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Un planeta tipo Tierra, caliente y con mucha agua

El objeto, descubierto en órbita de una estrella enana roja, ha sido detectado con telescopios de aficionado

A una distancia de unos 40 años luz hay una estrella enana roja, cinco veces más pequeña que el Sol, y a su alrededor gira a gran velocidad un planeta considerado del tipo de la Tierra por su tamaño, aunque es bastante más grande que nuestro planeta. Es un mundo raro porque, a partir de su radio y su tamaño y del efecto gravitatorio que tiene sobre su estrella, los científicos calculan que tiene una densidad baja y deducen que debe estar compuesto de agua helada, en sus tres cuartas partes, y roca el cuarto restante. Su temperatura superficial ronda los 200 grados centígrados, así que es un horno inhóspito, pero los científicos están entusiasmados porque creen que tiene una atmósfera a su alrededor de 200 kilómetros de espesor y, como está relativamente cerca, el telescopio Hubble puede intentar verla directamente y medir sus propiedades. El hallazgo se da a conocer en la revista Nature.

Hubblecast 12: Hubble sees an Extrasolar Planet’s Atmosphere (Murk on a monster planet).

Using the NASA/ESA Hubble Space Telescope, astronomers have found the first clear evidence of high altitude haze or clouds in the atmosphere of an extrasolar planet (exoplanet). This discovery reveals a deeper understanding of the class of giant planets that astronomers call ‘hot Jupiters’.

Over the last 15 years astronomers have discovered more than 270 planets around other stars. A burning question is naturally: What are these so called extrasolar planets like?

Most of them are actually giant gaseous worlds several times the size of Jupiter which is our own Solar Systems largest planet. Others are smaller rocky or icy worlds that are still several times larger than our own home planet Earth. We have yet to discover a world that is similar to our own planet.

Using the Advanced Camera for Surveys (or ACS), the Hubble Space Telescope has recently observed a fascinating large gaseous planet around the star HD 189733. Careful analysis of these very high precision observations by a team lead by Frédéric Pont from the Geneva University Observatory showed that this planet, designated HD 189733b, possesses a murky haze layer extending over an altitude range of about 1000 km in the planets upper atmosphere. The atmosphere of this gaseous planet is at around 800 degrees Centigrade.

This is due to its proximity to its parent star and is the reason that astronomers call this type of planet a hot Jupiter. These hazes are probably made of tiny condensed particles less that 1/1000th of a millimetre across, similar to those already known on Venus and Saturns moon, Titan. Their presence means that the sky over HD 189733b would look very much like a red hazy sunset viewed from an industrially polluted city on Earth.

Whats so special about this particular star and its planet? Bob Fosbury: “This is a rather special planetary system because the star itself is quite small, its only about 75% the size of our Sun and yet the planet itself is larger than Jupiter. So when the planet transits in front of the star it obscures quite a large fraction of light from the star which enables a very accurate measurement.”

What exactly made these observations so precise? Why was this only possible now? Bob Fosbury: “We have to do this from space because when we try and do this from the ground the atmosphere makes it very difficult to make precise measurements of brightness, so we do it from space. And the special thing about this particular observation is that the scientists spread the light out over many pixels in the detector. So rather than just having a little point of light representing the star on the detector the starlight was spread out into a spectrum using this so called grism mode of the Advanced Camera for Surveys.”

“Now, that enables a very precise measurement because you can measure over many many pixels (a large area of detector)but it also, by spreading out the colours, enables you to measure the brightness (or the reduction in brightness) of the starlight over many colours. So having these different measurements in different colours enables you to characterize the nature of the atmosphere.”

The reason why this work was possible at all was because from our vantage point the orbit of HD 189733b is seen almost exactly edge on. Now what that means is that every two days or so the planet actually moves across the face of its parent star as seen from here. Now when that happens some small fraction of the light from the star has to pass through the atmosphere of the planet in front of it in order to reach Earth. Because of this process the composition of the atmosphere of the planet is stamped onto the light like a unique fingerprint. Astronomers can then see this fingerprint in the spectrum of the stars light.

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Hubblecast features news and Images from the Hubble Space Telescope (HST).

The space-based observatory is a collaboration between NASA and ESA. The observations are carried out in visible, infrared and ultraviolet light. In many ways Hubble has revolutionised modern astronomy.

The Hubble Space Telescope has made some of the most dramatic discoveries in the history of astronomy. From its vantage point 600 km above the Earth, Hubble can detect light with “eyes” five times sharper than the best ground-based telescopes and looks deep into space where some of the most profound mysteries are still buried in the mists of time.

• http://www.eso.org
• http://www.spacetelescope.org
• http://hubblesite.org

Documental El universo

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