CfA: Alien World Looms Large in Its Neighbor's Sky

[bystander]

Planetrise: Alien World Looms Large in its Neighbor's Sky
Harvard Smithsonian Center for Astrophysics | 2012 June 21

[img3="In this artist's conception, a "hot Neptune" known as Kepler-36c looms in the sky of its neighbor, the rocky world Kepler-36b. The two planets have repeated close encounters, experiencing a conjunction every 97 days on average. At that time, they are separated by less than 5 Earth-Moon distances. Such close approaches stir up tremendous gravitational tides that squeeze and stretch both planets, which may promote active volcanism on Kepler-36b. (Credit: David A. Aguilar (CfA))"]http://www.cfa.harvard.edu/image_archiv ... /lores.jpg[/img3]

[i]This image, adapted by Eric Agol of the UW, depicts the view one might have of a rising Kepler-36c (represented by a NASA image of Neptune) if Seattle (shown in a skyline photograph by Frank Melchior, frankacaba.com) were placed on the surface of Kepler-36b. [/i]

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Few nighttime sights offer more drama than the full Moon rising over the horizon. Now imagine that instead of the Moon, a gas giant planet spanning three times more sky loomed over the molten landscape of a lava world. This alien vista exists in the newly discovered two-planet system of Kepler-36.

"These two worlds are having close encounters," said Josh Carter, a Hubble Fellow at the Harvard-Smithsonian Center for Astrophysics (CfA).

"They are the closest to each other of any planetary system we've found," added co-author Eric Agol of the University of Washington.

Carter, Agol and their colleagues report their discovery in the June 21st Science Express.

They spotted the planets in data from NASA's Kepler spacecraft, which can detect a planet when it passes in front of, and briefly reduces the light coming from, its parent star.

The newfound system contains two planets circling a subgiant star much like the Sun except several billion years older. The inner world, Kepler-36b, is a rocky planet 1.5 times the size of Earth and weighing 4.5 times as much. It orbits about every 14 days at an average distance of less than 11 million miles.

The outer world, Kepler-36c, is a gaseous planet 3.7 times the size of Earth and weighing 8 times as much. This "hot Neptune" orbits once each 16 days at a distance of 12 million miles.

The two planets experience a conjunction every 97 days on average. At that time, they are separated by less than 5 Earth-Moon distances. Since Kepler-36c is much larger than the Moon, it presents a spectacular view in its neighbor's sky. (Coincidentally, the smaller Kepler-36b would appear about the size of the Moon when viewed from Kepler-36c.) Such close approaches stir up tremendous gravitational tides that squeeze and stretch both planets.

Researchers are struggling to understand how these two very different worlds ended up in such close orbits. Within our solar system, rocky planets reside close to the Sun while the gas giants remain distant.

Although Kepler-36 is the first planetary system found to experience such close encounters, it undoubtedly won't be the last.

"We're wondering how many more like this are out there," said Agol.

"We found this one on a first quick look," added Carter. "We're now combing through the Kepler data to try to locate more."

This result was made possible with asteroseismology. Asteroseismology is the study of stars by observing their natural oscillations. Sunlike stars resonate like musical instruments, due to sound waves trapped in their interiors. And just like a musical instrument, the larger the star, the "deeper" are its resonances. This trapped sound makes the stars gently breathe in and out, or oscillate.

Co-author Bill Chaplin (University of Birmingham, UK) noted, "Kepler-36 shows beautiful oscillations. By measuring the oscillations we were able to measure the size, mass and age of the star to exquisite precision."

He added, "Without asteroseismology, it would not have been possible to place such tight constraints on the properties of the planets."

Astronomers Discover Planetary Odd Couple
NASA | JPL-Caltech | Ames Research Center | 2012 June 21

Alien World Looms Large in its Neighbor World's Sky
NASA Kepler Mission News | 2012 June 21

Astronomers spy two planets in tight quarters as they orbit a distant star
University of Washington | Vince Stricherz | 2012 June 21

Astronomers with NASA’s Kepler Mission find ‘puzzling pair of planets’
Iowa State University | Mike Krapfl | 2012 June 21

What a View! Exoplanet Odd Couple Orbit in Close Proximity
Universe Today | Nancy Atkinson | 2012 June 21

Exoplanet 'Odd Couple' Defy Formation Theories
Discovery News | Irene Klotz | 2012 June 21

Planetary Roommates Are an Odd Couple
Science Shot | Krystnell A. Storr | 2012 June 21

Kepler-36: A Pair of Planets with Neighboring Orbits and Dissimilar Densities - Joshua A. Carter et al

• Science (online 21 June 2012) DOI: 10.1126/science.1223269
  arXiv.org > astro-ph > arXiv:1206.4718 > 20 Jun 2012

Rapid dynamical chaos in an exoplanetary system - Katherine M. Deck et al

• arXiv.org > astro-ph > arXiv:1206.4695 > 20 Jun 2012

[bystander]

Exoplanetary Bedfellows Make Odd Couple
New Scientist | Astrophile | MacGregor Campbell | 2012 Jun 21

Object: Exoplanet pair
Location: 1200 light years from Earth

A deep blue arc peeks above the horizon of a lava-strewn alien world. As it slowly rises, it becomes a circle in the sky, looming three times as large as a full moon back on Earth. This is no moon, though. Despite its close proximity, it's a planet.

Such a "planet-rise" is what you would see if you were standing on the rockier half of a planetary odd couple that has just been spotted orbiting a sun-like star. One is rocky and dense: a super Earth. Its companion is airy and majestic: a hot Neptune. These close-orbiting yet radically different worlds are the strangest planetary pair yet discovered.

Until relatively recently, our only model for how a solar system is organised was our own: rocky planets inhabiting the inner circles, while gas giants slowly make their way around the outer reaches. Gas giants tend to form in the outer solar system because that's where water and other liquids stay solid, the theory went, ensuring there is enough raw materials to make the dense core of what will become a gas giant.

But in the late 1980s and early 1990s, this neat picture was disrupted by the discoveries of the first exoplanets. These were giants more massive than Jupiter, often orbiting closer to their stars than Mercury.

This seeming contradiction has since been resolved: such hot Jupiters are thought to form far out, only to hurtle inwards later. Indeed, hot Jupiters seem to be loners, having long ago trashed or gobbled up any other planets that had dared to orbit the same star.

Crazy composition

Now our view of what's possible has been broadened even further. Josh Carter of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and colleagues combed through data from the Kepler space telescope to spot a two-planet system.

This exo-planetary odd couple zoom around their host star, Kepler-36, in similar orbits. At their closest approach, they come within 1.2 million miles of each other. That's about five times the distance from Earth to the moon, and 20 times closer than Earth and Venus, the two closest planets in our solar system. Both planets orbit at around a third of the distance from Mercury to our sun, with the super Earth slightly closer than its gaseous companion.

While these orbits are the closest to each other found in any solar system to date, what really sets the pair apart are their differing compositions.

Migration likely

The super Earth, Kepler 36-b, is a rocky planet 4.5 times more massive than Earth, with a radius 1.5 times greater. Kepler 36-c is almost twice as massive as its partner, with a radius about 2.5 times as large. This means it must be less dense, most likely a small gas giant, or hot Neptune. "They have a density contrast comparable to that between Saturn and the Earth," says Carter.

Astronomers don't agree on how two disparate worlds ended up so close together. Greg Laughlin of the University of California, Santa Cruz, who was not involved with the research, thinks the two planets must have somehow formed near to their current orbits . Recent models have shown that Neptune-sized planets can form closer to their star than previously thought.

In contrast, Phil Armitage, an astrophysicist at the University of Colorado at Boulder, thinks they probably formed further apart in the system and migrated to their current positions, rather like hot Jupiters. "The exceptional data quality of this system tips the balance in favour of migration, but both theories are likely still viable," he says.

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