Astronomers Search For Rocky, Habitable Planets In Other Solar Systems

By Sarah Nelson

To most of us, "Twinkle, Twinkle Little Star" is just the opening of a familiar nursery rhyme. To astronomers looking for planets orbiting stars other than our own Sun, the twinkling of a distant star could mean something much more interesting. Finding planets outside our solar system, known as exoplanets, is an active area of research in the field of astronomy. While our own planetary system still harbors a wealth of mysteries, a new wave of research is aimed at detecting and characterizing these exoplanets.

Astronomers discovered many of the first exoplanets by indirectly observing the effect of these planets on their host stars. While in orbit around a star, a planet's gravitational pull induces slight wobble in the star's motion. Astronomers infer the presence of an exoplanet by observing this wobble and measuring its speed, known as the star's radial velocity. This method, however, is biased towards finding large planets with sizeable gravitational fields. Of the over 400 exoplanets discovered since 1995, most have been Jupiter-like gas giants due to the limitations of radial velocity and related methods. Astronomers are keenly interested in finding a planet in the so-called "habitable zone," a concept used to describe the distance a rocky planet would need to be from its host star in order for water to exist as a liquid. Too close and life would be baked by the star; too far and the planet would be too cold. So far, none of the known exoplanets falls into this category.

Astronomers more recently have started using an alternative approach. When a planet passes in front of a star, making its transit, the amount of light we see from that star drops by a tiny fraction. Inferring the presence of an exoplanet by detecting this dip in light is known as the transit method. According to University of Washington astronomer Rory Barnes, this technique was suggested a long time ago but only recently applied to exoplanetary discovery.

"Just seeing it once, you don't necessarily know if you saw a transit," says Barnes. The second time establishes a period equal to the length of the planet's orbit. A third observation is convincing evidence that there is indeed a planet orbiting that distant sun. In the case where the planet is about as far from its star as the Earth is from the Sun, it would take three years to observe three transits. "You have to be patient, that's the biggest challenge," says Barnes.

To detect transits of small and distant planets requires a very powerful telescope, one ideally orbiting out in space rather than ground-based, where its observations are literally clouded by the Earth's atmosphere. A telescope with this design was launched in December 2006 aboard the CoRoT spacecraft, in a joint mission of the French and European Space Agencies. The CoRoT (Convection, Rotation and planetary Transits) mission was designed specifically for the dual purpose of detecting exoplanets via the transit method and for probing the interior of stars.

In February of 2009, CoRoT detected the first rocky exoplanet, orbiting a star 500 light years away. Named CoRoT-7b, this planet has a diameter roughly half that of Earth's and is 50-60 times closer to its sun, far too hot to harbor life. "It is ‘Earth-like' only in the sense that it is mostly composed of rock," says Barnes, who for years has been doing theoretical research on what this type of small, terrestrial exoplanet might look like once detected. He was nonetheless excited. "This is the planet I'd been waiting for."

In studying its orbit over the past year, Barnes and his collaborators have predicted the surface of CoRoT-7b to be highly volcanic. The system is similar to that of Jupiter, its violently volcanic moon Io, and the next moon out, Europa. Io orbits Jupiter in an elliptical orbit, meaning that at any given time the side of Io closer to Jupiter is receiving a stronger gravitational pull compared to the far side, which is being pulled on slightly by Europa. This causes Io to stretch in and out of a football shape, producing the internal energy underlying its volcanism.

"The standard analogy is if you imagine holding a tennis ball in your hand and you just squished it," says Barnes. Pumping energy into the tennis ball is going to eventually create a lot of heat. "That's what you do to Io, that's why it's so volcanic," explains Barnes, who has extrapolated from this well-observed system to predict the likely volcanism of CoRoT-7b. He is wary, however, of taking the analogy too far. In terms of the amount of internal energy, "It's possible that CoRoT-7b could be as different from Io as Io is from the Earth," he says. It is hard to say for certain with the current methods of observation.

Down the hall from Barnes in the University of Washington astronomy department, post-doctoral researcher Nick Cowan is looking ahead to how exoplanets may be studied when the technology is 15-20 years more advanced. Cowan has used images of Earth taken from a distant spacecraft to roughly map out the location of continents and oceans, under the premise that water is a key ingredient for any planet to be habitable. In order to make his map, Cowan treated Earth as an exoplanet and analyzed how the brightness and color of Earth changed over time, a pattern called a light curve. The basis for his technique is that different features on Earth reflect light at different wavelengths. Oceans reflect blue light, which has a shorter wavelength, while land is reflective at red wavelengths.

Cowan started with images of Earth taken every 15 minutes over two 24 hour periods by the Deep Impact spacecraft. He then analyzed the light curve created by Earth as it spun on its axis. In order to better approximate the images we might someday get from an exoplanet, Cowan took the Deep Impact images of Earth and had them "smushed" into a single pixel. "All I knew were the time variations of that pixel so that the brightness and color of that single pixel was changing as a function of time," he says. Cowan interpreted these changes into a longitudinal map of the Earth, with approximate locations of oceans and continents.

"The map is limited in that we only get a picture of places that are well-illuminated, like near the equator where there's lots of sunlight," cautions Cowan. "You also only get a good look at where the spacecraft happened to be looking." The map they ended up with was "pretty decent near the equator but terrible at the poles." The features the map was able to pick out, however, can be verified against the oceans and continents that we know exist on Earth. The approximation is close enough for Cowan to foresee someday applying this method to an extrasolar planet.

"To do that you'd need a spacecraft that doesn't exist yet," Cowan points out. "You'd need a next generation spacecraft." The technology isn't there yet, but Cowan is optimistic: "it's inevitable."

The near future of exoplanetary research will be affected by NASA's Kepler Mission, which is aimed at finding Earth-sized, terrestrial exoplanets in the habitable zone. Launched in March 2009, Kepler has already enabled the detection of five exoplanets via the transit method, from its survey field of roughly 150,000 stars. The mission is expected to find hundreds, if not thousands, of exoplanets before its scheduled 2012 completion, according to Kepler co-investigator Alan Gould. While most telescopes have a field of view the size of a "grain of sand," says Gould, Kepler's is the size of a human hand held at arm's length - the largest field of view of any telescope yet launched into space.

Exoplanetary researchers like Barnes await Kepler's findings. "I think Kepler is going to prove some of the biggest advances in our field," says Barnes. "Up until this point, exoplanets have really been a case by case study, there have just been so few of them. Now we'll get to a point where we'll understand their general properties and how our solar system fits in. Probably ten years from now we'll have confirmed a rocky planet about the mass of the earth in the habitable zone of a star, although we probably won't be able to tell if it's inhabited. One thing that's for certain is that new discoveries are happening all the time." As the technology and methods continue to improve, so will our ability to satisfy the nursery rhyme's plea, "How I wonder what you are!"

Sarah Nelson is a research scientist at the University of Washington Genetics Coordinating Center and a freelance science writer.

Images

Top: As a planet transits in front of its parent star, the amount of light detectable by the CoRoT spacecraft drops by a tiny fraction. Image: CNES

Bottom: The moon beginning its transit in front of the Earth, as seen from the Deep Impact spacecraft. Images were taken on May 29, 2008 through three color filters: blue, green, and orange, when the spacecraft was approximately 30 million miles from Earth. Image: NASA/JPL-Caltech/UMD/GSFC