Fourth planet found in Solar System look-alike

An artist's impression of one of the planets in the HR 8799 system.

An artist's impression of one of the planets in the HR 8799 system. Credit: NASA, ESA, and G. Bacon (STScI).

  • Fourth planet found in system with three previously known planets
  • The planetary system resembles a supersized version of our Solar System
  • Origin of the four giant planets remains a puzzle

An international team of astronomers has discovered and imaged a fourth giant planet outside our Solar System, a discovery that further strengthens the remarkable resemblances between a distant planetary system and our own.

In a new paper in the journal Nature, the astronomers say the planetary system resembles 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 Benjamin Zuckerman, a UCLA professor of physics and astronomy and co-author of the Nature paper.

Our giant planets are Jupiter, Saturn, Uranus and Neptune, and our debris belts include the asteroid belt between the orbits of Mars and Jupiter and the Kuiper Belt, beyond Neptune’s orbit.

The newly discovered fourth planet (known as HR 8799e) orbits a bright star called HR 8799, which lies some 129 light years from Earth and is faintly visible to the naked eye. The mass of the HR 8799 planetary system is much greater than our own.

Astronomers estimate that the combined mass of the four giant planets may be 20 times greater than the mass of all the planets in our Solar System, and the debris belt counterparts also contain much more mass than our own.

Representation of HR 8799 planetary system and Solar System

A 3D representation of the HR 8799 planetary system (left) and our Solar System (right). All orbital diameters are greatly exaggerated in order to make them visible. (Background Milky Way image credit: Atlas image courtesy of 2MASS / Umass / IPAC-Caltech / NASA / NSF.)

Planets’ long-term fate uncertain

The new planet joins three previously discovered planets that were the subjects of a 2008 paper in the journal Science reporting the first-ever images of a planetary family orbiting a star other than our sun. Four of the co-authors of the new Nature paper, including Zuckerman, were also co-authors on that Science paper.

“This is the fourth imaged planet in this planetary system, and only a tiny percentage of known exoplanets (planets outside our Solar System) have been imaged; none has been imaged in multiple-planet systems other than those of HR 8799,” Zuckerman said.

All four planets orbiting HR 8799 are similar in size, likely between five and seven times the mass of Jupiter. The newly discovered planet orbits HR 8799 more closely than the other three. If it were in orbit around our Sun, astronomers say, it would lie between the orbits of Saturn and Uranus.

The astronomers used the Keck II telescope at Hawaii’s W.M. Keck Observatory to obtain images of the fourth planet. Zuckerman’s colleagues are from Canada’s National Research Council (NRC), Lawrence Livermore National Laboratory (LLNL) in California, and Lowell Observatory in Arizona.

“The images of this new inner planet are the culmination of 10 years’ worth of innovation, making steady progress to optimise every aspect of observation and analysis,” said Christian Marois, an NRC astronomer and lead author of the Nature paper. “This allows us to detect planets located ever closer to their stars and ever further from our own Solar System.”

HR 8799 planetary system

Actual image showing the three earlier planets (b, c, and d) and the newly discovered planet "e". Arrows show the next 10 years of motion. 20 AU is 20 times the distance from the Earth to the Sun. (Credit: NRC-HIA, C. Marois, and Keck Observatory)

“The four massive planets pull on each other gravitationally,” said co-author Quinn Konopacky, a postdoctoral 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.”

New instrument will help with future studies

The origin of these four giant planets remains a puzzle; neither of the two main models of planet formation can account for all four.

“There’s no simple model that can form all four planets at their current location,” said co-author Bruce Macintosh of LLNL. “It’s going to be a challenge for our theoretical colleagues.”

It is entirely plausible that this planetary system contains additional planets closer to the star than these four planets, quite possibly rocky, Earth-like planets, Zuckerman said. But such interior planets are far more difficult to detect, he added.

“Images like these bring the exoplanet field, which studies planets outside our Solar System, into an era of exoplanet characterisation,” said co-author Travis Barman, a Lowell Observatory exoplanet theorist.

“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 the planet’s relative faintness and its proximity to its star. To overcome those limitations, Macintosh is leading an effort to build an advanced exoplanet imager, called the Gemini Planet Imager, for the Gemini Observatory.

This new instrument will physically block the starlight and allow quick detection and detailed characterisation of planets similar to HR 8799e. UCLA and the NRC are also contributing to Gemini Planet Imager.

Adapted from information issued by Stuart Wolpert, UCLA.

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