An international team of astronomers has discovered a fourth giant planet, HR 8799e, outside our solar system. The new planet joins the three planets that were the subjects of the first-ever images of a planetary family orbiting a star other than our Sun. The planets orbit the star HR 8799, which lies about 129 light years from Earth and is faintly visible to the naked eye.

All four planets orbiting HR 8799 are similar in size: likely between five and seven times the mass of Jupiter, the largest planet in the Sun's own family. The newly revealed planet orbits HR 8799 more closely than the other three. If this newly discovered planet were in orbit around the Sun, it would lie between the orbits of Saturn and Uranus.

"We reached a milestone in the search for other worlds in 2008 with the discovery of the HR 8799 planetary system," said Dr. Christian Marois, an astronomer with NRC. Dr. Marois is the first author of the new paper and designed the improved image-processing software that made the new discovery possible.

"The images of this new inner planet are the culmination of ten years' worth of innovation, making steady progress to optimize every aspect of observation and analysis. Compared with what was previously possible, this allows us to detect planets located ever closer to their stars and ever further from our own solar system."

Discovery of this fourth giant planet strengthens the remarkable resemblance between the HR 8799 planetary system and our own - the HR 8799 system appears as a supersized version of our solar system.

"Besides having four giant planets, both systems also contain two 'debris belts,' composed of small rocky or icy objects along with lots of tiny dust particles," said co-author Ben Zuckerman, a professor of physics and astronomy at UCLA.

The mass of the HR 8799 planetary system is much more extreme than that of our own - the combined mass of the four giant planets may be twenty times higher, and the debris belt counterparts are also much larger than our own.

"The four massive planets pull on each other gravitationally," said co-author Quinn Konopacky, a post-doctoral researcher at LLNL. "We don't yet know if the system will last for billions of years, or fall apart in a few million more. As astronomers carefully follow the HR 8799 planets during the coming decades, the question of the stability of their orbits could become much clearer."

The origin of these four giant planets remains a puzzle - neither of the two main models of planet formation can account for all four objects. Dr. Bruce Macintosh of LLNL, a co-author, noted that there's no simple model that can form all four planets at their current location. It's going to be a challenge for our theoretical colleagues.

Travis Barman, a Lowell Observatory exoplanet theorist and study co-author stated images like these bring the exoplanet field, which studies planets outside our solar system, into an era of exoplanet characterization. Astronomers can now directly examine the atmospheric properties of four giant exoplanets that are all the same young age and that formed from the same building materials.

Detailed study of the properties of HR 8799e will be challenging due to its relative faintness and proximity to its star. To overcome these limitations, a team led by Dr. Macintosh, including NRC and several US institutions, is building an advanced new instrument, called the Gemini Planet Imager for the Gemini Observatory.

This new instrument will physically block the starlight and allow quick detection and detailed characterization of planets similar to HR 8799e. The Gemini Planet Imager is scheduled to arrive at the Gemini South telescope in Chile late in 2011.

"We can expect a tidal wave of new discoveries with the new planet imager. HR 8799 is really just the beginning, the tip of the iceberg," said Dr. Marois.

The international team included astronomer Dr. Christian Marois of the National Research Council Canada (NRC) in Victoria, B.C., as well as astronomers from the University of California Los Angeles (UCLA), Lawrence Livermore National Laboratory (LLNL), and the Lowell Observatory. Their discovery was published in Nature, and images of the fourth planet were captured at Hawaii's W. M. Keck Observatory.