Thu, 13 Aug 2009
WASP-17b is the first planet to be discovered in a retrograde orbit: it rotates about its star in the opposite direction to the spin of the star. (See the nice BBC News write-up, or the ArXiv preprint for the more technical details).
We think planetary systems are formed from a disk of debris as the star forms, and the disk and star would rotate the same way, so most planets rotate pro-grade, in the same direction of the star. For retrograde motion, it would have to have been hit early-on by some large object (another planetary embryo, for example, being ejected from the stars 'solar system' in the game of cosmic billiards as they coalesce to form planets).
Within our solar system we see retrograde motion in Venus as it spins (on its own axis) in the opposite direction of other planets, but this is the first time we've seen a planet orbit in retrograde. Similarly, Uranus' spin axis is tilted at 89 degrees, but it orbits in a pro-grade manner; its thought that Uranus was hit by a giant object causing its tilt. As we find more exoplanets, the statistics of how many are pro-grade or retrograde will help us learn about the formation of planetary systems.
WASP-17b is a "Hot Jupiter", discovered by the Wide Area Search for Planets (WASP) consortium of UK Universities by Radial velocity methods.
Thu, 15 Jan 2009
From Today's Astronomy & Astrophysics there were two papers from the Geneva Observatory / Grenoble team on The HARPS search for southern extra-solar planets:
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XIII. A planetary system with 3 super-Earths (4.2, 6.9, and 9.2 Me )
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XIV. Gl 176b, a super-Earth rather than a Neptune, and at a different period
These are probably part of the 45 super-Earths tentively announced by M. Mayor last June at the Nantes conference on super-Earths. He showed radial-velocity diagrams for 45 or so planets in an impressive display, but only gave details of three at the time, while the rest were being peer-reviewed, etc. Now they're drip-feeding out.Interestingly, this now makes 3 out of 7 super-Earths do far discovered as being around M dwarf stars, and another multiple planetary system. It looks more like the "metallicity" relation (larger and more frequent planets around more metallic stars) seems to be more true for gas giants than terrestrial planets. Super-Earths (potentially rocky planets, though heavier than Earth) look more abundant around low mass and less-metal stars.
Now, down to work examining the characteristics of these planets. Rory Barnes and colleagues at LLPL, U. of Arizona have also been looking at these three planets around HD 40307, to determine if they are super-Earths or mini-Neptunes. They have an interesting technique of modelling the tidal circularisation of the planet backwards in time, based on its current measured or estimated eccentiricity. Tidal circularisation makes the orbit of close-in planets more circular (decreasing the eccentricity, e, to zero), dumping the energy into tidal heating of the planet (and the star). Doing this for Gl 437, for example, they showed that it would be hotter than Io, with a volcanic surface and couldn't be habitable. For HD 403007b, Hence they believe the planets are not terrestrial-like, but may be more Neptune-like.
So, could these planets have oceans ?
Sun, 29 Jul 2007
Thanks to Centauri Dreams for the pointer to the ESA DARWIN proposal now here (high-resolution version also here). DARWIN is a space mission to detect the presence of life on other planets by spectroscopy: looking for biomarkers such as the presence of Ozone, water and methane together on terrestrial-sized planets. These are groups of gases that collectively would imply the presence of life.
Unfortunately it appears that DARWIN is slipping: its not currently hardware-funded, and has a planned launch-date of 2020, having slipped from 2012. This is when NASAs alternative Terrestrial Planet Finder and Space Interferometry Mission have been defunded indefinitely.
At the same time, the French COROT mission has just announced its first month-long series light-curves. COROT hunts for Earth-sized planets: it's a small space telescope that stares at the same patch of sky for long periods, looking for changes in the amount of light from stars that indicate a planet passing in front of a star. It has been much more successful than expected, having already announced its first exoplanet detection in April. NASA is also launching a similar mission, Kepler, in 2008.
So we face the prospect that any day now we may know of Earth-sized planets in a Habitable zone, and yet be over a decade from determining if they could contain life.
So do we have any other tricks up our sleeves? Other possibilities include the James Webb Space Telescope, the successor to the Hubble. With a launch planned in 2013, it would be able to determine some atmospheric features found in COROT detections, but would be too expensive to do the planet-hunting Darwin could do. Perhaps this could be imroved with occulters such as UMBRAS (thanks to enzo). Other novel detection missions include the Voyager Interstellar Mission (via), reusing exisiting 'end-of-life' spacecraft as telescopes.
And finally, could ground-based telescopes do it? tests have just started on a new 10M telescope in the Canaries that could help; in terms of larger scopes, colleagues of mine in NUI Galway have been collaborating on the design of Euro50, a proposed 50M telescope. Together with new techniques such as spectral deconvolution (via) may help.
Sat, 28 Apr 2007
Now that the weekend is here, I've been catching up on the news of this weeks big astronomy announcement: the discovery of a possible "Super-Earth" exo-planet, Gliese 581 c. This is the first exoplanet likely to be within the so-called "Habitable zone" around its star, where life as we know it could be possible. Unfortunately the Irish Times "World Apart" weekend supplement article has been quite disappointing.
Like Greg Laughlin at UCO Lick I get depressed when people give misleading information . It includes a very misleading "Artists impression" picture, simultaneously showing details we don't know and being incorrect about the few things we do know.
Its easy to get annoyed at the concentration of facts that are guesswork or probably wrong in this article. Gl 581c is not 5 Earth masses: thats its minimum mass: due to radial velocity measurements, we can only measure M sin(i), where i is the inclination, currently unknown. It could easily be far more, and hence all predictions of what the planet is like become moot.
Similarly, the radius is not necessarily 1.5 times Earths: thats a prediction, based on the mass, and current theories. Unfortunately, there is no mention that current theories of planetary size are coming under scrutiny as they fail to match other recent exoplanet discoveries. This makes other speculation about the planet such as the gravity on the surface quite misleading.
The BBC News summary is much better: it refers to the radius as "the radius should be", rather than "is". But it too includes the incorrect picture. While its article is couched in more conservative language, it doesn't explain what facts it quotes are speculation and why they may be wrong.
None of the media reports I have yet seen mention current missions, such as COROT, which is the most likely to find Earth-sized planets in the near future (launched late last year, it is due to finish its first observing run in April. Results awaited with anticipation). They also ignore Darwin and the The Terrestrial Planet Finder, the most likely missions to discover life on other planets, but currently sitting in funding limbo.
Concentration on purely the known facts would of course make the news reports short and boring. Instead they should emphasise more of the work of the scientists involved: we teach science as a set of cut-and-dried facts that often bore school students and drive them away.
Instead, when faced as the cutting edge of an important discovery like this, we could show how an amateur could be partially credited with getting this result; with the fact that, since the planet has a short "year" astronomers are already lining up to investigate transit measurements of it on May 7 and that with a magnitude of 10.6 (only 20 light years from Earth, after all) its within range of amateur astronomers to observe and determine its true size and details. How about getting people excited to get involved themselves?