Jul 10

Caltech Astronomer Finds Planets in Unusually Intimate Dance around Dying Star

Source: Caltech

Hundreds of extrasolar planets have been found over the past decade and a half, most of them solitary worlds orbiting their parent star in seeming isolation. With further observation, however, one in three of these systems have been found to have two or more planets. Planets, it appears, come in bunches. Most of these systems contain planets that orbit too far from one another to feel each other's gravity. In just a handful of cases, planets have been found near enough to one another to interact gravitationally.

Now, however, John A. Johnson, an assistant professor of astronomy at the California Institute of Technology (Caltech), and his colleagues have found two systems with pairs of gas giant planets locked in an orbital embrace.

In one system—a planetary pair orbiting the massive, dying star HD 200964, located roughly 223 light-years from Earth-the intimate dance is closer and tighter than any previously seen. "This new planet pair came in an unexpected package," says Johnson.

Adds Eric Ford of the University of Florida in Gainsville, "A planetary system with such closely spaced giant planets would be destroyed quickly if the planets weren't doing such a well synchronized dance. This makes it a real puzzle how the planets could have found their rhythm."

A paper by Johnson, Ford, and their collaborators describing the planets and their intriguing orbital dynamics has been accepted for publication in the Astronomical Journal (see http://arxiv.org/abs/1007.4552 for a preprint).

All of the four newly discovered exoplanets are gas giants more massive than Jupiter, and like most exoplanets were discovered by measuring the wobble, or Doppler shift, in the light emitted by their parent stars as the planets orbit around them. Surprisingly, however, the members of each pair are located remarkably close to one another.

For example, the distance between the planets orbiting HD 200964 occasionally is just .35 astronomical units (AU)—roughly 33 million miles—comparable to the distance between Earth and Mars. The planets orbiting the second star, 24 Sextanis (located 244 light-years from Earth) are .75 AU, or about 70 million miles. By comparison, Jupiter and Saturn are never less than 330 million miles apart.

Because of their large masses and close proximity, the exoplanet pairs exert a large gravitational force on each other. The gravitational tug between HD 200964's two planets, for example, is 3,000,000 times greater than the gravitational force between Earth and Mars, 700 times larger than that between the Earth and the moon, and 4 times larger than the pull of our sun on the Earth.

Unlike the gas giants in our own solar system, the new planets are located comparatively close to their stars. The planets orbiting 24 Sextanis have orbital periods of 455 days (1.25 years) and 910 days (2.5 years), and the companions to HD 200964 periods of 630 days (1.75 years) and 830 days (2.3 years). Jupiter, by contrast, takes just under 12 Earth years to make one pass around the sun.

Planets often move around after they form, in a process known as migration. Migration is thought to be commonplace—it even occurred to some extent within our own solar system—but it isn't orderly. Planets located farther out in the protoplanetary disk can migrate faster than those closer in, "so planets will cross paths and jostle each other around," Johnson says. "The only way they can 'get along' and become stable is if they enter an orbital resonance."

When planets are locked in an orbital resonance, their orbital periods are related by the ratio of two small integers. In a 2:1 resonance, for example, an outer planet will orbit its parent star once for every two orbits of the inner planet; in a 3:2 resonance, the outer planet will orbit two times for every three passes by the inner planet, and so forth. Such resonances are created by the gravitational influence of planets on one another.

"There are many locations in a protoplanetary disk where planets can form," says Johnson. "It's very unlikely, however, that two planets would just happen to form at locations where they have periods in one of these ratios."

A 2:1 resonance—which is the case for the planets orbiting 24 Sextanis—is the most stable and the most common pattern. "Planets tend to get stuck in the 2:1. It's like a really big pothole," Johnson says. "But if a planet is moving very fast"—racing in from the outer part of the protoplanetary disk, where it formed, toward its parent star—"it can pass over a 2:1. As it moves in closer, the next step is a 5:3, then a 3:2, and then a 4:3."

Johnson and his colleagues have found that the pair of planets orbiting HD 200964 is locked in just such a 4:3 resonance. "The closest analogy in our solar system is Titan and Hyperion, two moons of Saturn which also follow orbits synchronized in a 4:3 pattern," says Ford. "But the planets orbiting HD 200964 interact much more strongly, since each is around 20,000 times more massive than Titan and Hyperion combined."

"This is the tightest system that's ever been discovered," Johnson adds, "and we're at a loss to explain why this happened. This is the latest in a long line of strange discoveries about extrasolar planets, and it shows that exoplanets continuously have this ability to surprise us. Each time we think we can explain them, something else comes along."

Johnson and his colleagues found the two systems using data from the Keck Subgiants Planet Survey—a search for planets around stars from 40 to 100 percent larger than our own sun. Subgiants represent a class of stars that have evolved off the "main sequence," and have run out of hydrogen for nuclear fusion, causing their core to collapse and their outer envelope to swell. Subgiants eventually become red giants—voluminous stars with big, puffy atmospheres that pulsate, making it difficult to detect the subtle spectral shifts caused by orbiting planets.

"Subgiants are rotating very slowly and they're cool," unlike rapidly rotating, hot main sequence stars, "but they haven't expanded enough to be too fluffy and too jittery," Johnson says. "They're 'Goldilocks' stars: not too fast, not too hot, not too fluffy, not too jittery"—and, therefore, ideal for planet hunting.

"Right now, we're monitoring 450 of these massive stars, and we are finding swarms of planets," he says. "Around these stars, we are seeing three to four times more planets out to a distance of about 3 AU—the distance of our asteroid belt—than we see around main sequence stars. Stellar mass has a huge influence on frequency of planet occurrence, because the amount of raw material available to build planets scales with the mass of the star."

Eventually, perhaps 10 or 100 million years from now, subgiant stars like HD 200964 and 24 Sextanis will become red giants. They will throw off their outer atmospheres, swelling to the point where they could engulf the inner planet of their dancing pair, and will throw off mass, changing the gravitational dynamics of their whole system. "The planets will then move out, and their orbits will become unstable," Johnson says. "Most likely one of the planets will get flung out of the system completely"-and the dance will end.

The paper, "A Pair of Interacting Exoplanet Pairs Around the Subgiants 24 Sextanis and HD 200964," was coauthored by Matthew Payne and Eric B. Ford of the University of Florida; Andrew W. Howard and Geoffrey W. Marcy of the University of California, Berkeley; Kelsey Clubb of San Francisco State University; Brendan P. Bowler of the University of Hawai'i at Manoa,; Gregory W. Henry of Tennessee State University; Debra A. Fischer, John Brewer, and Christian Schwab of Yale University; Sabine Reffert of ZAH-Landessternwarte; and Thomas Lowe of the UCO/Lick Observatory.

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Jun 10

Hubble captures bubbles and baby stars

Credit: ESA/HST

A spectacular new NASA/ESA Hubble Space Telescope image — one of the largest ever released of a star-forming region — highlights N11, part of a complex network of gas clouds and star clusters within our neighbouring galaxy, the Large Magellanic Cloud. This region of energetic star formation is one of the most active in the nearby Universe.(read more)

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May 10

Herschel reveals new stars in a stellar cocoon

Source: ESA - oshi

This glowing core is the stellar equivalent of an insect’s cocoon. Nestled in the bright centre are two newly forming stars. When they reach maturity they will begin to generate their own energy and shine out across the Universe.

Small, isolated clouds of forming stars are known as Bok Globules after the 20th century astronomer Bart Bok. Back in the 1940s, their identification was an important step towards the realisation that stars form from the condensation of gas clouds in space.

It was not until the Infrared Astronomical Satellite was launched in the 1980s, and the era of space-based infrared astronomy began, that the idea of Bok Globules as stellar cocoons was confirmed by João Yun and co-authors. (read more)

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May 10

Herschel unveils rare massive stars in the act of forming

Source: ESA Science and Technology

RCW 120 as seen by Herschel.
Credit: ESA, PACS & SPIRE Consortia, A. Zavagno

Massive stars are the rare birds of astrophysics. With a mass over eight times that of the Sun, these stars are much less common than their lower-mass counterparts. In addition, they are short-lived, consuming their nuclear fuel at a rapid rate before ending their life in spectacular manner as a supernova. Their scarcity means that observations of these rare giants can prove difficult to obtain, but characterising these elusive objects is essential for understanding the chemical and dynamical evolution of galaxies.

The mechanism leading to the formation of massive stars is still largely debated. Detecting these objects in their earliest phases is a highly challenging task, since they are embedded in dusty cocoons that hide them from view. However, the dust that absorbs their light re-emits it at infrared wavelengths, making an infrared observatory such as Herschel a unique tool for locating newborn massive stars in their natal nests.

New images from ESA's Herschel space observatory reveal high-mass protostars around two ionised regions in our Galaxy. The detection of these rare stars in an early phase of evolution is key to understanding the mysterious formation of massive stars.(read more)

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Apr 10

Planck highlights the complexity of star formation

Source: ESA Space Science

A low activity, star-formation region in the constellation Perseus, as seen with Planck.
Credit: ESA/Planck

New images from ESA’s Planck space observatory reveal the forces driving star formation and give astronomers a way to understand the complex physics that shape the dust and gas in our Galaxy.

Star formation takes place hidden behind veils of dust but that doesn’t mean we can’t see through them. Where optical telescopes see only black space, Planck’s microwave eyes reveal myriad glowing structures of dust and gas. Now, Planck has used this ability to probe two relatively nearby star-forming regions in our Galaxy.(read more)

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Apr 10

Herschel enhances knowledge of stellar formation

Source: ESA/Herschel

Herschel's latest image reveals the formation of previously unseen large stars, each one up to ten times the mass of our Sun. These are the stars that will influence where and how the next generation of stars are formed. The image is a new release of 'OSHI', ESA's Online Showcase of Herschel Images.(learn more)

Related links:

Space Daily

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Mar 10

APEX Snaps First Close-up of Star Factories in Distant Universe

Source: ESO Science Release eso1012

For the first time, astronomers have made direct measurements of the size and brightness of regions of star-birth in a very distant galaxy, thanks to a chance discovery with the APEX telescope. The galaxy is so distant, and its light has taken so long to reach us, that we see it as it was 10 billion years ago. A cosmic "gravitational lens" is magnifying the galaxy, giving us a close-up view that would otherwise be impossible. This lucky break reveals a hectic and vigorous star-forming life for galaxies in the early Universe, with stellar nurseries forming one hundred times faster than in more recent galaxies. The research is published online today in the journal Nature.(read more)

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Mar 10

The Many Colors of Star Birth

Source: Gemini Observatory

A dramatic image from the Gemini North telescope illustrates the dynamic and sometimes violent process of star birth. It also demonstrates the capabilities of new filters available to researchers using the Gemini Multi-Object Spectrograph (GMOS).

Known as Sharpless 2-106 (Sh2-106), the hourglass-shaped (bipolar) nebula in the new Gemini image is a stellar nursery made up of glowing gas and light-scattering dust. The material shrouds a natal high-mass star thought to be mostly responsible for the hourglass shape of the nebula due to high-speed winds (more than 200 kilometers/second) which eject material from the forming star deep within (see the recent Gemini press release on the birth of a massive star which exhibits evidence of similar processes). Research also indicates that many sub-stellar objects are forming within the cloud and may someday result in a cluster of 50 to 150 stars in this region. (read more)

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Feb 10

After all it seems size doesn't matter

Source: Universe Today

Artist's impression of stellar formation.

Astronomers have managed to peer past obscuring dust clouds to gain their first peek at the gestation of a massive proto-star W33A which is about 12,000 light years away within the Sagittarius constellation.

There has been a standing debate in astronomical circles about whether or not massive stars form in the same way as smaller stars. The issue has been hampered by a lack of observational data on just how massive stars form – as they develop so quickly they are generally only seen in an already fully formed state when they pop out of the obscuring dust clouds of their stellar nursery. (read more)

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Jan 10

Astronomers are finding millisecond pulsars faster than ever

Credit: ASTRON

Astronomers of an international team that is discovering the exotic stars known as "millisecond pulsars" at an astonishing rate. Whereas in the last 30 years only 60 millisecond pulsars have been identified in the disk of our Galaxy, 17 new millisecond pulsars have been found in just the last 3 months by using large radio telescopes to target sources of high-energy gamma-rays recently found with NASA's Fermi Gamma-ray Space Telescope. (read more)

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Dec 09

Close-up Photos of Dying Star Show Our Sun's Fate

Credit: Harvard-Smithsonian Center for Astrophysics

Chi Cygni, a red giant star as shown in this artist's
conception of Betelgeuse, is nearing the end of its life.
Credit: ESO/L. Calçada

One day our Sun will eventually die. But it won't be tomorrow ... in fact, it will only happen in 5000 million years.


Chi Cygni changes brightness dramatically and regularly
every 408 days due to in-and-out pulsations.
Credit: Sylvestre Lacour, Observatoire de Paris

About 550 light-years from Earth, a star like our Sun is writhing in its death throes. Chi Cygni has swollen in size to become a red giant star so large that it would swallow every planet out to Mars in our solar system. Moreover, it has begun to pulse dramatically in and out, beating like a giant heart. New close-up photos of the surface of this distant star show its throbbing motions in unprecedented detail. (read more)

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