Is there an Earth's twin?

Twinkle, twinkle little star, goes the nursery rhyme, but what scientists are really wanting to find is planet’s with a particular twinkle. That’s because sunlight glinting off extraterrestrial oceans could help astronomers spot liquid surface water on other worlds.

Researchers have modeled the way a distant planet would reflect light towards Earth as it orbits its star. They found that when a watery planet appears as a crescent, light striking the smooth surface of large bodies of water would make it appear brighter. But light reflected off of the surface of a drier planet would not look brighter in this way.

Oceans cover over 70 percent of Earth's surface, and surely there are other watery worlds out there like ours surmise astronomers. The problem is that there are currently no telescopes capable of specifically identifying planets with water on their surfaces. However, this new method gives researchers a way to find water worlds like Earth solely by observing its “twinkle”. The method, reported in an upcoming issue of Icarus, relies on the specific reflective properties of water and sunlight.

"A planet like Venus, with a dense atmosphere, will scatter the sunlight in all directions," says Darren M. Williams, associate professor of physics and astronomy, Penn State Erie, the Behrend College. "If you look at Venus in phases, when it is full, it is brightest and when it is crescent, it is faintest."

But if a planet with water on its surface were full, in respect to its sun, with the whole disk illuminated, water would actually look darker than dirt. Conversely, when the planet is in crescent phase, with the sun glancing off of the watery surface, the reflection would be brightest.

By monitoring the light curve of a distant planet through an optical space telescope as the planet spins on its axis and moves around its star, researchers can observe the changes in brightness, correlated to the planet's phase, which should tell them precisely whether or not the planet has liquid oceans. If the temperatures match up, they can then be fairly certain that the liquid is water. It sounds simple enough, but if the research pans out it would represent a big step forward in finding Earthlike planets. It’s one thing to find a planet that lies in the “sweet spot” distance from it’s star, but to know whether or not there is water on the planet would take the search for extraterrestrial life to a whole new level.

"We are looking for Earth-like planets in the habitable zone of their star, a band not too hot nor too cold for life to exist," says Williams. "We also want to know if there is water on these planets."

For life as we know it to exist, planets must have habitable temperatures throughout a period long enough for life to evolve. As far as we know, it would also help our search to find a significant amount of water on the planet’s surface. Scientists already know how to determine the distance a planet orbits from its star, and analysis of light interacting with molecules in the atmosphere can indicate if water exists in some form. However, Williams and Eric Gaidos, associate professor of geobiology, University of Hawaii, want to specially identify truly Earthlike planets with liquid water on their surfaces.

The image of the Blue Marble, taken by Apollo 17 in December 1972, is striking because the Earth is mostly covered in water. The researchers believe that large enough amounts of water will provide a glint of light visible in the infrared and visible spectrum if they watch the planet for long enough.

"We are going to look at the planets for a long time," says Williams. "They reflect one billionth or one ten billionth of their sun. To gain enough light to see a dot requires observation over two weeks with the kinds of telescopes we are imagining. If we stare that long, unless the planet is rotating very slowly, different sides of the planet will come through our field of view. If the planet is a mix of water, we are going to see the mix travel around the planet."

Astronomers are hopeful that a terrestrial planet finder telescope will orbit the earth in the next 10 to 20 years, which will make the search much easier. But until then, Williams has arranged for the current Mars Express and Venus Express missions of the European Space Agency to look back at the Earth occasionally from great distances and observe what our watery planet looks like in various phases.

"Any time that the Earth is in a crescent phase as viewed by a distant space vehicle, we should take advantage of the situation and look back at the Earth," says Williams.

This will help researchers verify and fine-tune the method, and with any luck we’ll soon be able to identify planets truly worthy of the “Earth’s Twin” title.

Rebecca Sato.