For the first time, astronomers have been able to directly observe reflected light from a planet circling a distant star, allowing them to ascertain unprecedented details about the “hot Jupiter.” In the past, researchers have been able to obtain some information about the atmosphere of such planets when the planets pass between their star and the Earth, briefly dimming the star’s output. But the newly analyzed planet, called Tau Bootis b, does not transit its star, making such observations impossible. Furthermore, the light from the planet accounts for only about 0.01% of the light emanating from the star system, making its detection extremely difficult.

Tau Bootis b was discovered in 1996, making it one of the first exoplanets known. It was identified by observing fluctuations in the path of its parent star, Tau Bootis, caused by the gravitational influence of its orbit. Tau Bootis is a yellow-white dwarf in the constallation Bootes. At 51 light-years from Earth, it is the nearest star known to have an exoplanet. The star is visible to the naked eye, but astronomers have been trying to see the planet for 15 years.

A team headed by astronomer Matteo Brogi of the Leiden Observatory in the Netherlands used the European Southern Observatory’s Very Large Telescope (VLT) at the Paranal Observatory in Chile to observe the star system. The VLT consists of four telescopes, each with a mirror 8.2 meters (about 27 feet) in diameter. The telescopes can be used individually or can all be focused on one object to provide a very high resolution. The team examined the planet in the infra-red because the star emits less light in that wavelength region, and used new technology to tease out the signal from the planet.

Brogi and his colleagues reported Wednesday in the journal Nature that the planet’s mass is 5.95 times that of Jupiter and that it is orbiting Tau Bootis at an angle of 44.5 degrees as seen from Earth. It orbits its star at a distance less than 5% of that between our sun and the Earth. The planet’s atmosphere has an unusually high concentration of carbon monoxide, and its temperature profile is different from that of other gaseous exoplanets. Previous studies of such planets have shown that the atmosphere gets hottest at the highest altitudes, but Tau Bootis b’s atmosphere cools with altitude. The reason for the difference is not yet clear, but researchers suspect that it is due to the strong ultraviolet emission of the planet.


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