Sunday, February 6, 2011
This week I saw a couple of articles in Nature about new exoplanets. I'm generally interested in new anything but didn't stop to read the articles as astronomy jargon can sometimes make me feel like I couldn't understand a third grade solar system diorama. But I noticed these articles getting picked up by the news media around the world and so decided to check them out.
Exoplanets, or extrasolar planets, are planets that reside outside of our solar system and do not orbit around our Sun/star. In the search for extrasolar planets we are, in effect, searching for evidence that our solar system is not unique, that there are other planets out there that are similar to our own. As of February 1, 2011 there are 526 confirmed exoplanets and 1,235 that are waiting for confirmation. Of these, 68 are approximately Earth-size, 288 are super-Earth-size, 662 are Neptune-size, 165 are Jupiter-size, and 19 are super-Jupiter-size.
How do you detect a planet orbiting a very (and in astronomical terms that 'very' is oh-so-huge!) distant star? You can start by looking for the wobble. When you have two large bodies (such as planets and stars) they orbit a common center of gravity and gravitationally push and pull on each other, causing them to wobble back and forth. By measuring the wobble of stars you can gain all sorts of information, like planet size and mass, even though you can't actually see the planet. This gravitational push and pull can be utilized for planet hunting in other ways as well. Astronomers also use the Doppler shift to find planets. Basically, they are measuring the velocities of the stars by measuring the changes in the light that comes from a star moving towards us versus away from us. If the star is wobbling then the light emitting from that star will shift too. Scientists can also look for radial velocity changes, looking at transitions in atomic lines. As atoms transition between energy levels they either absorb or emit a photon in constant ways, add that up with an entire star and we can see bright and dark lines in the continuum of light. When a star moves it causes these lines to shift back and forth in color - toward us blue and away from us red.
Getting away from the wobble, there is also the planetary transit method. This is when the planet, as it passes in front of the star, blocks some of the light from that star. Now, planets are tiny compared to stars, so they don't block a lot of light but they can block enough to be detectable. Of course, you need to be looking at a solar system edge-on for this to work. There is also the gravitational micro-lensing technique. Gravity affects light, just like it affects just about everything else. So if you put a high-mass object between you and the light source (the star) then the gravity from that object will bend the light behind it into your field of view. This will make the objects appear to suddenly brighten as the object passes in front of the light source/star.
Recently new imaging technologies have become available. Technologies that will allow astronomers to take the first images of planets circling other stars. One such technology is coronagraphs. This was originally invented to study our Sun by blocking light coming from the solar disk in order to see the Sun's corona. This is being refined and adapted to find exoplanets, but it still has its problems (see Planet Quest). Another technique is using mirrors. Replace one large mirror with a lot of smaller mirrors and combine their light using a process called interferometry. This way you can have the small mirrors, which obtain a resolution equal to a single telescope as big as the largest separation between the individual telescopes, and gather information to build a larger picture.
The Kepler spacecraft was launched in March 2010 in order to explore the structure and diversity of planetary systems. To find extrasolar planets, particularly Earth-size terrestrial planets in habitable zones. It's speciality is the transit method and it observes 156,000 stars in the constellation Cygnus looking for those little blinks of light. The new papers in Nature reveal that the Kepler spacecraft has located a solar system with 6 known planets orbiting the star Kepler-11, 2000 light-years away. The planets are named Kepler-11b, Kepler-11c, Kepler-11d, Kepler-11e, Kepler-11f, and Kepler-11g, going from innermost to outermost. None of the planets is identified as Earth-like (bummer), but they are close, ranging from 2 to 4.5 times the radius of Earth. The planets are mostly mixtures of rock and gases, possibly including water, with the planets closest to the star having the highest densities and so likely having the most rock and water. The planets range in size between the masses of Earth and Uranus, with three being gas giants with thick hydrogen and helium atmospheres.
Alright, so why all of the excitement? First of all, a whole solar system discovered is always an exciting event, and this is the largest group of transiting planets orbiting a single star to be discovered. Kepler-9 has been found to have 3 transiting planets, and the star HD 10180 has at least 5 Neptune-like planets orbiting. Additionally, these planets are among the smallest found for which both mass and size have been measured. The Kepler-11 solar system's structure is also interesting and unique. Five of the six planets orbit very close to their star. I'm talking closer to their star than Mercury orbits to our star. The sixth planet orbits at about the distance of Venus to our Sun. But, as Kepler-11 is a smaller, cooler star than our Sun that means that these planets fall within the system's habitatable zone. This structure also means that it is a really densely packed system. It is unclear as to why the planets are so densely packed, but it may shed some light on planet formation and solar system evolution. It is pretty much accepted that planets form from a cloud of dust around a newly formed star. However, how and where they form within this protoplanetary disc is still debated. Do they form far from their parent star (at the distance of Jupiter and Saturn) and move inward over time? Or do they form in place? Kepler-11d, Kepler-11e, and Kepler-11f have a significant amount of light gas which indicates that they formed within a few million years of the system's formation. How do we know this? Well, we know that free hydrogen only lasts around 5 million years around a star before it gets dispersed by the solar wind. If these planets have a significant amount of hydrogen gas then they must have formed within 5 million years of the star igniting. The team suggests that "the small eccentricities and inclinations of all five inner planets imply...that gas and/or numerous bodies much less massive than the current planets were present," and that "the lack of strong orbital resonances argues against slow, convergent migration of the planets, which would lead to trapping in such configurations." For the planets for form in place it would require a massive protoplanetary disk of solids near the star and the accretion of large amounts of gas by hot small rocky cores, but the very high temperatures close to a growing star would have been too high for ices to have condensed. And so the solar system's evolution is still unclear. The closely packed nature of the Kepler-11 system does allow for lots of gravitational tugging and so good information for further study. It's a mystery, for now.
Now for lots and lots of read-it and look-it-up on your own links. Many of which have fantastic information, infographics, and links. Let's start with the actual paper:
Lissauer, Jack J., et al. (2011) A closely packed system of low-mass, low-density planets transiting Kepler-11. Nature: 470 (7332), 53. (DOI: 10.1038/nature09760)
And here is an editorial and article about extrasolar planets, also published in Nature.
The hunt is on for a distant planet similar to our own. Astronomers should decide just how similar it needs to be, before the candidates start pouring in. Nature: 470(7332), 5. (DOI: 10.1038/470005a)
Reich, Eugenie S. (2011) Beyond the Stars. Nature: 470(7332), 24-26. (DOI: 10.1038/470024a)
And here's a short video from NASA Ames Research Center about the Kepler Mission, exoplanets, and specifically the Kepler-11 system:
Here are also some lay-people-friendly articles from news outlets and the like about this story:
Some information on exoplanets and how we find them:
Browse the Kepler Mission website: http://kepler.nasa.gov/
And explore the Planet Quest website from the NASA Jet Propulsion Laboratory CalTech:
Become a planet hunter! Help find extrasolar planets from the comfort of your own living room (or wherever your computer lives). The citizen science project from the people behind Galaxy Zoo (www.galaxyzoo.org/) allows anyone with an Internet connection to contribute to astronomical research by sorting through data from NASA's Kepler mission. All you have to do is log in to the site and answer some simple questions about a star's brightness over time. If the star shows repetitive dimming then it is flagged as possibly having an exoplanet. Easy right? Check it out and contribute!
Planet Hunters: http://www.planethunters.org/
(image from the above National Geographic source and courtesy of Tim Pyle, NASA)