Author: Heather Mispagel Ganio
Institution: University of Connecticut
Date: March 2007
Late astronomer Carl Sagan compared our universe to a vast cosmic ocean consisting of about 100 billion galaxies, each galaxy comprising some 100 billion stars; according to Sagan, there are more stars in the universe than there are grains of sand on all the beaches of Earth. In this practically infinite sea of stars, we have stood on only one secluded shore: that of our own planet. Are we Earthlings the only inhabitants of the universe, or could beings from another cosmic island be gazing in our direction with a similar sense of curiosity?
Although many scientists believe the universe could be teeming with exotic life forms, most agree that there are probably no intelligent beings on the other planets in our own solar system; if there had been, we would probably have noticed them by now. In searching for extraterrestrial intelligence, then, we must turn to extrasolar planets, or exoplanets, which orbit other distant stars.
Spying on the Neighbors
Of course, finding planets so far away is no small feat. Like the Moon, planets emit no light of their own, and so are only directly visible through the starlight they reflect. Unfortunately, the relatively small and dim planets are usually drowned out by the more intense light of the stars they orbit. As a result, astronomers must often detect planets indirectly. For example, they may observe how a star "wobbles" in space from the gravitational pull of an unseen companion, such as a large planet, as it orbits the star.
Using these clever techniques, scientists have managed to detect hundreds of exoplanets, most of them in the last decade. The first discovery of an exoplanet, orbiting a star called Gamma Cephei, was made in 1988 by Canadian astronomers Bruce Campbell, G.A.H Walker, and S. Yang, though it was not confirmed until 2003. The first exoplanet detection to be confirmed was that of Aleksander Wolszczan and Dale Frail in 1992. In the five-hundred-year history of modern astronomy, finding other solar systems is a very recent field.
Until recently, these searches have been confined to just one small part of our galaxy. Part of the reason for this is the huge area of space the Milky Way covers; while it takes light only eight minutes to reach the Earth from the sun, it takes 100,000 years for light to travel across the disk of our galaxy! Across such distances, even the sparse clouds of gas and dust in space make it difficult to see other stars clearly. As a result, it has been hard to guess how common planets are in other parts of the galaxy.
Artist's conception of an ultra-short period planet (USPP), a type of exoplanet that is so close to its star (750,000 miles, or about three times the distance from the Earth to the Moon) that it completes a full orbit in 10.5 hours. (Credit: NASA, ESA and A. Schaller for STScI)
Sweeping the Sky
However, in 2006, a team of astronomers published new findings about exoplanets from a survey harnessing the power of the Hubble Space Telescope. The team, led by Kailash Sahu of the Space Telescope Science Institute in Baltimore, Maryland, discovered 16 new candidate planets around stars in the central bulge of our galaxy, a whopping 25,000 light years from Earth. They are referred to as candidate planets because the scientists have not been able to confirm most of the planetary masses.
The study, referred to as SWEEPS (Sagittarius Window Eclipsing Extrasolar Planet Search), was the first to show that planets are just as common in other regions of the Milky Way as they are near our solar system.
The only exoplanet to have been directly visualized: planet 2M1207b (red) orbits its star 2M1207 (blue). This image was captured by the European Southern Observatory's Very Large Telescope in 2005.
"The SWEEPS results showed that the frequency of planets is similar to that in our solar neighborhood, even in the Galactic bulge," Sahu said. "This implies that there are indeed billions of planets in the galaxy." In fact, the team now estimates that there may be around 6 billion Jupiter-sized planets in the Milky Way.
Sahu and his colleagues detected the planet candidates by measuring how much a star darkens as a planet passes in front of it, an event called a transit. This is similar to a solar eclipse, in which the Moon passes in front of the Sun and blocks out most of its light. Since the solar systems are so far away, though, the planets only dim the star by 1 to 10 percent. To make sure the dimming represented a regularly orbiting planet, the team detected between two and fifteen transits per planet candidate.
The SWEEPS survey also revealed a new type of planet: ultra-short period planets (USPPs). These exotic objects hug their stars very closely, whirling around them in less than one day. For example, the planet candidate SWEEPS-10 travels through one cycle or "year" in only ten hours! Furthermore, since it is so close to its star, the planet is estimated to have a surface temperature of about 1650 degrees Celsius, hot enough to melt iron.
One-half of the field of view of the Hubble Space Telescope in the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS), containing about 150,000 stars. The field is so crowded because the telescope must look past many stars to see the center of the galaxy, 26,000 light years away. (Credit: NASA, ESA, K. Sahu (STScI) and the SWEEPS science team)
Before SWEEPS-10, astronomers had never discovered a planet orbiting a star so quickly. "The lowest orbital period previously known was less than 1.2 days," Sahu explains. "This was mainly because the previous studies were not very sensitive to detecting planets around low-mass stars." This is important because, in order to have such a short orbit around a larger star, a planet would have to be so close to the star that it would probably evaporate.
Also, because the SWEEPS survey was able to detect planets around smaller stars than ever before, astronomers have learned something about how common planets are around more lightweight suns. "Our results showed that planets are also equally abundant around low-mass stars," Sahu said.
Finally, Sahu and his team found that all of the discovered planet candidates revolve around stars containing chemical elements heavier than hydrogen and helium, like carbon. This supports the belief among most astronomers that stars must be rich in such heavy elements in order for planets to form.
There's No Place Like Home. or Is There?
Although most of the candidate planets from SWEEPS are unconfirmed, two of them revolve around stars bright enough to determine the approximate mass of the planets, promoting them from "candidate planets" to "planets." One of them is about ten times as massive as Jupiter, while the other is less than four times as massive as Jupiter; both are quite large by our standards, but still small enough to be considered planets. The rest of the candidates' masses may be confirmed by the planned James Webb Space Telescope, a powerful observatory that will "see" in invisible infrared light. The Webb Telescope is currently scheduled to launch in June 2013.
But what about smaller bodies rocky planets like Earth that might harbor life? Detection of such planets will be the goal of the French CNES's COROT mission, set to launch in December 2006, and NASA's Kepler mission, scheduled for launch in October 2008. "Kepler," explains Sahu, "will monitor some 100,000 stars in the Galactic disk for four or five years. These observations will be sensitive to detecting Earth-sized planets." In fact, the Kepler and COROT missions may detect hundreds of Earth-sized planet candidates within the habitable zone the region within which liquid water can exist on a planet's surface around their stars. With this knowledge, scientists may be able to estimate how many habitable planets there are in the galaxy. "Many of Kepler's concepts are very similar to that of the SWEEPS project," Sahu adds, making SWEEPS a good proof-of-concept for future searches like Kepler.
As it is, the SWEEPS survey has shown that planets are probably just as common throughout the galaxy as they are in our solar neighborhood. With several billion planets now estimated to exist in the Milky Way, the possibility of life elsewhere becomes more tangible. Sahu notes that, because of this, endeavors like the Search for Extraterrestrial Intelligence (SETI), which monitors radio frequencies for signs of intelligence in outer space, now have a "stronger purpose and meaning."
So, although physical travel to other solar systems remains a distant dream, perhaps to be realized by our distant descendents, astronomers are now catching glimpses of planets in other solar systems; we are finally spying other islands in the cosmic sea.