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."

"Ground-based K-band detection of thermal emission from the exoplanet TrES-3b" (15 January 2009, Volume 493, Issue No.2, pp L31-34); and "Ground-based secondary eclipse detection of the very-hot Jupiter OGLE-TR-56b" also 15 January 2009, Volume 493, Issue No.2)