sun.’

The new results are based on observations taken with the world’s most sensitive submillimetre camera, SCUBA. The camera, built by the Royal Observatory, Edinburgh, is operated on the James Clerk Maxwell Telescope in Hawaii. The SCUBA image shows a disk of very cold dust (-210 degrees C) in orbit around the star. The dust is produced by collisions between larger comets and asteroids that break them down into smaller and smaller pieces.

Original Source: NASA Astrobiology Story

Image credit: NASA/JPL
NASA has announced new findings from the Spitzer Space Telescope, including the discovery of significant amounts of icy organic materials sprinkled throughout several “planetary construction zones,” or dusty planet-forming discs, which circle infant stars.

These materials, icy dust particles coated with water, methanol and carbon dioxide, may help explain the origin of icy planetoids like comets. Scientists believe these comets may have endowed Earth with some of its water and many of its biogenic, life-enabling materials.

Drs. Dan Watson and William Forrest of the University of Rochester, N.Y, identified the ices. They surveyed five very young stars in the constellation Taurus, 420 light-years from Earth. Previous studies identified similar organic materials in space, but this is the first time they were seen unambiguously in the dust making up planet- forming discs.

In another finding, Spitzer surveyed a group of young stars and found intriguing evidence that one of them may have the youngest planet detected. The observatory found a clearing in the disc around the star CoKu Tau 4. This might indicate an orbiting planet swept away the disc material, like a vacuum leaving a cleared trail on a dirty carpet. The new findings reveal the structure of the gap more clearly than ever before. Because CoKu Tau 4 is about one million years old, the possible planet would be even younger. As a comparison, Earth is approximately 4.5-billion years old.

“These early results show Spitzer will dramatically expand our understanding of how stars and planets form, which ultimately helps us understand our origins,” said Dr. Michael Werner, Spitzer project scientist at NASA’s Jet Propulsion Laboratory , Pasadena, Calif., which manages the mission.

Spitzer also discovered two of the farthest and faintest planet- forming discs ever observed. These discs surround two of more than 300 newborn stars uncovered for the first time in a stunning new image of the dusty stellar nursery called RCW 49. It is approximately 13,700 light-years from Earth in the constellation Centaurus.

“Preliminary data suggest that all 300 or more stars harbor discs, but so far we’ve only looked closely at two. Both were found to have discs,” said Dr. Ed Churchwell of the University of Wisconsin, Madison, Wis., principal investigator of the RCW 49 research, with Dr. Barbara Whitney of Space Science Institute, Boulder, Colo.

Planet-forming, or “protoplanetary,” discs are a natural phase in a star’s life. A star is born inside a dense envelope of gas and dust. Within this envelope, and circling the star, is a flat, dusty disc, where planets are born.

“By seeing what’s behind the dust, Spitzer has shown us star and planet formation is a very active process in our galaxy,” Churchwell said.

Spitzer’s exquisitely sensitive infrared eyes can see planet-forming discs in great detail. “Previously, scientists could study only a small sample of discs, but Spitzer is already on its way toward analyzing thousands of discs,” Werner said.

Spitzer’s infrared spectrograph instrument, which breaks apart infrared light to see the signatures of various chemicals, was used to observe the organic ices and the clearing within CoKu Tau 4’s disc. Spitzer’s infrared array camera found the new stars in RCW 49. Papers on the research will appear in the September 1 issue of the journal Astrophysical Journal Supplements. For images and information about the research on the Internet, visit: http://www.spitzer.caltech.edu/ and http://photojournal.jpl.nasa.gov .

Original Source: NASA News Release

Image credit: NASA/JPL
Included in the nation’s new vision for space is a plan for NASA to “conduct advanced telescope searches for Earth-like planets and habitable environments around other stars.” To meet this challenge, NASA has chosen to fly two separate missions with distinct and complementary architectures to achieve the goal of the Terrestrial Planet Finder. The purpose will be to take family portraits of stars and their orbiting planets, and to study those planets to see which, if any, might be habitable, or might even have life. Both missions would launch within the next 10 to 15 years.

The two missions are:

* Terrestrial Planet Finder-C: a moderate-sized visible-light telescope, similar to the 4- by 6-meter (13.1- by 19.6-foot) version currently under study, to launch around 2014. Onboard coronagraph instrumentation will use a central disc and other specialized techniques to block the glare of a star, allowing detection and characterization of dimmer planets around it.

* Terrestrial Planet Finder-I: multiple spacecraft carrying 3 to 4 meter (9 to 13 foot) infrared telescopes flying in precise formation, to launch before 2020, and to be conducted jointly with the European Space Agency. Combining the infrared, or heat radiation gathered by the multiple telescopes, using a technique called interferometry, will simulate a much larger telescope. This will enable the mission to detect and study individual planets orbiting a parent star observed by TPF-C and also new ones beyond the reach of TPF-C.

Observing extra-solar planets in both visible and infrared light allows scientists to obtain a rich set of data to understand what chemical processes may be going on at various levels in a planet’s atmosphere and surface. That leads to understanding of whether a planet ever could or actually does harbor life. A review of these two plans will be conducted over the summer by NASA and the National Academy of Sciences Committee on Astronomy and Astrophysics. Two other architectures that were studied, the large visible coronagraph and the structurally connected infrared interferometer, will be documented and further studies concluded this summer.

Terrestrial Planet Finder is managed by NASA’s Jet Propulsion Laboratory, Pasadena, Calif., for NASA’s Office of Space Science, Washington, D.C. It is part of NASA’s Origins program, a series of missions and studies designed to answer the questions: Where did we come from? Are we alone?

Original Source: NASA/JPL News Release

Image credit: ESO
A European team of astronomers [1] are announcing the discovery and study of two new extra-solar planets (exoplanets). They belong to the OGLE transit candidate objects and could be characterized in detail. This trebles the number of exoplanets discovered by the transit method; three such objects are now known.

The observations were performed in March 2004 with the FLAMES multi-fiber spectrograph on the 8.2-m VLT Kueyen telescope at the ESO Paranal Observatory (Chile). They enabled the astronomers to measure accurate radial velocities for forty-one stars for which a temporary brightness “dip” had been detected by the OGLE survey. This effect might be the signature of the transit in front of the star of an orbiting planet, but may also be caused by a small stellar companion.

For two of the stars (OGLE-TR-113 and OGLE-TR-132), the measured velocity changes revealed the presence of planetary-mass companions in extremely short-period orbits.

This result confirms the existence of a new class of giant planets, designated “very hot Jupiters” because of their size and very high surface temperature. They are extremely close to their host stars, orbiting them in less than 2 (Earth) days.

The transit method for detecting exoplanets will be “demonstrated” for a wide public on June 8, 2004, when planet Venus passes in front of the solar disc, cf. the VT-2004 programme.

Discovering other Worlds
During the past decade, astronomers have learned that our Solar System is not unique, as more than 120 giant planets orbiting other stars were discovered by radial-velocity surveys (cf. ESO PR 13/00, ESO PR 07/01, and ESO PR 03/03).

However, the radial-velocity technique is not the only tool for the detection of exoplanets. When a planet happens to pass in front of its parent star (as seen from the Earth), it blocks a small fraction of the star’s light from our view. The larger the planet is, relative to the star, the larger is the fraction of the light that is blocked.

It is exactly the same effect when Venus transits the Solar disc on June 8, 2004, cf. ESO PR 03/04 and the VT-2004 programme website. In the past centuries such events were used to estimate the Sun-Earth distance, with extremely useful implications for astrophysics and celestial mechanics.

Nowadays, planetary transits are gaining renewed importance. Several surveys are attempting to find the faint signatures of other worlds, by means of stellar photometric measurements, searching for the periodic dimming of a star as a planet passes in front of its disc.

One of these, the OGLE survey, was originally devised to detect microlensing events by monitoring the brightness of a very large number of stars at regular intervals. For the past four years, it has also included a search for periodical shallow “dips” of the brightness of stars, caused by the regular transit of small orbiting objects (small stars, brown dwarfs or Jupiter-size planets). The OGLE team has since announced 137 “planetary transit candidates” from their survey of about 155,000 stars in two southern sky fields, one in the direction of the Galactic Centre, the other within the Carina constellation.

Resolving the nature of the OGLE transits
The OGLE transit candidates were detected by the presence of a periodic decrease of a few percent in brightness of the observed stars. The radius of a Jupiter-size planet is about 10 times smaller than that of a solar-type star [2], i.e. it covers about 1/100 of the surface of that star and hence it blocks about 1 % of the stellar light during the transit.

The presence of a transit event alone, however, does not reveal the nature of the transiting body. This is because a low-mass star or a brown dwarf, as well as the variable brightness of a background eclipsing binary system seen in the same direction, may result in brightness variations that simulate the ones produced by an orbiting giant planet.

However, the nature of the transiting object may be established by radial-velocity observations of the parent star. The size of the velocity variations (the amplitude) are directly related to the mass of the companion object and therefore allow to discriminate between stars and planets as the cause of the observed brightness “dip”.

In this way, photometric transit searches and radial-velocity measurements combine to become a very powerful technique to detect new exoplanets. Moreover, it is particularly useful for elucidating their characteristics. While the detection of a planet by the radial velocity method only yields a lower estimate of its mass, the measurement of the transit makes it possible to determine the exact mass, radius, and density of the planet.

The follow-up radial-velocity observations of the 137 OGLE transit candidates is not an easy task as the stars are comparatively faint (visual magnitudes around 16). This can only be done by using a telescope in the 8-10m class with a high-resolution spectrograph.

The nature of the two new exoplanets
A European team of astronomers [1] therefore made use of the 8.2-m VLT Kueyen telescope. In March 2004, they followed 41 OGLE “top transit candidate stars” during 8 half-nights. They profited from the multiplex capacity of the FLAMES/UVES fiber link facility that permits to obtain high-resolution spectra of 8 objects simultaneously and measures stellar velocities with an accuracy of about 50 m/s.

While the vast majority of OGLE transit candidates turned out to be binary stars (mostly small, cool stars transiting in front of solar-type stars), two of the objects, known as OGLE-TR-113 and OGLE-TR-132, were found to exhibit small velocity variations. When all available observations – light variations, the stellar spectrum and radial-velocity changes – were combined, the astronomers were able to determine that for these two stars, the transiting objects have masses compatible with those of a giant planet like Jupiter.

Interestingly, both new planets were detected around rather remote stars in the Milky Way galaxy, in the direction of the southern constellation Carina. For OGLE-TR-113, the parent star is of F-type (slightly hotter and more massive than the Sun) and is located at a distance of about 6000 light-years. The orbiting planet is about 35% heavier and its diameter is 10% larger than that of Jupiter, the largest planet in the solar system. It orbits the star once every 1.43 days at a distance of only 3.4 million km (0.0228 AU). In the solar system, Mercury is 17 times farther away from the Sun. The surface temperature of that planet, which like Jupiter is a gaseous giant, is correspondingly higher, probably above 1800 ?C.

The distance to the OGLE-TR-132 system is about 1200 light-years. This planet is about as heavy as Jupiter and about 15% larger (its size is still somewhat uncertain). It orbits a K-dwarf star (cooler and less massive than the Sun) once every 1.69 days at a distance of 4.6 million km (0.0306 AU). Also this planet must be very hot.

A new class of exoplanets
With the previously found planetary transit object OGLE-TR-56 [3], the two new OGLE objects define a new class of exoplanets, still not detected by current radial velocity surveys: planets with extremely short periods and correspondingly small orbits. The distribution of orbital periods for “hot Jupiters” detected from radial velocity surveys seems to drop off below 3 days, and no planet had previously been found with an orbital period shorter than about 2.5 days.

The existence of the three OGLE planets now shows that “very hot Jupiters” do exist, even though they may be quite rare; probably about one such object for every 2500 to 7000 stars. Astronomers are truly puzzled how planetary objects manage to end up in such small orbits, so near their central stars.

Contrary to the radial velocity method which is responsible for the large majority of planet detections around normal stars, the combination of transit and radial-velocity observations makes it possible to determine the true mass, radius and thus the mean density of these planets.

Great expectations
The two new objects double the number of exoplanets with known mass and radius (the three OGLE objects plus HD209458b, which was detected by the radial velocity surveys but for which a photometric transit was later observed). The new information about the exact masses and radii is essential for understanding the internal physics of these planets.

The complementarity of the transit and radial velocity techniques now opens the door towards a detailed study of the true characteristics of exoplanets. Space-based searches for planetary transits – like the COROT and KEPLER missions – together with ground-based radial velocity follow-up observations will in the future lead to the characterization of other worlds as small as our Earth.

Original Source: ESO News Release