In this day and age, it seems like everyone is trying to add some excitement to their lives. Now that we no longer have animals trying to hunt us and have enough infrastructure that one bad harvest won’t wipe us out, we’re looking for something to spend our excess energy on. People have taken to jumping off bridges with giant rubber bands, getting charged by bulls and jumping out of the way, or even eating fugu every chance they can.
Some of us less adventurous (and less crazy) folks have other ideas about how to heighten the adrenaline we get out of our hobbies. One of the more superficial ways to do this is to just add the word “extreme” (or, if we are hardcore enough, we might leave out the e and be “x-treme”). It started out with extreme sports. Then it went to x-treme makeovers and weight loss. And now, it has passed on to x-treme couponing. I expect x-treme snail fighting will be coming along some time soon.
While we could put the x-treme in front of astronomy to make it even more exciting, astronomy beat us to it with an “X” of its own: Extrasolar planets (called exoplanets or x-planets for short).
An x-planet is a planet outside our sun system. Once thought to be purely fiction, there are over 860 such identified planets (and by the time you read this, the number will have gone up). The first definitive finding of an x-planet was in April of 1992 in orbit of PSR B1257+12. (Unfortunately there are a lot of stars out there without names and only unwieldy catalog numbers.) The first multiple planetary system was found in 1999, in the Upsilon Andromedae star system — only 44 light years away.
But the search is still on for other habitable planets. Alpha Centauri has a planet the right size, but far too hot. OGLE-2005-BLG-390Lb (try pronouncing it, it’s a fun series of sounds) is larger than our planet, but too cold to support life. We have yet to find one that’s just right. When we do, it will still be a long drive to check and see if we have some neighbors.
Until then, the best way to experience what an “alien world” might be like is the X-Planet show in the HMNS Burke Baker Planetarium. And what better way to experience it than after-hours with the creators of the show, Dr. Carolyn Sumners and Adam Barnes?
Since its launch in 2009, NASA’s planet-hunting Kepler Telescope has uncovered 2,740 new extra-solar planets, also known as exoplanets or X-planets. Now scientists are working to identify gases in the exoplanets’ atmospheres that can support life. It is just a matter of time before an “alien Earth ” is found. Join Dr. Carolyn Sumners and Adam Barnes of the Museum’s astronomy department for a behind-the-scenes look at the science behind X-Planets, the making of the film The X-Planets and a viewing of the film in the Burke Baker Planetarium. To purchase tickets and read more about the film, click here.
When you look up at the night sky, do you ever think you’re seeing other solar systems? Do you ever wonder if any of the stars you see have planets like Earth in orbit around them?
We have discovered that seven planets and more than a hundred moons in our solar system are simply not enough like Earth to foster the development of life or to make colonization easy. We now realize that our search for an alien Earth must occur in solar systems around other stars.
As we approach a thousand confirmed exoplanets, we are becoming better at identifying Earth-like worlds. Sensitive measurements are required to detect the small wobble in a star caused by an orbiting planet or the drop in light caused by a planet crossing in front of a star.
NASA’s Kepler telescope, a planet-hunting mission, has uncovered 2,740 potential alien worlds since its 2009 launch. Of these, more than 350 are about the size of Earth. Observatories on Earth’s mountaintops are also identifying planets around other worlds and confirming the discoveries of Kepler.
Now we are working on detecting more than an exoplanet’s mass, diameter, and distance from its star by developing sensors that can identify gases in the planet’s atmosphere. This way, we can look for the oxygen and water vapor that support life on Earth.
It is just a matter of time before we find a world that is truly Earth’s twin. Studies suggest small planets like Earth are probably common in the universe — easily over 10 billion in our Milky Way Galaxy. Will the discovery of an alien Earth change the way we think about the universe and our place in it? Will we then realize that our planet is not unique, and that perhaps life on Earth is not unique either? Does this change how we think of our home planet and ourselves?
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 Trekmovie, and by the critically acclaimed Battlestar Galacticaseries 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.
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.
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.