Venus is uninhabitable for humans yet from a distance, using statistical wobbles, a Venus-like planet and an Earth-like planet are very similar.

While the Earth has oceans of water and relatively moderate temperatures, Venus has no liquid
water and exists in a runaway greenhouse scenario where the levels of carbon dioxide are so high that the atmosphere traps all the heat. The planet is torrid but it is 95 percent the size of Earth. Right now, size is what astronomers are looking for when searching for exoplanets but size alone is not a distinguishing characteristic when sorting for a habitable one. 

NASA's Kepler space telescope can identify planets close to their suns by recording the shadow that a planet makes when crossing in front of its star. Multiple views over time not only sort out the errant sunspot from a potential planet, but also provide the planet's orbit -- the all-important distance from the star. Now astronomers are zeroing in on a way to better tell them apart. 

"We want to know how common Venus-like planets are," said Ravi Kumar Kopparapu, research associate in geosciences, Penn State. "We also want to know how common solar systems like ours are. Just as stars have habitable zones -- the orbital belt where planets have water that can remain liquid -- stars have Venus zones," said Kopparapu.

Kopparapu, working with Stephen Kane, assistant professor of physics and astronomy, San Francisco State University and Shawn Domagal-Goldman, research assistant, planetary studies, NASA Goddard Space Flight Center, developed parameters for a Venus zone, the closest distance from a star where a planet can retain its atmosphere to the furthest distance that will sustain a runaway greenhouse.

The researchers looked at potential Venus-like planets located in the Kepler database. They searched for planets that were between half Earth's size and almost one and a half times the size of Earth that fell within the Venus zone, locating 43 potential Venus-like planets. Their findings will be posted on ArXiv on Sept. 9 and will appear in an upcoming issue of Astrophysical Journal Letters.

These are only potential Venus-like planets because while they are the correct size and fall in the Venus zone, the Kepler space telescope cannot determine if these planets have an atmosphere, which is necessary for a Venus analog.

"We would need to use something like the planned James Webb Space Telescope to see if planets have an atmosphere," said Kopparapu. "If a planet has an atmosphere dominated by a greenhouse gas such as carbon dioxide, is the right size and has the correct amount of light falling on it, then it might be a Venus-like planet."

Some planets with the proper size and location will be devoid of atmosphere, some planets in the Venus zone will be gas giants, not Venus-like at all, but some will mimic the atmospheric scenario that currently exists on Venus.

Not all stars are the same as the sun, so both the Venus zone and habitable zone shift depending on the energy output of individual star types. Also, the Kepler Space Telescope's method of identifying planets is biased toward planets closer to their stars -- more Venus zone planets will be identified than Earth-like planets. And not all of the planets identified by Kepler have been determined to be actual planets.

The researchers want to understand how terrestrial planets like Venus, Earth and Mars and gas giant planets like Jupiter are distributed in planetary systems around other stars. Understanding how planetary systems form and where different types of planets form can shed light on whether our solar system is unique, or more likely, is a "standard" form of planetary system formation.