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Sunday, June 24, 2012

Absolutely everything about Kepler-36b and Kepler-36c

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)

Planetrise: Alien World Looms Large in its Neighbor's Sky

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."

Figure S21 in a supplementary document associated with the research paper published on Science Express. Distribution of densities and effective temperatures for super-Earths and mini-Neptunes (Mp < 10M⊕). The authors have indicated the lack of hot mini-Neptunes (ρ < 3.5 g/cc, T < 1200K) with a dashed blue box. Credit: Joshua A. Carter, Eric Agol et al. / Science
By: Harvard-Smithsonian Center for Astrophysics

Astronomers spy two planets in tight quarters as they orbit a distant star

A research team led by astronomers at the University of Washington and Harvard University has discovered a bigger version of Earth locked in an orbital tug-of-war with a much larger, Neptune-sized planet as they orbit very close to each other around the same star about 1,200 light years from Earth.

The planets occupy nearly the same orbital plane and on their closest approach come within about 1.2 million miles of each other – just five times the Earth-moon distance and about 20 times closer to one another than any two planets in our solar system.

But the timing of their orbits means they'll never collide, said Eric Agol, a UW astronomy professor and co-lead author of a paper documenting the discovery published June 21 by Science Express, the online edition of the journal Science.

"These are the closest two planets to one another that have ever been found," Agol said. "The bigger planet is pushing the smaller planet around more, so the smaller planet was harder to find."

Orbiting a star in the Cygnus constellation referred to as Kepler-36a, the planets are designated Kepler-36b and Kepler-36c. Planet b is a rocky planet like Earth, though 4.5 times more massive and with a radius 1.5 times greater. Kepler-36c, which could be either gaseous like Jupiter or watery, is 8.1 times more massive than Earth and has a radius 3.7 times greater.

The larger planet was originally spotted in data from NASA's Kepler spacecraft, which uses a photometer to measure light from distant celestial objects and can detect a planet when it transits, or passes in front of, and briefly reduces the light coming from, its parent star.

The team wanted to try finding a second planet in a system where it was already known that there was one planet. Agol suggested applying an algorithm called quasi-periodic pulse detection to examine data from Kepler.

Joshua Carter, a Hubble fellow at the Harvard-Smithsonian Center for Astrophysics and the other co-lead author of the Science paper, used the algorithm to begin methodically checking planetary systems already in the Kepler data and saw a clear signal in the Kepler-36a system.

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

The data revealed a slight dimming of light coming from Kepler-36a every 16 days, the length of time it takes the larger Kepler-36c to circle its star. Kepler-36b circles the star seven times for each six orbits of 36c, but it was not discovered initially because of its small size and the gravitational jostling by its orbital companion. But when the algorithm was applied to the data, the signal was unmistakable.

"If you look at the transit time pattern for the large planet and the transit time pattern for the smaller planet, they are mirror images of one another," Agol said.

The fact that the two planets are so close to each other and exhibit specific orbital patterns allowed the scientists to make fairly precise estimates of each planet's characteristics, based on their gravitational effects on each other and the resulting variations in the orbits. To date, this is the best-characterized system with small planets, the researchers said.

They believe the smaller planet is 30 percent iron, less than 1 percent atmospheric hydrogen and helium and probably no more than 15 percent water. The larger planet, on the other hand, likely has a rocky core surrounded by a substantial amount of atmospheric hydrogen and helium.

The planets' densities differ by a factor of eight but their orbits differ by only 10 percent, which makes the differences in composition difficult for the scientists to explain using current models of planet formation.

The team also calculated specific information for the star itself, determining that Kepler-36a is about the same mass as the sun but is just 25 percent as dense. It also is slightly hotter and has slightly less metal content. The researchers concluded that the star is a few billion years older than the sun and no longer burns hydrogen at its core, so has entered a sub-giant phase in which its radius is 60 percent greater than the sun's.

Long cadence Kepler data used in the analysis for the research paper published on Science Express (Figure S6 in a supplementary document). Each panel shows data for a contiguous set of cadences near transits of either Planet b or Planet c. The red curve is the best-fitting photometric-dynamical model. The time and flux axis is the same in all panels. Credit: Joshua A. Carter, Eric Agol et al. / Science
By: University of Washington

Alien World Looms Large in its Neighbor World's Sky

Today NASA Kepler Mission astronomers are publishing in the journal Science Express the discovery of a star, Kepler-36, about 1200 light-years away, with two planets orbiting very close to each other but with very dissimilar densities. "These two worlds are having close encounters," said Josh Carter, a Hubble Fellow at the Harvard-Smithsonian Center for Astrophysics (CfA). Co-author Eric Agol of the University of Washington added, "They are the closest to each other of any planetary system we've found."

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, whereas the outer world, Kepler-36c, orbits at a distance of 12 million miles, about every 16 days, and is a gaseous planet 3.7 times the size of Earth. The authors believe the smaller planet is 30 percent iron, less than 1 percent atmospheric hydrogen and helium and probably no more than 15 percent water. The larger planet, by contrast, likely has a rocky core surrounded by a substantial amount of atmospheric hydrogen and helium, a "hot Neptune."

The two planets have closest approach (conjunction) about every 97 days, when they are separated by less than 5 Earth-Moon distances. At those times Kepler-36c would present a spectacular view in the sky of the smaller rocky Kepler-36b and both planets would experience significant tidal forces.

This system presents a puzzle as to 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.

This result was made possible by the incredibly precise brightness-measuring capability of the Kepler telescope which allows Kepler asteroseismologists to the study of oscillations stars. Stars resonate like musical instruments, with larger stars, having "deeper sounds" are its resonances.

Figure S20 in a supplementary document associated with the research paper. Distribution of fractional separation difference (a2/a1 − 1) and density ratios (ρ2/ρ1) for adjacent planets in our Solar System (blue) and in Kepler systems (black). The labels use the first letter of the solar system planets, and the Kepler planet number, (i.e. 11e,b plots the ratio of Kepler-11f to Kepler 11e). Kepler-36 is plotted as a red point. Credit: Joshua A. Carter, Eric Agol et al. / Science
By: Kepler site at Ames Research Center

Astronomers Discover Planetary Odd Couple

Astronomers have discovered a pair of neighboring planets with dissimilar densities orbiting very close to each other. The planets are too close to their star to be in the so-called "habitable zone," the region in a system where liquid water might exist on the surface, but they have the closest-spaced orbits ever confirmed. The findings are published today in the journal Science.

The research team, led by Josh Carter, a Hubble fellow at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and Eric Agol, a professor of astronomy at the University of Washington in Seattle, used data from NASA's Kepler space telescope, which measures dips in the brightness of more than 150,000 stars, to search for transiting planets.

The inner planet, Kepler-36b, orbits its host star every 13.8 days and the outer planet, Kepler-36c, every 16.2 days. On their closest approach, the neighboring duo comes within about 1.2 million miles of each other. This is only five times the Earth-moon distance and about 20 times closer to one another than any two planets in our solar system.

Kepler-36b is a rocky world measuring 1.5 times the radius and 4.5 times the mass of Earth. Kepler-36c is a gaseous giant measuring 3.7 times the radius and eight times the mass of Earth. The planetary odd couple orbits a star slightly hotter and a couple billion years older than our sun, located 1,200 light-years from Earth.

Figure S15 in a supplementary document associated with the research paper. Plot of the two components of the eccentricity vector versus one another. Kepler-36b is plotted in blue, while Kepler-36c is plotted in red. Credit: Joshua A. Carter, Eric Agol et al. / Science
By: NASA

Astronomers with NASA’s Kepler Mission find ‘puzzling pair of planets’

Two planets with very different densities and compositions are locked in surprisingly close orbits around their host star, according to astronomers working with NASA’s Kepler Mission.

One planet is a rocky super-Earth about 1.5 times the size of our planet and 4.5 times the mass. The other is a Neptune-like gaseous planet 3.7 times the size of Earth and eight times the mass. The planets approach each other 30 times closer than any pair of planets in our solar system.

The discovery of the Kepler-36 planetary system about 1,200 light years from Earth is an example of planets breaking with the planetary pattern of our solar system: rocky planets orbiting close to the sun and gas giants orbiting farther away.

The discovery is reported June 21 in the online Science Express. Lead authors of the study are Joshua Carter, a Hubble Fellow at the Harvard-Smithsonian Center for Astrophysics, and Eric Agol, an associate professor of astronomy at the University of Washington.

“The planetary system reported in this paper is another example of an ‘extreme’ planetary system that will serve as a stimulus to theories of planet migration and orbital rearrangement,” researchers wrote in the paper.

Steve Kawaler, an Iowa State University professor of physics and astronomy, was part of the research team that provided information about the properties of the planets’ host star. He and other researchers measured changes in the star’s brightness to precisely identify the size, mass and age of the host star.

Kawaler explained the importance of the discovery:

“Small, rocky planets should form in the hot part of the solar system, close to their host star – like Mercury, Venus and Earth in our Solar System. Bigger, less dense planets – Jupiter, Uranus – can only form farther away from their host, where it is cool enough for volatile material like water ice, and methane ice to collect. In some cases, these large planets can migrate close in after they form, during the last stages of planet formation, but in so doing they should eject or destroy the low-mass inner planets.

“Here, we have a pair of planets in nearby orbits but with very different densities. How they both got there and survived is a mystery.”

The discovery was made possible by NASA’s Kepler Mission, a spacecraft launched in 2009 that’s carrying a photometer to measure changes in star brightness. Its primary job is to use tiny variations in the brightness of the stars within its view to find earth-like planets that might be able to support life.

The Kepler Asteroseismic Investigation is also using data from that photometer to study star oscillations, or changes in brightness, that offer clues to a star’s interior structure.

Kawaler said the Kepler spacecraft was essential to discovering what the researchers called in their paper “this puzzling pair of planets.”

“The seismic signal is very small, and only Kepler has the sensitivity and persistence to reveal it,” Kawaler said. “Also, the transit signal from the planets crossing in front of the star is very small, and only visible with Kepler’s level of sensitivity.”

Figure S4 in a supplementary document associated with the research paper. Plot of the transits of planet b (left) and planet c (right). Top is data; middle is best-fit photometric-dynamical model; bottom is the residuals after subtracting the model. The greyscale ranges from 0.9994 (black) to 1.0001 (white) on a common scale between the top four panels. Credit: Joshua A. Carter, Eric Agol et al. / Science
By: Iowa State University

Astronomers discover pair of planets surprisingly close to each other

NASA’s Kepler mission, along with astronomers from Harvard, University of Washington and University of Florida, has discovered a rocky planet orbiting very close to a much larger planet around the same star, about 1,200 light years from Earth, according to research being published online today in Science Express.

Orbiting a star in the Cygnus constellation referred to as Kepler-36a, the planets are designated Kepler-36b and Kepler-36c. Planet b is a rocky planet, but 4.5 times more massive and with a radius nearly 50 percent larger than that of the Earth. Planet c is 8.1 times more massive than Earth and has a radius 3.7 times greater than our home planet. 

The planets occupy nearly the same orbital plane and on their closest approach come within about 1.2 million miles of each other — just five times the distance from Earth to the moon. But their orbits are timed so that they do not collide. 

The larger planet was originally spotted in data from NASA’s Kepler satellite, which is monitoring more than 160,000 stars, measuring their apparent brightness so precisely that it can detect when a distant planet passes in front of its host star, briefly reducing the light detected by Kepler’s instruments. 

“The two planets pass so close to each other than Kepler has already observed changes in their orbits,” said Eric Ford, associate astronomy professor at UF. “Initially, those interactions made it difficult to detect the second planet. Once we recognized the rapid changes in the orbit of the first planet, we suspect there was likely another planet.” Eric Agol, an associate professor at the University of Washington, and Joshua Carter, a Hubble fellow at the Harvard-Smithsonian Center for Astrophysics and lead author of the Science paper, developed methods to search for signals that are not perfectly periodic. The rocky planet was discovered once they applied those methods to the Kepler-36 system. 

“Once we had identified the planets, the gravitational tug-of-war between the two closely spaced planets made it possible for Kepler to measure their masses with unusual precision,” said Ford. “This is the best-characterized system of small planets orbiting a distant Sun-like star.” 

Astronomers estimate that the smaller planet likely contains significant iron and a small fraction of water, but less than 1 percent atmospheric hydrogen and helium. On the other hand, the larger planet could be either a Neptune-like body with a 50 percent of water by mass or a rocky core covered by a hydrogen and helium gas envelope. 

The planet’s densities differ by a factor of 8 but their orbits differ by only 10 percent. Explaining the difference in the planet’s compositions is a new challenge for planet formation models. 

Ford recognized the Kepler-36 system as particularly interesting in summer 2010. For most of the more than 160,000 stars that the Kepler spacecraft observes, astronomers only receive one measurement every 30 minutes. Because Ford identified the system as a particularly interesting target, Kepler has been sending back one measurement per minute, a capability reserved for just a few hundred stars at a time. The additional data enabled the precise measurements of the planets’ masses and densities.

Figure S5 in a supplementary document associated with the research paper. Correlation between the photometric-dynamical model flux and normalized Kepler photometry (i.e. after removal of a local linear fit about the transit). The red points are data, sorted in flux, averaged in bins with an equal number of data samples. The blue line denotes exact correspondence between model and data. Credit: Joshua A. Carter, Eric Agol et al. / Science
By: University of Florida

Proximity of New Planets Stuns Even Astronomers

One is a rocky planet 1.5 times the size of Earth. The other is a gaseous world nearly four times Earth's size. Together they form a spectacular system in which two planets orbit closer to each other than any yet discovered.

"We've never known of planets like this," said Yale University astronomer Sarbani Basu, a member of the research team that analyzed the system. "If you were on the smaller planet looking up, the larger planet would seem more than twice the size of Earth's full moon. It would be jaw-dropping."

Basu's research focused on determining the properties of the planets' host star -- work that was essential for discerning the characteristics of the orbiting planets.

The 46-member, international team, led by astronomers at Harvard and the University of Washington, report their discovery June 21 in Science Express, the early release version of the journal Science.

"These two worlds are having close encounters," said Josh Carter, lead author of the paper and a Hubble Fellow at the Harvard-Smithsonian Center for Astrophysics.

Located about 1200 light years away, the two-planet system – now called Kepler-36 – orbits a star similar to Earth's sun, but bigger and older.

The larger outer planet, Kepler-36c, is a hot, gaseous, Neptune-like planet. The smaller inner planet, Kepler-36b, is rocky and subject to quakes and volcanic eruptions caused by the interplay of the planets' gravitational forces on each other.

Like our sun, Kepler-36 pulsates constantly. Data on its quakes enabled the team to determine its size, weight, and age (all greater than those of our sun). Knowing the star's radius and mass enabled the calculation of the sizes and masses of the planets. From this information, astronomers could determine the planets' densities and characteristics: the smaller planet is denser than Earth and hence must be rocky; the larger planet is much less dense, in fact less dense than water, suggesting it is gaseous.

"The precise determination of the planets' properties was possible because the star around which they revolve could be characterized precisely," said Basu.

The planets' proximity to each other is astonishing, according to the researchers. The rocky inner planet orbits its star every 14 days, at an average distance of 11 million miles. The outer gaseous planet orbits every 16 days, at an average distance of 12 million miles.

Every 97 days they move into perfect alignment, a position known as conjunction. At that point they are separated by a mere 1.2 million miles – less than five times the distance between Earth and its moon. By contrast, Venus, Earth's nearest neighbor, never comes closer than 26 million miles.

The research team identified the planets by analyzing data from NASA's Kepler satellite. Kepler detects planets by measuring variations in the brightness of stars; dips in brightness may indicate a planet passing in front the star.

This discovery poses new challenges to the theories of planet formation. Astronomers are now trying to understand how planets with markedly different compositions and densities fell into remarkably close orbit.

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. Credit: NASA / Frank Melchior / Eric Agol
Credit: Joshua A. Carter, Eric Agol et al. / Science
Credit: Joshua A. Carter, Eric Agol et al. / Science
By: Yale University

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