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Astronomers Observe Heat From Hot Jupiter EMBARGOED UNTIL 14 January, 2009, 1000 HST/1500 EST/2000 WET/2100 CET:

Issued by: Inge Heyer, Public Information Officer
Joint Astronomy Centre
Tel: +1 808 969 6524
Fax: +1 808 961 6516

Images, notes, and contact details appear below.

14 January 2009

Astronomers Observe Heat From Hot Jupiter

Two teams of astronomers have measured light emitted from extrasolar planets around sun-like stars for the first time ever using ground-based telescopes. These results were obtained simultaneously and independently by the two teams for two different planets. These landmark observations open new possibilities for studying exoplanets and their atmospheres.

The measurements were conducted by a team of Astronomers from the University of Leiden, using the William Herschel Telescope (WHT) on La Palma (Canary Islands, Spain) and the United Kingdom Infrared Telescope (UKIRT) on Mauna Kea in Hawai`i. The planet, named TrES-3b, is in a very tight orbit around its host star, TrES-3, transiting the stellar disk once per 31 hours. For comparison, Mercury orbits the sun once every 88 days. TrES-3b is just a little larger than Jupiter, yet orbits around its parent star much closer than Mercury does, making it a "hot jupiter." UKIRT observations caught the transit, from which the size of the planet has been worked out extremely precisely. The WHT observations show the moment the planet moves behind the star, and allow the strength of the planet light to be measured. Astronomers have been trying to observe this effect from the ground for many years, and this is the first success.

Ernst de Mooij, leader of the research team, emphasises, “while a few such observations have been conducted previously from space, they involved measurements at long wavelengths, where the contrast in brightness between the planet and the star is much higher. These are not only the first ground-based observations of this kind, they are also the first to be conducted in the near-infrared, at wavelengths of 2 micron for this planet, where it emits most of its radiation.” Fellow researcher Dr Ignas Snellen adds, “we have been able to measure the temperature of TrES-3b to be a bit over 2000 Kelvin. Since we know how much energy it should receive by the type of its host star, this gives us insights into the thermal structure of the planet's atmosphere, which is consistent with the prediction that this planet should have a so-called 'inversion layer.' It is absolutely amazing that we can now really probe the properties of such a distant world”.

An atmospheric inversion layer is a layer of air where the normal change of temperature with altitude reverses. For example, while we are all familiar with the general decrease of the air temperature as we rise above the ground, often there is a point (usually at a good few thousand feet) where the temperature starts to increase again. This inversion layer prevents air below the inversion layer from escaping to higher altitude. Many places on Earth have strong inversion layers, such as big cities with lots of pollution. Mauna Kea in Hawai`i has a tropical inversion layer about 2,000 feet thick, which usually sits well below the summit. It is this inversion layer that isolates the upper atmosphere from the moist maritime air at lower levels, ensuring that the summit skies are dry and clear, making Mauna Kea such an excellent observing site. It is interesting that the world's great observatories are situated above inversion layers and are now being used to study inversions in planetary atmospheres outside our own solar system.

Current theory says that there are two types of "hot jupiters," one with an inversion layer, and one without. The type is predicted to depend on the amount of light the planet receives from its star. If the inversion layer could be confirmed, for example by measurements at other wavelengths, these observations would fit in perfectly with this theory.

Measuring the emitted light from a planet at different wavelengths reveals the planet's spectrum. This spectrum can be used to determine the planet's day-side temperature. In addition, this spectrum will depend on many physical processes in the planet's atmosphere, such as absorption by molecules like water, carbon monoxide and methane, redistribution of heat around the planet, and temperature structure as a function of height (the aforementioned inversion layer). It will be very useful to be able to compare these for different planets in different environments. "The shorter infrared wavelength targeted in our work is where the planet emits most of its energy and where the molecules have the most influence on the spectrum," says de Mooij.

Alongside the discovery of de Mooij and Snellen, a second team has made a ground-based detection of a different extrasolar planet, OGLE-TR-56b, at the wavelength of 1 micron. Both landmark observations will open up a new window for studying exoplanets and their atmospheres using ground-based telescopes. They show great promise for using future extremely large telescopes which will have much higher sensitivity than the telescopes used today.

Professor Gary Davis, Director of UKIRT, said "this first direct detection of light emitted by another planet, using existing telescopes on the ground, is a major milestone in the study of planets beyond our own Solar System. This is a very exciting scientific discovery, and it nicely demonstrates that existing telescopes like UKIRT and WHT continue to deliver results at the forefront of astronomical research."


TrES-3b is a gas giant like Jupiter, but with an orbit much closer to its star than Mercury is to our Sun. That puts it into the category of "hot jupiter" planets. This graphic illustrates the concept of a "hot jupiter". Credit: Leiden Observatory.

This image shows a comparison between the sizes of the orbits of TrES-3b and Mercury around the primary star. Note that while the orbits are to scale, the sizes of the planets and the star are not.

This image shows the star eclipsing the planet. As the planet disappears behind the star, the light coming from the whole system decreases because of the absence of the planet's light. This allows for precise measurements of the light emitted by the planet.

The William Herschel Telescope. Photo Courtesy of the Isaac Newton Group of Telescopes, La Palma.

The United Kingdom Infrared Telescope on Mauna Kea, Hawaii.

Notes for Editors


The planet TrES-3b was discovered in 2007 by the Transatlantic Exoplanet Survey (TrES). It can be found in the constellation of Hercules at a distance of about 800 light years from the Earth. The planet is about 1.3 times the size of Jupiter.


The planet OGLE-TR-56b was discovered by the Optical Gravitation Lensing Experiment (OGLE) survey in 2002. It can be found in the constellation of Sagittarius at a distance of about 5000 light years from the Earth.

Light Year

One light year is about 10 million million kilometres or 6 million million miles.

Infrared Light

Infrared wavelengths are longer wavelengths than light waves. They are typically measured in microns, also called micrometres. One micron is one millionth of a metre, one 10000th of a centimetre, or one 25000th of an inch.

Planetary Light

Planets give off light in two ways. They can reflect the light they receive from their primary star, usually shortward of 0.5 micron. This has not yet been measured. Planets can also emit their own light, usually longward of 1 micron in the infrared. This has only been observed from space before.


There are three temperatures scales in use: Celsius, Fahrenheit, and kelvin. Water freezes at 0° Celsius, 32° Fahrenheit, and 273 kelvin. Water boils at 100° Celsius, 212° Fahrenheit, and 373 kelvin. 2000 kelvin is 1727° Celsius and 3140° Fahrenheit. 0 kelvin is called "absolute zero," the coldest temperature attainable in the Universe.


The world's largest telescope dedicated solely to infrared astronomy, the 3.8-metre (12.5-foot) UK Infrared Telescope (UKIRT) is sited near the summit of Mauna Kea, Hawaii, at an altitude of 4194 metres (13760 feet) above sea level. It is operated by the Joint Astronomy Centre in Hilo, Hawaii, on behalf of the UK Science and Technology Facilities Council. UKIRT's technical innovation and privileged position on the high, dry Mauna Kea site have placed it at the forefront of infrared astronomy since its opening in 1979. UKIRT is currently engaged in a world-leading infrared sky survey as well as the type of innovative individual programmes described in this press release. More about the UK Infrared Telescope:

Science and Technology Facilities Council

The Science and Technology Facilities Council is an independent, non-departmental public body of the Office of Science and Innovation which itself is part of the Department of Innovation, Universities and Skills. It was formed as a new Research Council on 1 April 2007 through a merger of the Council for the Central Laboratory of the Research Councils (CCLRC) and the Particle Physics and Astronomy Research Council (PPARC) and the transfer of responsibility for nuclear physics from the Engineering and Physical Sciences Research Council (EPSRC). We are one of seven national research councils in the UK. The Science and Technology Facilities Council is government funded and provides research grants and studentships to scientists in British universities, gives researchers access to world-class facilities and funds the UK membership of international bodies such as the European Organisation for Nuclear Research, CERN, the European Space Agency and the European Southern Observatory. It also contributes money for the UK telescopes overseas on La Palma, Hawaii, Australia and in Chile, the UK Astronomy Technology Centre at the Royal Observatory, Edinburgh and the MERLIN/VLBI National Facility.

The William Herschel Telescope

The William Herschel Telescope (WHT) is the largest optical and near-infrared telescope of its kind in Europe, with a primary mirror of diameter 4.2 metres. Its versatile and state-of-the-art instrumentation together with the superb sky quality of the Observatorio del Roque de los Muchachos on La Palma, Canary Islands, have made the WHT one of the most scientifically productive ground-based telescopes in the world. The telescope has played an important role in discovering the accelerated expansion of the Universe, and it has made relevant contributions in the fields of observational cosmology, large-scale structure, galaxy dynamics, star evolution, solar system bodies, and gamma-ray bursts. The WHT is part of the Isaac Newton Group of Telescopes (ING). The ING is owned and operated jointly by the Science and Technology Facilities Council (STFC) of the United Kingdom, the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) of the Netherlands and the Instituto de Astrofísica de Canarias (IAC) of Spain. The telescope is located in the Spanish Observatorio del Roque de los Muchachos which is operated by the IAC.

Netherlands Organisation for Scientific Research

The Netherlands Organisation for Scientific Research (NWO) funds thousands of top researchers at universities and institutes and steers the course of Dutch science by means of subsidies and research programmes. NWO is responsible for enhancing the quality and innovative nature of scientific research as equally initiating and stimulating new developments in scientific research, mainly fulfils its task by allocating resources, facilitates, for the benefit of society, the dissemination of knowledge from the results of research that it has initiated and stimulated, and mainly focuses on university research in performing its task. In fulfilling its responsibilities NWO pays due attention to the aspect of coordination and facilitates this where necessary. NWO wants to ensure that Dutch science continues to be amongst the best in the world and that the currently strong position is further strengthened.

Instituto de Astrofísica de Canarias

The Instituto de Astrofísica de Canarias(IAC) is an international research centre in Spain which comprises: the Instituto de Astrofísica, the headquarters, which is in La Laguna (Tenerife); the Centro de Astrofísica en La Palma (CALP); the Observatorio del Teide (OT), in Izaña (Tenerife); the Observatorio del Roque de los Muchachos (ORM), in Garafía (La Palma). These centres, with all the facilities they bring together, make up the European Northern Observatory (ENO). The IAC is constituted administratively as a Public Consortium, created by statute in 1982, with involvement from the Spanish Government, the Government of the Canary Islands, the University of La Laguna and Spain's Science Research Council (CSIC). The International Scientific Committee (CCI) manages participation in the observatories by institutions from other countries. The IAC's research programme includes astrophysical research and technological development projects. The IAC is also involved in researcher training, university teaching and outreach activities. The IAC has devoted much energy to developming technology for the design and construction of a large 10.4 metre diameter telescope, the (Gran Telescopio CANARIAS, GTC), which is sited at the Observatorio del Roque de los Muchachos.

Media Contacts

Please note that it is best to contact these individuals by email.
  • Inge Heyer, Public Information Officer
    Joint Astronomy Centre
    Desk: +1 808 969 6524
    Fax: +1 808 961 6516
  • Javier Méndez, Public Relations Officer
    Isaac Newton Group of Telescopes
    Desk: +34 922 425 464
    Fax: +34 922 425 401
  • Julia Maddock, Senior Press Officer
    Science and Technology Facilities Council
    Desk: +44 (0)1793 442094
    Fax: +44 (0)1793 442002

Science Contacts

Please note that it is best to contact these individuals by email.
  • Dr Ignas Snellen
    Leiden University
    Desk: +31-715275838
  • Ernst de Mooij
    Leiden University
    Desk: +31-715278492
  • Dr Andy Adamson
    Joint Astronomy Centre
    Desk: +1 808 969 6511
  • Prof. Gary Davis
    Joint Astronomy Centre
    Desk: +1 808 969 6504


This press release refers to two papers published in Astronomy&Astrophysics:
"Ground-based K-band detection of thermal emission from the exoplanet TrES-3b"
(15 January 2009, Volume 493, Issue No.2, pp L31-34, Online Paper PDF Link)
"Ground-based secondary eclipse detection of the very-hot Jupiter OGLE-TR-56b"
(also 15 January 2009, Volume 493, Issue No.2, Online Paper PDF Link).

Web Links

Joint Astronomy Centre - UKIRT
Joint Astronomy Centre Public Outreach Site
Science and Technology Facilities Council
William Herschel Telescope
Netherlands Organisation for Scientific Research
Instituto de Astrofísica de Canarias
This press release
Contact: JAC outreach. Updated: Wed Jul 14 16:30:37 HST 2010

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