Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Glowing Hot Transiting Exoplanet Discovered

22.04.2003


VLT Spectra Indicate Shortest-Known-Period Planet Orbiting OGLE-TR-3

More than 100 exoplanets in orbit around stars other than the Sun have been found so far. But while their orbital periods and distances from their central stars are well known, their true masses cannot be determined with certainty, only lower limits.

This fundamental limitation is inherent in the common observational method to discover exoplanets - the measurements of small and regular changes in the central star’’s velocity, caused by the planet’’s gravitational pull as it orbits the star.



However, in two cases so far, it has been found that the exoplanet’’s orbit happens to be positioned in such a way that the planet moves in front of the stellar disk, as seen from the Earth. This "transit" event causes a small and temporary dip in the star’’s brightness, as the planet covers a small part of its surface, which can be observed. The additional knowledge of the spatial orientation of the planetary orbit then permits a direct determination of the planet’s true mass.

Now, a group of German astronomers [1] have found a third star in which a planet, somewhat larger than Jupiter, but only half as massive, moves in front of the central star every 28.5 hours. The crucial observation of this solar-type star, designated OGLE-TR-3 [2] was made with the high-dispersion UVES spectrograph on the Very Large Telescope (VLT) at the ESO Paranal Observatory (Chile).

It is the exoplanet with the shortest period found so far and it is very close to the star, only 3.5 million km away. The hemisphere that faces the star must be extremely hot, about 2000 degrees and the planet is obviously losing its atmosphere at high rate.

The search for exoplanets

More than 100 planets in orbit around stars other than the Sun have been found so far. These "exoplanets" come in many different sizes and they move in a great variety of orbits at different distances from their central star, some nearly round and others quite elongated. Some planets are five to ten times more massive than the largest one in the solar system, Jupiter - the lightest exoplanets known at this moment are about half as massive as Saturn, i.e. about 50 times more massive than the Earth.

Astronomers are hunting exoplanets not just to discover more such objects, but also to learn more about the apparent diversity of planetary systems. The current main research goal is to eventually discover an Earth-like exoplanet, but the available telescopes and instrumentation are still not "sensitive" enough for this daunting task.

However, also in this context, it is highly desirable to know not only the orbits of the observable exoplanets, but also their true masses. But this is not an easy task.

Masses of exoplanets

Virtually all 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.

This technique is rapidly improving: the new HARPS spectrograph (High Accuracy Radial Velocity Planet Searcher), now being tested on the 3.6-m telescope at the ESO La Silla Observatory, can measure such stellar motions with an unrivalled accuracy of about 1 metre per second (m/s), cf. ESO PR 06/03. It will shortly be able to search for exoplanets only a few times more massive than the Earth.

However, velocity measurements alone do not allow to determine the true mass of the orbiting planet. Because of the unknown inclination of the planetary orbit (to the line-of-sight), they only provide a lower limit to this mass. Additional information about this orbital inclination is therefore needed to derive the true mass of an exoplanet.

The transit method

Fortunately, this information becomes available if the exoplanet is known to move across ("transit") the star’s disk, as seen from the Earth; the orbital plane must then necessarily be very near the line-of-sight. This phenomenon is exactly the same that happens in our own solar system, when the inner planets Mercury and Venus pass in front of the solar disk, as seen from the Earth [3]. A solar eclipse (caused by the Moon moving in front of the Sun) is a more extreme case of the same type of event.

During such an exoplanet transit, the observed brightness of the star will decrease slightly because the planet blocks a part of the stellar light. The larger the planet, the more of the light is blocked and the more the brightness of the star will decrease. A study of the way this brightness changes with time (astronomers refer to the "light curve"), when combined with radial velocity measurements, allows a complete determination of the planetary orbit, including the exact inclination. It also provides accurate information about the planet’s size, true mass and hence, density.

The chances that a particular exoplanet passes in front of the disk of its central star as seen from the Earth are small. However, because of the crucial importance of such events in order to characterize exoplanets fully, astronomers have for some time been actively searching for stars that experience small regularly occurring "brightness dips" that might possibly be caused by exoplanetary transits.

The OGLE list

Last year, a first list of 59 such possible cases of stars with transiting planets was announced by the Optical Gravitational Lensing Experiment (OGLE) [2]. These stars were found - within a sample of about 5 million stars observed during a 32-day period - to exhibit small and regular brightness dips that might possibly be caused by transits of an exoplanet.

For one of these stars, OGLE-TR-56, a team of American astronomers soon thereafter observed slight variations of the velocity, strongly indicating the presence of an exoplanet around that star.

UVES spectra of OGLE-TR-3

Now, a team of German and ESO astronomers [1] have used the UVES High-Dispersion Spectrograph on the 8.2-m VLT KUEYEN telescope at the Paranal Observatory (Chile) to obtain very detailed spectra of another star on that list, OGLE-TR-3, cf. PR Photos 10a-b/03.

Over a period of one month, a total of ten high-resolution spectra - each with an exposure time of about one hour - were obtained of the 16.5-mag object, i.e. its brightness is about 16,000 fainter that what can be perceived with the unaided eye. A careful evaluation shows that OGLE-TR-3 is very similar to the Sun, with a temperature of about 5800 centigrades (6100 K). And most interestingly, it undergoes velocity variations of the order of 120 m/s.

The exoplanet at OGLE-TR-3

The 2 per cent dip in the brightness of OGLE-TR-3, as observed during the OGLE programme, occurs every 28 hours 33 minutes (1.1899 days), cf. PR Photo 10e/03. The UVES velocity measurements (PR Photo 10d/03) fit this period well and reveal, with high probability, the presence of an exoplanet orbiting OGLE-TR-3 with this period. In any case, the observations firmly exclude that the well observed brightness variations could be due to a small stellar companion. A red dwarf star would have caused velocity variations of 15 km/s and a brown dwarf star 2.5 km/s; both would have been easy to observe with UVES, and it is clear that such variations can be excluded.

Although the available observations are still insufficient to allow an accurate determination of the planetary properties, the astronomers provisionally deduce a true mass of the planet of the order of one half of that of Jupiter. The density is found to be about 250 kg/m3, only one-quarter of that of water or one-fifth of that of Jupiter, so the planet is quite big for this mass - a bit "blown up". It is obviously a planet of the gaseous type.

A very hot planet

The orbital period, 28 hours 33 minutes (1.1899 days), is the shortest known for any exoplanet and the distance between the star and the planet is correspondingly small, only 3.5 million kilometres. The temperature of the side of the planet facing the star must therefore be very high, of the order of 2000 °C. Clearly, the planet must be losing its atmosphere by evaporation. The astronomers also conclude that it might in fact be possible to observe this exoplanet directly because of its comparatively strong infrared radiation. An attempt to do so will soon be made.

As only the third exoplanet found this way (after those at the stars HD209458 and OGLE-TR-56), the new object confirms the current impression that a considerable number of stars may possess giant planets in close orbits. Since such planets cannot form so close to their parent star, they must have migrated inwards to the current orbit from a much larger, initial distance. It is not known at this time with certainty how this might happen.

Future prospects

It is expected that more observational campaigns will be made to search for transiting planets around other stars. There is good hope that OGLE-TR-3 and OGLE-TR-56 are just the first two of a substantial number of exoplanets to be discovered this way.

Some years from now, searches will also begin from dedicated space observatories, e.g. ESA’s Eddington and Darwin, and NASA’s Kepler.

Richard West | alfa
Further information:
http://www.eso.org/outreach/press-rel/pr-2003/pr-09-03.html

More articles from Physics and Astronomy:

nachricht NASA's Fermi catches gamma-ray flashes from tropical storms
25.04.2017 | NASA/Goddard Space Flight Center

nachricht DGIST develops 20 times faster biosensor
24.04.2017 | DGIST (Daegu Gyeongbuk Institute of Science and Technology)

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Making lightweight construction suitable for series production

More and more automobile companies are focusing on body parts made of carbon fiber reinforced plastics (CFRP). However, manufacturing and repair costs must be further reduced in order to make CFRP more economical in use. Together with the Volkswagen AG and five other partners in the project HolQueSt 3D, the Laser Zentrum Hannover e.V. (LZH) has developed laser processes for the automatic trimming, drilling and repair of three-dimensional components.

Automated manufacturing processes are the basis for ultimately establishing the series production of CFRP components. In the project HolQueSt 3D, the LZH has...

Im Focus: Wonder material? Novel nanotube structure strengthens thin films for flexible electronics

Reflecting the structure of composites found in nature and the ancient world, researchers at the University of Illinois at Urbana-Champaign have synthesized thin carbon nanotube (CNT) textiles that exhibit both high electrical conductivity and a level of toughness that is about fifty times higher than copper films, currently used in electronics.

"The structural robustness of thin metal films has significant importance for the reliable operation of smart skin and flexible electronics including...

Im Focus: Deep inside Galaxy M87

The nearby, giant radio galaxy M87 hosts a supermassive black hole (BH) and is well-known for its bright jet dominating the spectrum over ten orders of magnitude in frequency. Due to its proximity, jet prominence, and the large black hole mass, M87 is the best laboratory for investigating the formation, acceleration, and collimation of relativistic jets. A research team led by Silke Britzen from the Max Planck Institute for Radio Astronomy in Bonn, Germany, has found strong indication for turbulent processes connecting the accretion disk and the jet of that galaxy providing insights into the longstanding problem of the origin of astrophysical jets.

Supermassive black holes form some of the most enigmatic phenomena in astrophysics. Their enormous energy output is supposed to be generated by the...

Im Focus: A Quantum Low Pass for Photons

Physicists in Garching observe novel quantum effect that limits the number of emitted photons.

The probability to find a certain number of photons inside a laser pulse usually corresponds to a classical distribution of independent events, the so-called...

Im Focus: Microprocessors based on a layer of just three atoms

Microprocessors based on atomically thin materials hold the promise of the evolution of traditional processors as well as new applications in the field of flexible electronics. Now, a TU Wien research team led by Thomas Müller has made a breakthrough in this field as part of an ongoing research project.

Two-dimensional materials, or 2D materials for short, are extremely versatile, although – or often more precisely because – they are made up of just one or a...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Expert meeting “Health Business Connect” will connect international medical technology companies

20.04.2017 | Event News

Wenn der Computer das Gehirn austrickst

18.04.2017 | Event News

7th International Conference on Crystalline Silicon Photovoltaics in Freiburg on April 3-5, 2017

03.04.2017 | Event News

 
Latest News

NASA's Fermi catches gamma-ray flashes from tropical storms

25.04.2017 | Physics and Astronomy

Researchers invent process to make sustainable rubber, plastics

25.04.2017 | Materials Sciences

Transfecting cells gently – the LZH presents a GNOME prototype at the Labvolution 2017

25.04.2017 | Life Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>