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Planetary atmospheres may hold the key to identifying Earth-like worlds

When NASA's Kepler spacecraft launched in 2009, its mission was to find other worlds akin to Earth orbiting the immense number of stars in the Milky Way galaxy.

In their pursuit to find other habitable worlds, astronomers look at a planet's size, the distance from its star and the planet's atmosphere - three components which are essential in sustaining life.

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However, the study of planetary atmospheres remains a challenge because of the vast distances between planets and our observation instruments, Pennsylvania State University NASA Sagan Postdoctoral Fellow Paul Robertson said.

In February, San Francisco State University Astronomer Stephen Kane published a report online regarding light reflected from an exoplanet located 117 light-years from Earth. Kane and his research team were able to detect a signal of reflected starlight bouncing off the planet's atmosphere as it made its closest approach to its star.

"This is a window into planetary atmospheres," Robertson said, adding that wavelengths of reflected starlight and the degree of reflectivity can allow scientists to infer more information about a planet's unique characteristics.

A very small amount of light is reflected from planetary atmospheres, and scientists have to analyze what is being reflected, break it up and if possible determine what wavelengths are being absorbed versus reflected, Robertson said. He cited that this particular method of study is challenging due to the limited light reflected.

In the case of HD 20782, the exoplanet observed by Kane's team, researchers speculate the reflectivity, which is determined by atmospheric composition, may indicate it has an atmosphere similar to Jupiter, boasting thick clouds of icy particles.

"If [light] is reflected, you may have some kind of molecule in the atmosphere, which you can look at and then infer [more information] from," Robertson said.

Unlike the planets in our solar system, HD 20782 has the most eccentric orbit of any planet yet discovered, which may have preserved its atmospheric reflectivity as it passed the star, according to Kane's report.

"In some of the extrasolar, Jupiter-sized planets that tread short, circular orbits, this phenomenon does appear to strip the atmospheres of reflective particles, making the planets appear 'dark,' Kane said in his report. "But in the case of HD 20782, the atmosphere of the planet doesn't have a chance to respond. The time it takes to swing around the star is so quick that there isn't time to remove all the icy materials that make the atmosphere so reflective."

Robertson compared the highly elliptical orbit of HD 20782 to the orbits of comets, stretching far out from the star, before they slingshot quickly around the star at the closest approach.

"HD 20782 offers a particularly lucrative observing opportunity for studying the planetary atmosphere of an eccentric-orbit planet - a type not seen in our own solar system. By studying the reflected light from HD 20782, astronomers may learn more about the structure and composition of a planetary atmosphere that can withstand a brief but blistering exposure to its star," Kane said in his report.

While the composition of HD 20782 was not able to be determined by Kane's team, the reflectivity is one element of ongoing research of the unique exoplanet and its eccentric orbit. On Sept. 3, the planet will make its next closest approach to the star, Robertson said, adding that eyes will be fixed on HD 20782 for further research.

As a planet crosses in front of its star or transits at its observable angle, it produces a change in the star's brightness. The probability for an Earth-like orbit around a star similar to the sun is very low, approximately 0.5 percent, according to NASA, making this method of detection even more difficult.

Since its foundation, the Kepler mission has led to the discovery of more than 1,000 planets, some of which are Earth-sized and located within the habitable zone from their stars.

"Kepler has shown us that there are planets everywhere," Robertson said, adding NASA will continue its search for habitable worlds in the future. Two new initiatives will launch to further the search in the next three years.

A science team for NASA's Transiting Exoplanet Survey Satellite (TESS), including Kane, will have its sights set on HD 20782 after it launches no later than June of 2018.

"In a two-year survey of the solar neighborhood, TESS will monitor more than 200,000 stars for temporary drops in brightness caused by planetary transits," according to NASA.

Through a joint initiative NASA and the National Science Foundation called NN-Explore, another new instrument will be launched with help from Pennsylvania State University researchers, Robertson said. The researchers will help to develop a spectrograph called NEID, which is a high resolution tool that will be used to find additional exoplanets. The mission is set to launch in 2019.

In March, a Penn State-led research group was selected by NASA to build the $10-million instrument. The research group will create a target list of one's which may be habitable, Robertson said.

"TESS and this project are part of the next generation [of extra-solar research]," Robertson said.