The idea that other life-bearing worlds are out there continues to fire our imaginations, as attested by the success of the recently-opened Star Trek movie, and by the critically acclaimed Battlestar Galactica series which concluded earlier this spring.
In 1995, astronomers identified the first exoplanet around the star 51 Pegasi, nicknamed Bellerophon. Since then, we’ve found over 300 planets around other stars. For many years, though, we were finding only ‘hot Jupiters’ – gas giants extremely close to the host star (such as Bellerophon.) These are not logical places to search for Mr. Spock, or for that matter any kind of life as we know it. However, the search for extra-solar planets or exoplanets (planets around stars other than our Sun) is now entering a new phase. As we refine our methods and our tools, we are at last beginning to find planets much smaller than Jupiter, approaching Earth in size. And we’re starting to find some planets in the habitable zones of stars, regions where the temperature is neither too hot nor too cold for life. Although we don’t really expect to find another Vulcan or Caprica, two recent announcements can give us some insight into how the search is done.
In April, astronomers announced the discovery of Gliese 581 e, the fourth planet found around the star Gliese 581. At around two Earth masses, this is the least massive planet ever found outside our solar system. Astronomers also announced that Gliese 581 d(the third planet found in the system) is within the star’s habitable zone. (‘A’ would designate the star itself; the planets are b, c, d, and e.) This is star #581 in Wilhelm Gliese’s Gliese Catalogue of Nearby Stars, an effort to list all stars less than 25 parsecsfrom the Sun. Gliese 581 is about 20 light years away, located in the constellation Libra.
Astronomers found Gliese 581’s planets using the radial velocity method. Perhaps you are familiar with the Doppler effect, in which a sound changes in frequency when a source that had been approaching begins to move away. We see the same effect with receding and approaching sources of light. When a light source is receding from us, the wavelength of its light gets longer (and therefore redder.) When a light source is approaching, the wavelength of its light gets shorter (and therefore bluer.) The spectra of stars show dark absorption lines, indicating wavelengths of light absorbed by gases in the star. By observing these lines over time we see that some stars show a slight redshift, then a slight blueshift, then a slight redshift…. Such a periodic variation indicates that the star is being tugged by something orbiting it. The size and period of the tug gives us an idea of the tugger’s mass. A mass much less than our Sun and comparable instead to Jupiter indicates a planet.
To understand how hard it is to find Earth-sized planets this way, imagine if a crewman on Galactica had to find Earth with this method. Our observer needs to see an entire oscillation to recognize the periodic tug of a planet, so (s)he must observe the Sun for a full year (Earth’s entire orbital period) to detect our planet. Further, Jupiter’s tug on our Sun overwhelms Earth’s by about a factor of 12. Any distant observer studying our own Sun’s radial velocity would probably notice only Jupiter’s influence on our Sun. And that would take about 12 years of observing, since Jupiter takes about that long to orbit the Sun. Finally, the observer needs to see our solar system roughly edge on, such that planets tug the Sun towards and away from the observer. Fortunately for Starbuck et. al., Galactica has access to much better technology than we do today.
Gliese581 is type M3V. Here ‘V’ is the Roman numeral five, representing the fifth luminosity class, which is the main sequence of stars that includes our Sun. ‘M3’ indicates a reddish star significantly smaller and cooler than our Sun. In particular, Gliese 581 has less than one-third our Sun’s mass and is more than 2000K (3600 oF) cooler than our Sun. Therefore, the habitable zone around Glises 581 is much closer to the star than ours is to our Sun. Gliese 581 d, orbiting in that zone, orbits once in 67 Earth days. Although Gliese 581 e takes only about 3 days to orbit its star once, is the planet closest to Earth’s mass we have yet identified. The Gliese 581 system brings us closer to finding planets like ours and to understanding solar systems like our own.
© Photo credit: Xaethyx
Just days ago (May 13,) NASA announced that its Kepler telescope, launched March 6, is ready to begin observations. This is NASA’s first mission capable of finding Earth-sized and smaller planets around stars other than our Sun. Unlike the Hubble telescope which orbits Earth, this telescope is in orbit around the Sun. It is roughly at Earth’s distance from the Sun, but on an orbit where it lags slightly more behind Earth’s position as time passes. After 4 years, Kepler will be about 0.5 AU, or half the Earth-Sun distance, behind Earth on its orbit.
Kepler will stare continuously at the same small region of the sky for three and a half years. Scientists did not want this steady gaze interrupted by day-night cycles or by passage behind the Earth, as would happen if the telescope were in Earth’s orbit. Further, Kepler is looking at a region of space far above the plane of our solar system, so the Sun, Moon, and other solar system bodies never come near the field of view. That area of space is also in the galactic plane roughly in the direction the Sun itself is traveling. This means we are observing stars at the Sun’s approximate distance from the galactic core.
Kepler will detect extrasolar planets using the transit method. This method involves looking at stars continually for long periods of time to see if the light ever gets slightly dimmer. If the slight dimming occurs on a regular basis, it might be because a planet is orbiting the star and regularly passing in front of it from our perspective. Such a passage is called a transit. When a planet as small as our Earth transits its star, the star dims by only a factor of 1/10,000. Only now, with Kepler, do we have an instrument powerful enough to detect such a tiny change in a star’s brightness. Of course, we need to be fortunate enough to observe the planetary system edge-on, otherwise no transit will occur. However, the chosen field of view contains about 100,000 stars, so odds are at least a few are oriented favorably.
|© Photo credit: stéfan|
Even if we do find other Earth-sized planets, however, we are still far from finding alien cultures, much less interacting with them as in science fiction. Aliens in science fiction can interact because writers cheat on the laws of physics by introducing a parallel dimension. This dimension is called ‘subspace’ in Star Trek and ‘hyperspace’ in Star Wars and Babylon 5. To travel from one planetary system to the next, a starship leaves space, enters subspace/hyperspace, travels through that dimension, and reemerges into space at its destination. As far as we know, however, no subspace or hyperspace exists to shorten space travel; real spacecraft must travel through space. That makes the speed of light (300,000,000 m/s) an inviolable speed limit. For example, nothing can travel between our system and that of Gliese 581 in less than 20 years, because Gliese 581 is 20 light years away.
What’s more, any life we hope to find and interact with has to exist at the same time we do. Given our short existence compared to the universe as a whole (equivalent to eight minutes out of a year), this could be the single biggest limitation on our ability to find other worlds with life.
But even as we leave green-blooded aliens and warp drive at the theater, a quite real adventure remains before us. No matter how much we explore and study our solar system, we cannot truly understand it until we can place it in a larger context. The Kepler mission promises to show us more systems like our own, or to show us just how rare systems like ours are. Either way, we will be able to appreciate our own Earth and familiar planets like never before. I find that as thrilling as a ride with Captain Kirk.