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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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Backward planets turn theory upside down

Artist's impression of an exoplanet in a retrograde orbit around a star

Up to now it was expected that exoplanets would all orbit in more or less the same plane, and that they would move along their orbits in the same direction as the star’s rotation—as they do in our Solar System. However, new results unexpectedly show that with some, the orbit is completely reversed.

The discovery of nine new exoplanets as announced today at the Royal Astronomical Society’s National Astronomy Meeting in Glasgow. Exoplanet are ones that orbit stars beyond our Solar System.

When these new results were combined with earlier observations, the astronomers were surprised to find that six out of a larger sample of 27 were found to be orbiting in the opposite direction to the rotation of their host star — the exact reverse of what is seen in our own Solar System.

The new discoveries provide an unexpected and serious challenge to current theories of planet formation. They also suggest that systems with exoplanets of the type known as “hot Jupiters” are unlikely to contain Earth-like planets.

“This is a real bomb we are dropping into the field of exoplanets,” says Amaury Triaud, a PhD student at the Geneva Observatory who, with Andrew Cameron and Didier Queloz, leads a major part of the observational campaign.

Planets are thought to form in the flattened, disc-like cloud of gas and dust encircling a young star. This proto-planetary disc rotates in the same direction as the star itself, and up to now it was expected that planets that form from the disc would all orbit in more or less the same plane, and that they would move along their orbits in the same direction as the star’s rotation. This is the case for the planets in the Solar System.

After the initial detection of the nine new exoplanets with, the WASP (Wide Angle Search for Planets) team of astronomers used the 3.6-metre European Southern Observatory (ESO) telescope at the La Silla observatory in Chile, along with data from the Swiss Euler telescope, also at La Silla, and data from other telescopes to confirm the discoveries and characterise the exoplanets found in both the new and older surveys.

Surprisingly, when the team combined the new data with older observations they found that more than half of all the hot Jupiters studied have orbits that are misaligned with the rotation axis of their parent stars.

They even found that six exoplanets in this extended study (of which two are new discoveries) have “retrograde motion”—they orbit their star in the “wrong” direction.

“The new results really challenge the conventional wisdom that planets should always orbit in the same direction as their stars spin,” says Andrew Cameron of the University of St Andrews.

Artist's impressions of the exoplanets that have "backward" orbits.

Artist's impressions of the exoplanets that have "backward" orbits. The exoplanets are shown moving across the face of their parent stars, which is how they were found. The object at the lower right is for comparison and has a “normal” orbital direction.

How do “hot Jupiter’s” form?

In the 15 years since the first hot Jupiters were discovered, their origin has been a puzzle. These are planets with masses similar to or greater than that of Jupiter, but that orbit very close to their suns.

The cores of giant planets are thought to form from a mix of rock and ice particles found only in the cold outer reaches of planetary systems. Hot Jupiters must therefore form far from their star and subsequently migrate inwards to orbits much closer to the parent star.

Many astronomers believed this was due to gravitational interactions with the disc of dust from which they formed. This scenario takes place over a few million years and results in an orbit aligned with the rotation axis of the parent star.

It would also allow Earth-like rocky planets to form subsequently, but unfortunately it cannot account for the new observations.

To account for the new retrograde exoplanets an alternative migration theory suggests that the proximity of hot Jupiters to their stars is not due to interactions with the dust disc at all, but to a slower evolution process involving a gravitational tug-of-war with more distant planets or stars over hundreds of millions of years.

After these disturbances have bounced a giant exoplanet into a tilted and elongated orbit it would suffer tidal friction, losing energy every time it swung close to the star. It would eventually become parked in a near circular, but randomly tilted, orbit close to the star.

“A dramatic side-effect of this process is that it would wipe out any other smaller Earth-like planet in these systems,” says Didier Queloz of Geneva Observatory.

Two of the newly discovered retrograde planets have already been found to have more distant, massive companions that could potentially be the cause of the upset. These new results will trigger an intensive search for additional bodies in other planetary systems.

The current count of known exoplanets is 452.

Adapted from information issued by ESO / A.C. Cameron / L. Calçada.