Is the solar system unique?

"Is Our Solar System Unique or the 'Standard'?" --Kepler Mission Scientists

Kepler 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.

"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." The team 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.

Identification of planets orbiting distant stars is spurring the search for an Earth-like planet. Now a team of researchers has developed a way to distinguish a distant Venus-like planet from an Earth-like one. 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 and the planet is torrid. Venus is 95 percent the size of Earth so size is not a distinguishing characteristic when sorting Earth- and Venus-like planets. Distance from a star, however, is what potentially makes a planet Earth-like or Venus-like.

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.

"Just as stars have habitable zones -- the orbital belt where planets have water that can remain liquid -- stars have Venus zones," said 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.

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.

This past December, a team of European astrophysicists discovered the most extensive planetary system to date, orbiting star KOI-351. The star system has seven planets, more than in other known planetary systems arranged in a similar fashion to the eight planets in the Solar System, with small rocky planets close to the parent star and gas giant planets at greater distances.

Although the planetary system around KOI-351 is packed together more tightly, “We cannot stress just how important this discovery is. It is a big step in the search for a ‘twin’ to the Solar System, and thus also in finding a second Earth,” said Juan Cabrera, an astrophysicist at the DLR Institute of Planetary Research in Berlin-Adlershof.

KOI is the abbreviation for ‘Kepler Object of Interest’, which means the star was observed by NASA’s Kepler space telescope, between 2008 and 2013, and classified as a candidate for the existence of exoplanets. At present, KOI-351 is the star with the most extrasolar planets, or exoplanets for short. The star is 2500 light years away from Earth.

Astrophysicists around the world have been searching for a star system similar to our own for a long time. Now, the team led by Cabrera has taken a major step in this direction. Three of the seven planets in orbit around the star KOI-351 were discovered in recent years, and have periods of 331, 211 and 60 days, similar to those of Earth, Venus and Mercury.

Reference:

http://www.dailygalaxy.com/my_weblog/2014/09/is-our-solar-system-unique-or-the-standard-kepler-mission-researchers-ask-space-researchers-want-to-understand-how-terres.html

https://en.wikipedia.org/wiki/Earth_analog

http://www.jwst.nasa.gov/

http://kepler.nasa.gov/