Possible water on HD 209458b
Published on Apr 18, 2007 at 1:16 pm.
2 Comments.
Filed under extrasolar planets.
Yesterday’s Astronomy Picture of the Day was an artists impression of the planet HD 209458b. The short blurb accompanying the picture said that water may have been found it the planet’s atmosphere. Then it went on to make a statement about what this means in the search for life in the universe. I thought that I’d extend this short entry a bit.
First of all, I don’t think that anyone would ever question that there is water out there. Water is one of the most common substances in the universe. Hydrogen and helium are by far the most common elements. But, carbon and oxygen are very common end products of stellar fusion, and the galaxy has a lot of those elements. Oxygen and hydrogen together make water. So, water is common. Even in our own Solar System, there is a lot of water. Now, it isn’t all liquid. Most of it is frozen in the outer parts of the Solar System. Ice being more stable in the outer Solar System allowed there to be more things available to build planets from, so it is no surprise that Uranus and Neptune are as big as they are. It is believed that water makes up a major portion of those worlds. Most of the moons of the outer Solar System are composed of water ice. The comets and Kuiper Belt objects are composed of large amounts of water ice (even Pluto!). It is believed that the deeper cloud layers of Jupiter and Saturn are water vapor clouds. So, water being a part of extrasolar planets is not really a surprise. However, its detection would be a milestone, since it has not previously been detected as such.
Now, a word about this planet. HD 209458b was discovered in 1999 by the subtle shift back and forth that it was having on its parent star (HD 209458). Subsequently, the planet was observed passing directly in front of the star (an event that we call a transit since it did not block much of the star’s light like an eclipse). HD 209458b is a gas giant, having about 70% the mass of Jupiter. But, it is what we call a “hot Jupiter,” being only 0.045 AU from its star. That’s closer than 1/8 of the distance that Mercury is from our own Sun! At that distance, this poor planet is heated to probably well in excess of 1000K, causing it to balloon outward, making it have a very low density (probably less than 0.4 grams per cubic centimeter). The outermost portions of the planets atmosphere would be so distended and heated to such an extend that the planet would have a difficult time holding onto its gasses. Basically, it would slowly be evaporating. That is what the artist’s impression above is trying to show.
The claim short note accompanying APOD’s image refers to a paper by T. Barman soon to be published in the Astrophysical Journal Letters. If you want to read the paper yourself, it is available at the Arxiv:astro-ph preprint site. In this paper, Barman shows that theoretical models of the atmosphere including water match very well the observed infrared spectrum of the transiting planet. Note, however, that the claim is for water clouds, not oceans of water. Furthermore, this planet is far too hot for any life to exist. The comment at APOD is not to be taken that there might be life on this planet, but rather that is shows that water exists on planets outside the Solar System, and that said water existing on just the right sort of planet would be needed for there to be life.
However, the claim of water in the atmosphere of HD 209458b predates this latest paper by at least three years. In 2004, a paper published in the Astrophysical Journal (also available at Arxiv:astro-ph) reported the detection of elemental oxygen in the escaping atmosphere of the planet. This was not taken to mean that the planet had an oxygen rich atmosphere like Earth, but rather that the element was present. The most likely source of the oxygen was from water. The water was being broken apart in the upper parts of the planet’s atmosphere as it was escaping. So, there are no plants making oxygen! They also found a very long hydrogen tail extending from the planet. That is most likely due to the fact that the atmosphere itself is probably composed mostly of hydrogen, as is the atmosphere of the gas giants of our Solar System.
So the new claim that water in the atmosphere is consistent with the observed spectrum is not quite as startling or monumental as they made it out to sound like. However, it is a milestone if further investigations support the claim.
-Astroprof
Image Credit:Â ESA, NASA, Alfred Vidal-Madjar
A Ler…-- Rastos de Luz on April 19, 2007 at 12:32 pm: 1
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dawn kelley on April 20, 2007 at 6:44 pm: 2
I believe that there is life on other planets. That’s basicly what the claim is! we found water we found life! but it’s not on the planetHD209458b. so we must keep on searching.