counting

The last time we ventured outside the confines of the solar system in this space was 2005, when we looked at the prospects for finding intelligent life in the universe: SETI and Such <http://www.asimovs.com/_issue_0508/Onthenet.shtml>. At that time I was croggled to report that astronomers had discovered 136 planets in addition to our local nine . . . er . . . eight. You will recall that in 2006 the International Astronomical Union famously banished Pluto <http://www.howstuff works.com/pluto-planet.htm> to the planetary minor leagues. It is a pleasure to report that, as I type this, we now know of 429 extrasolar planets (aka exoplanets) in orbit around 362 stars, according to Jet Propulsion Laboratory’s PlanetQuest <http://www.planetquest.jpl.nasa.gov/index.cfm> site. Alas, not one of them is a likely site for a condo development. Indeed, the smallest exoplanet <http://www.sciencenews.org/view/generic/id/43038/title/Smallest_exoplanet_yet_is_found> we know of is at least twice the size of our world and orbits COROT 7, a yellow dwarf star some 489-light years away. Unfortunately this rocky world hurtles around its star every twenty hours and its hellish surface temperature is between 1000 and 1500°C. Bring your sunblock.
Oh, and for those of you keeping score at home, SETI has found exactly 0 (zero) extraterrestrial civilizations since 2005.

discoverers

I have to say that I lose patience with those who claim that humanity’s efforts to probe the cosmos are flagging. We live in a golden age of space exploration; it’s just that our satellites are doing all the heavy lifting. Consider, for example, COROT <http://www.sci.esa.int/science-e/www/area/index.cfm?fareaid=39> which discovered that smallest exoplanet. COROT—short for Convection, Rotation and planetary Transits—is the poster child (can a space telescope be a poster child?) for international cooperation in planetary exploration. Led by the French Centre National d’Etudes Spatiales <http://www.cnes.fr> with help from the European Space Agency <http://www.esa.int>, Austria, Belgium, Germany, Spain, and Brazil, it was launched in 2006 aboard the Russian built Soyuz-Fregat <http://www.orbireport.com/Launchers/Soyuz_U-Fregat> rocket carrier. It was the first space mission specifically designed for exoplanetary research. Since its 27 cm telescope reported first light in 2007, it has discovered seven new worlds, most of them as big or bigger than Jupiter.
In March 2009, the Kepler <http://www.kepler.nasa.gov> Space Observatory was launched into an Earth-trailing orbit around the sun. This was necessary so that Earth does not block its view. Its huge 1.4 meter primary mirror, the largest of any telescope outside of Earth orbit, is specifically designed to spot terrestrial planets and so is much more likely to find earthlike planets than the Hubble Space Telescope <http://www.hubblesite.org>. Kepler has a much larger field of view and will look continuously at one starfield, estimated to be about 156,000 stars. In January 2010, scientists announced Kepler’s first discoveries: five new planets, four bigger than Jupiter, one the size of Neptune. Although none are terrestrial, it is yet early days in what is expected to be a historic mission.
But how exactly do we find an exoplanet? The obstacles to detection are many, which is why the first published observation didn’t occur until 1992, when two planets were detected around the pulsar PSR B1257 +12 <http://www.nasa.gov/multimedia/imagegallery/image_feature_574.html>. (Later a third planet was discovered in this exotic system.) For one thing, planets do not produce any light of their own. For another, they are lost in the brightness of their star. And don’t forget that they are mostly many, many light years away from us. However, scientists have found several ingenious ways to work around these difficulties.
Radial velocity or Doppler spectroscopy <http://www.planetary.org/explore/topics/extrasolar/radial_velocity.html> is currently the most successful tool in the planet hunters’ toolbox, although Kepler is expected to change this. Planets exert a gravitational tug on their stars, causing them to wobble ever so slightly. As a star moves toward us or away, we can make very precise measurements of its periodic Doppler shift, if any. These cycles are an almost certain indication that something is orbiting the star.
Astrometry <http://www.planetary.org/explore/topics/extrasolar/astrometry.html> is another method for detecting a star’s periodic wobble, and thus the presence and mass of orbiting bodies, by pinpointing its position in the star field. Unfortunately we have yet to deploy instruments precise enough to yield definitive astrometric data for planet hunting and, although there have been many claims of planets discovered by astrometry, none have been confirmed. It is possible that NASA’s Space Interferometry Mission <http://www.nexsci.caltech.edu/missions/SIMPQ>, delayed many times and now scheduled for 2015, may meet this need.
Microlensing <http://www.planetary.org/explore/topics/extrasolar_planets/extrasolar/microlensing.html> allows us to detect the most distant planets, as far away as the center of our galaxy. When stars align, the light from the distant star is bent by the closer one, resulting in an observable magnification of brightness. This effect was predicted by Albert Einstein <http://www.westegg.com/einstein> in the General Theory of Relativity <http://www.pbs.org/wgbh/nova/einstein/relativity>. If the distant star has a planet orbiting it, it will show up as a deviation from the standard magnification event. Although this is the only method currently available to us for detecting the most distant planets, it has the serious disadvantage of relying on the passage of a near star in front of a distant star as seen from Earth, a rare event indeed. 
Transit photometry <http://www.planetary.org/explore/topics/extrasolar_planets/extrasolar/transit_photometry.html> works by measuring the dimming of light from a star when one of its planets passes between it and us. If this transit follows a regular pattern, we can infer the size—but not the mass—and orbit of the planet. Both COROT and Kepler make use of this technique and it is expected that, in the coming years, they will discover many more planets than all the other techniques put together. A huge drawback with transit photometry is that it requires that a transit occur. If the orbital plane of the distant solar system isn’t edge-on to us, we will see no transit. Alas, the majority of planets are not likely to be conveniently situated for this technique.
It is almost impossible to see an exoplanet by optical detection, but it has happened at least twice. Maybe. In 2004, astronomers using the European Very Large Telescope array <http://www.eso.org/public/astronomy/teles-instr/vlt.html> claimed to have imaged a planetary “object” <http://www.en.wikipedia.org/wiki/2M1207b> near the brown dwarf star 2M1207. The claim is in dispute since the object may not be a planet at all, but rather a sub-brown dwarf, something between a gas giant planet and a brown dwarf star. In 2008, scientists using the Hubble announced that they had imaged a planet orbiting Fomalhaut and called it Fomalhaut B <http://www.en.wikipedia.org/wiki/Fomalhaut_b>. However, direct imaging is a dicey proposition that depends on unusual conditions, and a bit of luck. Its chief benefit is not so much scientific as it is psychological: an image makes a new world easier to imagine.

planet shopping

There are several excellent online catalogues of the known exoplanets. The Extrasolar Planets Encyclopaedia <http://exoplanet.eu> was established in 1995 and continues to be updated as new worlds are discovered. I was particularly impressed by the huge list—with links—of ongoing search programs. The Geneva Extrasolar Planet Search Programmes <http://www.exoplanets.ch> catalogues the 291 planets discovered or confirmed by the radial velocity technique. JPL’s Center for Exoplanet Science <http://www.exoplanets.jpl.nasa.gov/index.cfm> is a huge site well worth exploring. The Exoplanet Exploration Program <http://www.exep.jpl.nasa.gov> pages are a superb resource for the technically minded, while PlanetQuest succeeds in its mission of community outreach. Its New Worlds Atlas <http://www.planetquest.jpl.nasa.gov/atlas/atlas_index.cfm> is a database of all know exoplanets that allows you to filter searches so that you can find all the hot Neptunes or pulsar planets or list planets with host stars visible to the naked eye. Why not take the kids out for skywatch tonight?
I can also recommend an amazing site called The Neighborhood <http://www.deepfly.org/TheNeighborhood/Index.html>. This would appear to be the handiwork of one Raymond Harris, who tells us, “I’m not an astronomer, but I do love space and all the dark and shiny stuff out there. I try to ensure that all the information presented on this site conforms with current astronomical knowledge.” After you get done poking around Planets Around Other Stars <http://www.deepfly.org/TheNeighborhood/7b-ExoplanetaryOverview.html>, click to the home page and journey from the far edge of the cosmos as revealed by Hubble Ultra Deep Field to the local group of galaxies to the Milky Way to our local solar neighborhood to our solar system. Mr. Harris is an excellent tour guide and this is a must see site for any budding science fiction writer.

exit

Although I have a layman’s interest in space exploration—healthy but unfortunately constrained by a severe lack of math—I also have a professional interest in exoplanets, since from time to time I find it necessary to build one of my own. Luckily there are marvelous resources online suited to this task. Back in my 2005 column, I recommended World Builders Home Page <http://www.world-builders.org>, a great resource that collected materials for a course taught by Elizabeth Anne Viau at California State University, Los Angeles. Alas, although still useful, the site hasn’t been updated since 2006. I was also a big fan of StarGen <http://www.fast-times.eldacur.com/StarGen/RunStarGen.html>, and it is still one of the best worldbuilders around. For a wide range of science fiction and fantasy world construction advice, try SpecFicWorld’s World-Building Resources <http://www.specficworld.com/resources/world.aspx>. (And while you’re at it, explore the rest of SpecFicWorld, a busy and ambitious site that features fiction and non-fiction.) Similar to World-Building Resources is World Builder Projects <http://www.hiddenway.tripod.com/world>, although some of the links have expired.
I highly recommend Planet Science <http://www.planet-science.com>, a wonderful site funded by the British National Endowment for Science, Technology and the Arts <http://www.nesta.org.uk>, which “is for all children who study science and the teachers and other adults who support them.” Just the thing for autodidacts and English majors like yours truly. The Shockwave-based Planet 10 Simulator <http://www.planet-science.com/randomise/index.html?page=/planet10> is a hoot.
If I can do it, so can you. Isn’t it time to build a world of your own?