Hubble Probes Layer-cake Structure of Alien World's Atmosphere

The powerful vision of NASA's Hubble Space Telescope has allowed astronomers to study for the first time the layer-cake structure of the atmosphere of a planet orbiting another star. Hubble discovered a dense upper layer of hot hydrogen gas where the super-hot planet's atmosphere is bleeding off into space.
The planet, designated HD 209458b, is unlike any world in our solar system. It orbits so close to its star and gets so hot that its gas is streaming into space, making the planet appear to have a comet-like tail. This new research reveals the layer in the planet's upper atmosphere where the gas becomes so heated it escapes, like steam rising from a boiler.
Image Type: Illustration/Artwork Credit: NASA, ESA, and A. Feild (STScI)

"The layer we studied is actually a transition zone where the temperature skyrockets from about 1,340 degrees Fahrenheit (1,000 Kelvin) to about 25,540 degrees (15,000 Kelvin), which is hotter than the Sun," said Gilda Ballester of the University of Arizona in Tucson, leader of the research team. "With this detection we see the details of how a
planet loses its atmosphere."


The findings by Ballester, David K. Sing of the University of Arizona and the Institut d'Astrophysique de Paris, and Floyd Herbert of the University of Arizona will appear Feb. 1 in a letter to the journal Nature.
The Hubble data show how intense ultraviolet radiation from the host star heats the gas in the upper atmosphere, inflating the atmosphere like a balloon. The gas is so hot that it moves very fast and escapes the planet's gravitational pull at a rate of 10,000 tons a second, more than three times the rate of water flowing over Niagara Falls. The planet, however, will not wither away any time soon. Astronomers estimate its lifetime is more than 5 billion years.

The scorched planet is a big puffy version of Jupiter. In fact, it is called a "hot Jupiter," a large gaseous planet orbiting very close to its parent star. Jupiter might even look like HD 209458b if it were close to the Sun, Ballester said.
The planet completes an orbit around its star every 3.5 days. It orbits 4.7 million miles from its host, 20 times closer than the Earth is to the Sun. By comparison, Mercury, the closest planet to our Sun, is 10 times farther away from the Sun than HD 209458b is from its star. Unlike HD 209458b, Mercury is a small ball of iron with a rocky crust.
"This planet's extreme atmosphere could yield insights into the atmospheres of other hot Jupiters," Ballester said.

Although HD 209458b does not have a twin in our solar system, it has plenty of relatives beyond our solar system. About 10 to 15 percent of the more than 200 known extrasolar planets are hot Jupiters. A recent Hubble survey netted 16 hot Jupiter candidates in the central region of our Milky Way Galaxy, suggesting that there may be billions of these gas-giant star huggers in our galaxy.
HD 209458b is one of the most intensely studied extrasolar planets because it is one of the few known alien worlds that can be seen passing in front of, or transiting, its star, causing the star to dim slightly. In fact, the gas giant is the first such alien world discovered to transit its star. HD 209458b is 150 light-years from Earth in the constellation Pegasus.
The planet's transits allow astronomers to analyze the structure and chemical makeup of the gas giant's atmosphere by sampling the starlight that passes through it. The effect is similar to finding fingerprints on a window by watching how sunlight filters through the glass.
Previous Hubble observatoins revealed oxygen, carbon, and sodium in the planet's atmosphere, as well as a huge hydrogen upper atmosphere with a comet-like tail. These landmark studies provided the first detection of the chemical makeup of an extrasolar planet's atmosphere.
Additional observations by NASA's Spitzer Space Telescope captured the infrared glow from the planet's hot atmosphere.
The new study by Ballester and her team is based on an analysis of archival observations made in 2003 with Hubble's Space Telescope Imaging Spectrograph by David Charbonneau of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. Ballester's team analyzed spectra from hot hydrogen atoms in the planet's upper atmosphere, a region not studied by Charbonneau's group.

More information: hubblesite.org/newscenter/archive/releases/2007/07/full/

Hubble Sees Star Cluster Infant Mortality

Astronomers using NASA's Hubble Space Telescope have found that young stellar nurseries, called open star clusters, have very short lives.
Hubble's Advanced Camera for Surveys gleaned these new observations during a "Where's Waldo" search for blue stars tossed out of their open cluster "nest" in the nearby galaxy known as NGC 1313.
Only Hubble has the resolution needed to distinguish individual stars in galaxies at NGC 1313's distance about 14 million light-years.
Astronomers have long known that young or "open" star clusters must eventually disrupt and dissolve into the host galaxy. They simply don't have enough gravity to hold them together, unlike their much more
massive cousins, the globular star clusters.
Image Credit: NASA, ESA, and A. Pellerin (STScI)

Before Hubble, astronomers have had very few observational clues. It's been difficult to observe exactly how star clusters dissolve due to the fact that they are easily lost in the cluttered star field background of the host galaxy.
A team led by Anne Pellerin of the Space Telescope Science Institute in Baltimore used Hubble to observe the barred spiral galaxy NGC 1313 and found that a large number of very young massive blue (B-type) stars are not associated with compact star clusters anymore. B stars burn out quickly due to the quick rate at which they use up hydrogen fuel.
Because B stars have very short lives (a few tens of millions of years), the presence of a large number of massive B-type stars suggests to astronomers that star clusters may dissolve very rapidly, within 25 million years. This is brief compared to the lifetime of the galaxy, which is measured in billions of years.
The rapid disintegration of open clusters is reinforced by the fact that the team found that the B stars are significantly more spread in the galaxy than even the more massive O-type. The O stars are so short lived (a few million years or even less), they explode as supernovae before they can be scattered outside the cluster.
In fact, the supernovae explosions of O stars could be the reason for a cluster's rapid disintegration, say researchers. Supernovae are capable of blasting out residual dust and gas from star formation inside a cluster. This could abruptly leave an open cluster with too little mass to gravitationally hold together for very long. In this scenario, the cluster stars drift off as other stars in the galaxy gravitationally tug on them.
Previous research based on the Hubble images of the Antennae galaxies, a colliding pair of galaxies, showed that 90 percent of the clusters are dissolved in this way during the first 10 million years of their existence. However, NGC 1313 is the first example of this happening in a normal spiral galaxy.
By using the analogy of star formation in open clusters in NGC 1313, we can infer that stars formed in a similar manner in the Milky Way, and so can help us better understand the way the Sun was formed.


Astronomers Map a Hypergiant Star's Massive Outbursts


Using NASA's Hubble Space Telescope and the W.M. Keck Observatory, Kameula, Hawaii, astronomers have learned that the gaseous outflow from one of the brightest super-sized stars in the sky is more complex than originally thought.

The outbursts are from VY Canis Majoris, a red supergiant star that is also classified as a hypergiant because of its very high luminosity. The eruptions have formed loops, arcs, and knots of material moving at various speeds and in many different directions. The star has had many outbursts over the past 1,000 years as it nears the end of its life.

Image Credit: NASA, ESA, R. Humphreys


A team of astronomers led by Roberta Humphreys of the University of Minnesota used NASA's Hubble Space Telescope and the W.M. Keck Observatory to measure the motions of the ejected material and to map the distribution of the highly polarized dust, which reflects light at a specific orientation. The polarized light shows how the dust is distributed. Astronomers combined the Hubble and Keck information to produce a three-dimensional image of the matter emitted from VY Canis Majoris.
"We thought mass loss in red supergiants was a simple, spherical, and uniform outflow, but in this star it is very complex," Humphreys said. "VY Canis Majoris is ejecting large amounts of gas at a prodigious rate and is consequently one of our most important stars for understanding the high-mass loss episodes near the end of massive star evolution. During the outbursts, the star loses about 10 times more mass than its normal rate.  "With these observations, we have a complete picture of the motions and directions of the outflows, and their spatial distribution, which confirms their origin from eruptions at different times from separate regions on the star." Humphreys and her collaborators presented their findings today (Jan. 8) at the American Astronomical Society meeting in Seattle, Wash.Astronomers have studied VY Canis Majoris for more than a century. The star is located 5,000 light-years away. It is 500,000 times brighter and about 30 to 40 times more massive than the Sun. If the Sun were replaced with the bloated VY Canis Majoris, its surface could extend to the orbit of Saturn.Images with Hubble's Wide Field and Planetary Camera 2 revealed for the first time the complexity of the star's ejecta. The first images provided evidence that the brightest arcs and knots were created during several outbursts. The random orientations of the arcs also suggested that they were produced by localized eruptions from active regions on the star's surface.With spectroscopy obtained using the Keck Telescope, Humphreys and her team learned more about the shape, motion, and origin of the star's outflow. Line of sight velocities, measured from the spectra, showed that the arcs and knots were expanding relative to the star. With recently obtained Hubble images, the group measured the ejecta's motions across the line of sight. The team found that the numerous arcs, loops, and knots were moving at different speeds and in various directions, confirming they were produced from separate events and from different locations on the star. The astronomers also used the measurements to determine when the outbursts happened. The outermost material was ejected about 1,000 years ago, while a knot near the star may have been ejected as recently as 50 years ago.The arcs and knots represent massive outflows of gas probably ejected from large star spots or convective cells on the star's surface, analogous to the Sun's activity with sunspots and prominences associated with magnetic fields, but on a vastly larger scale. Magnetic fields have been measured in VY Canis Majoris's ejecta that correspond to field strengths on its surface comparable to the magnetic fields on the Sun. These measurements show that the supergiant star's magnetic fields would supply sufficient energy for these massive outflows. The astronomers used the measurements to map the velocity and direction of the outflows with respect to the embedded star. When combined with the dust distribution map, they also determined the location of the arcs and knots, yielding the three-dimensional shape of VY Canis Majoris and its ejecta. "
With these observations, we may have captured a short-lived phase in the life of a massive star," Humphreys said. "The most luminous red supergiants may all eventually experience high-mass loss episodes like VY Canis Majoris before ending their lives." The typical red supergiant phase lasts about 500,000 years. A massive star becomes a red supergiant near the end of its life, when it exhausts the hydrogen fuel at its core. As the core contracts under gravity, the outer layers expand, the star gets 100 times larger, and it begins to lose mass at a higher rate. VY Canis Majoris has probably already shed about half of its mass, and it will eventually explode as a supernova.

Astronomers Map a Hypergiant Star's Massive Outbursts

New observations from NASA's Hubble Space Telescope have begun to fill gaps in the early stages of planet birth.
Hubble observed a "blizzard" of particles in a disk around a young star revealing the process by which planets grow from tiny dust grains. The particles are as fluffy as snowflakes and are roughly ten times larger than typical interstellar dust grains. They were detected in a disk encircling the 12-million-year-old star AU Microscopii. The star is 32 light-years away in the southern constellation of Microscopium, the Microscope.
The particles' fluffiness suggests that they were shed by much larger, but unseen snowball-sized particles that had gently collided with each other. These unseen pieces are believed to reside in a region dubbed the "birth ring," first hypothesized in 2005 by Berkeley astronomers Linda Strubbe and Eugene Chiang. The ring is between 3.7 billion and 4.6 billion miles from the star. As the larger pieces bump into each other, they release fluffy particles that are propelled outward by the intense pressure from starlight.

"We have seen many seeds of planets and we have seen many planets, but how they go from one to the other is a mystery," said astronomer James Graham of the University of California at Berkeley and leader of the Hubble observations. "These observations begin to help us fill in that gap."
Graham and his colleagues, who include Paul Kalas of the University of California at Berkeley and Brenda Matthews of the Herzberg Institute of Astrophysics in Victoria, B.C., presented their results today (Jan. 7) at the American Astronomical Association meeting in Seattle, Wash.
The astronomers used the Advanced Camera for Survey's coronagraph and polarizing filters to analyze the starlight reflecting off the debris disk. The coronagraph blocked out the bright light from the star so astronomers saw only the reflected light from the nearly edge-on debris disk.

The polarizing filters allowed astronomers to study how dust is reflecting starlight. Dust in our atmosphere reflects sunlight so that only light waves vibrating at a certain angle are reflected toward us. Polarizing sunglasses take advantage of this effect to block out all reflections except those that align to the polarizing filter material.
The astronomers used the polarized light from AU Microscopii's disk to deduce information about the size, shape, and other physical properties of the dust. Astronomers have observed other circumstellar disks in polarized light, but this is the first comprehensive study of the size and structure of the grains within a debris disk.
Hubble is well-suited to make these observations because of its sharpness and its ability to precisely analyze polarized light without it being degraded by Earth's atmosphere. The disk observed by Hubble formed later in the star's life from debris shed through the collisions of small bodies. The bodies grew from dust in the primeval disk that encircled the newborn star.

Graham and his colleagues were surprised that fluffy particles could even form in a disk.
"Circumstellar disks are thought to be vigorous, turbulent places," he explained. "It is hard to understand why fluffy particles could grow and survive under such circumstances. But maybe there are some backwater places where turbulence had subsided and porous particles can form and grow. Planet-forming disks are undoubtedly much more complex than we currently envision."
The evidence Hubble captured of events in AU Microscopii's debris disk may parallel developments in our early solar system.
"If we ran our solar system clock back nearly 4.5 billion years, then the infant Kuiper Belt would probably look like AU Microscopii's birth ring," said Kalas, an adjunct professor of astronomy at Berkeley. The Kuiper Belt is a reservoir of leftover icy material from the birth of our solar system.
The planets in our solar system lie inside the Kuiper Belt. Likewise, inside AU Microscopii's version of the Kuiper Belt is a gap that may have been carved out by one or more as-yet unseen planets.
AU Microscopii is a red dwarf, the most common star in our Milky Way Galaxy. It is the perfect laboratory, therefore, for studying how planets form around everyday stars. Red dwarfs are fainter, cooler, and less massive than the Sun.
The results also appeared in the Jan. 1, 2007 issue of the Astrophysical Journal.


Hubble Yields Direct Proof of Stellar Sorting in a Globular Cluster

NASA's Hubble Space Telescope has provided astronomers with the best observational evidence to date that globular clusters sort out stars according to their mass, governed by a gravitational billiard ball game between stars. Heavier stars slow down and sink to the cluster's core, while lighter stars pick up speed and move across the cluster to its periphery. This process, called "mass segregation," has long been suspected for globular star clusters, but has never before been directly seen in action.

 
Hubble Captures Galaxy in the Making
Images from NASA's Hubble Space Telescope have provided a dramatic glimpse of a large and massive galaxy under assembly by the merging of smaller, lighter galaxies. Astrophysicists believe that this is the way galaxies grew in the young universe. Now, Hubble observations of the radio galaxy MRC 1138-262, nicknamed the "Spiderweb Galaxy" show dozens of star-forming satellite galaxies as individual clumpy features in the process of merging. Because the galaxy is 10.6 billion light-years away, astronomers are seeing it as it looked in the universe's early formative years, only 3 billion years after the Big Bang.


Mars May Be Cozy Place for Hardy Microbes

A class of especially hardy microbes that live in some of the harshest Earthly environments could flourish on cold Mars and other chilly planets, according to a research team of astronomers and microbiologists. In a two-year laboratory study, the researchers discovered that some cold-adapted microorganisms not only survived but reproduced at 30 degrees Fahrenheit, just below the freezing point of water. The microbes also developed a defense mechanism that protected them from cold temperatures. These close-up images, taken by an electron microscope, reveal the tiny one-cell organisms, called halophiles and methanogens, that were used in the study.

These news release and its supporting materials are permanently archived at:

hubblesite.org/news/


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