Category: Extrasolar Planets

How many planets like the Earth are there among the 130 or so known planetary systems beyond our own? How many of these ?Earths? could be habitable?

Recent theoretical work by Barrie Jones, Nick Sleep, and David Underwood at the Open University in Milton Keynes indicates that as many as half of the known systems could be harbouring habitable ?Earths? today.

Unfortunately, existing telescopes are not powerful enough to see these relatively small, distant ?Earths?. Orbiting close to a much brighter star, these very faint worlds resemble glow-worms hidden in the glare of a searchlight.

All of the planets that have been detected so far are giants the mass of Neptune or larger. Even so, they cannot be directly seen with ground-based instruments. Almost all of the known exoplanets have been found through the ?wobbling? motion they induce in their star as they orbit it, like a twirling dumb-bell in which the mass at one end (the star) is much greater than the mass at the other end (the giant planet).

Speaking today at the RAS National Astronomy Meeting in Birmingham, Professor Jones explained how his team used computer models to see if ?Earths? could be present in any of the currently known exoplanetary systems, and whether the gravitational buffeting from one or more giant planets in those systems would have torn them out of their orbits.

?We were particularly interested in the possible survival of ?Earths? in the habitable zone,? said Professor Jones. ?This is often called the ?Goldilocks zone?, where the temperature of an ?Earth? is just right for water to be liquid at its surface. If liquid water can exist, so could life as we know it.?

The Open University team created a mathematical model of a known exoplanetary system, with its star and giant planet(s), then launched an Earth-sized planet at some distance from the star to see if it survived.

By detailed study of a few representative exoplanetary systems, they found that each giant planet is accompanied by two ?disaster zones? – one exterior to the giant, and one interior. Within these zones, the giant?s gravity will cause a catastrophic change in the Earth-like planet?s orbit. The dramatic outcome is a collision with either the giant planet or the star, or ejection into the cold outer reaches of the system.

The team found that the locations of these disaster zones depend not only on the mass of the giant planet (a well known result) but also on the eccentricity of its orbit. They thus established rules for determining the extent of the disaster zone.

Having found the rules, they applied them to all of the known exoplanetary systems – a much quicker method than studying each system in detail. The range of distances from the star covered by its habitable zone was compared to the locations of the disaster zones to see if there was a full or partial safe haven for an Earth-like planet.

They discovered that about half of the known exoplanetary systems offer a safe haven for a period extending from the present into the past that is at least long enough for life to have developed on any such planets. This assumes that ?Earths? could have formed in the first place, which seems quite likely.

However, the situation is complicated by the fact that the habitable zone migrates outwards as the star ages, and in some cases this changes the potential for life to evolve. Thus, in some cases a safe haven might have been available only in the past, while in other cases it might exist only in the future.

These scenarios of past extinction and future birth increase to about two-thirds the proportion of the known exoplanetary systems that are potentially habitable at some time during the main-sequence lifetime of their central star.

Original Source: RAS News Release

NASA’s Spitzer Space Telescope has for the first time captured the light from two known planets orbiting stars other than our Sun. The findings mark the beginning of a new age of planetary science, in which “extrasolar” planets can be directly measured and compared.

“Spitzer has provided us with a powerful new tool for learning about the temperatures, atmospheres and orbits of planets hundreds of light-years from Earth,” said Dr. Drake Deming of NASA’s Goddard Space Flight Center, Greenbelt, Md., lead author of a new study on one of the planets.

“It’s fantastic,” said Dr. David Charbonneau of the Harvard- Smithsonian Center for Astrophysics, Cambridge, Mass., lead author of a separate study on a different planet. “We’ve been hunting for this light for almost 10 years, ever since extrasolar planets were first discovered.” The Deming paper appears today in Nature’s online publication; the Charbonneau paper will be published in an upcoming issue of the Astrophysical Journal.

So far, all confirmed extrasolar planets, including the two recently observed by Spitzer, have been discovered indirectly, mainly by the “wobble” technique and more recently, the “transit” technique. In the first method, a planet is detected by the gravitational tug it exerts on its parent star, which makes the star wobble. In the second, a planet’s presence is inferred when it passes in front of its star, causing the star to dim, or blink. Both strategies use visible-light telescopes and indirectly reveal the mass and size of planets, respectively.

In the new studies, Spitzer has directly observed the warm infrared glows of two previously detected “hot Jupiter” planets, designated HD 209458b and TrES-1. Hot Jupiters are extrasolar gas giants that zip closely around their parent stars. From their toasty orbits, they soak up ample starlight and shine brightly in infrared wavelengths.

To distinguish this planet glow from that of the fiery hot stars, the astronomers used a simple trick. First, they used Spitzer to collect the total infrared light from both the stars and planets. Then, when the planets dipped behind the stars as part of their regular orbit, the astronomers measured the infrared light coming from just the stars. This pinpointed exactly how much infrared light belonged to the planets. “In visible light, the glare of the star completely overwhelms the glimmer of light reflected by the planet,” said Charbonneau. “In infrared, the star-planet contrast is more favorable because the planet emits its own light.”

The Spitzer data told the astronomers that both planets are at least a steaming 1,000 Kelvin (727 degrees Celsius, 1340 Fahrenheit). These measurements confirm that hot Jupiters are indeed hot. Upcoming Spitzer observations using a range of infrared wavelengths are expected to provide more information about the planets’ winds and atmospheric compositions.

The findings also reawaken a mystery that some astronomers had laid to rest. Planet HD 209458b is unusually puffy, or large for its mass, which some scientists thought was the result of an unseen planet’s gravitational pull. If this theory had been correct, HD 209458b would have a non-circular orbit. Spitzer discovered that the planet does in fact follow a circular path. “We’re back to square one,” said Dr. Sara Seager, Carnegie Institution of Washington, Washington, co-author of the Deming paper. “For us theorists, that’s fun.”

Spitzer is ideally suited for studying extrasolar planets known to transit, or cross, stars the size of our Sun out to distances of 500 light-years. Of the seven known transiting planets, only the two mentioned here meet those criteria. As more are discovered, Spitzer will be able to collect their light – a bonus for the observatory, considering it was not originally designed to see extrasolar planets. NASA’s future Terrestrial Planet Finder coronagraph, set to launch in 2016, will be able to directly image extrasolar planets as small as Earth.

Shortly after its discovery in 1999, HD 209458b became the first planet detected via the transit method. That result came from two teams, one led by Charbonneau. TrES-1 was found via the transit method in 2004 as part of the NASA-funded Trans-Atlantic Exoplanet Survey, a ground-based telescope program established in part by Charbonneau.

Original Source: NASA/JPL News Release

The past four weeks have been heady ones in the planet-finding world: Three teams of astronomers announced the discovery of 12 previously unknown worlds, bringing the total count of planets outside our solar system to 145.

Just a decade ago, scientists knew of only the nine planets – those in our local solar system. In 1995, improved detection techniques produced the first solid evidence of a planet circling another star. A proliferation of discoveries followed, and now dozens of ongoing search efforts around the globe add steadily to the roster of worlds. Most of these planets differ markedly from the planets in our own solar system. They are more similar to Jupiter or Saturn than to Earth, and are considered unlikely to support life as we know it.

The news of the past four weeks has included:

* The discovery of six new gas-giant planets by two teams of European planet-hunters was announced this week. Two of these planets are similar in mass to Saturn; three belong to a class known as “hot jupiters” because of their close proximity to the host stars. The sixth is a gas giant at least four-and-a-half times the mass of Jupiter.

All were discovered as part of the High Accuracy Radial velocity Planet Search (HARPS), an ongoing search program based at La Silla Observatory in Chile.

* On January 20, a paper posted in the online edition of the Astrophysical Journal described five new gas-giant type planets detected by a team of U.S. astronomers. These planets provide further statistical information about the distribution and properties of planetary systems, according to the paper.

The U.S. team based its finding on observations obtained at the W.M. Keck Observatory in Hawaii, which is jointly operated by the University of California and Caltech. Observation time was granted by both NASA and the University of California.

* Last week, Penn State’s Alex Wolszczan and Caltech’s Maciej Konacki announced the discovery of the smallest planet-like body detected beyond our solar system. The object belongs to a strange class known as “pulsar planets.” It is about one-fifth the size of Pluto and orbits a rapidly spinning neutron star, called a pulsar.

A pulsar is a dense and compact star that forms from the collapsing core left over from the death of a massive star. The new pulsar planet is the fourth to be discovered; all orbit the same pulsar, named PSR B1257+12.

Because the planets around the pulsar are continually strafed by high-energy radiation, they are considered extremely inhospitable to life. (Note: The current planet count posted on this website includes only planets around normal stars.)

Two methods of detection
The pulsar planet was discovered by observing the neutron star’s pulse arrival times, called pulsar timing. Variations in these pulses give astronomers an extremely precise method for detecting the phenomena that occur within a pulsar’s environment.

The gas-giant planets were detected using the radial velocity method, which infers the presence of an unseen companion because of the back-and-forth movement induced in the host star. This movement is detectable as a periodic red shift and blue shift in the star’s spectral lines. (For more about this method, see the article Finding Planets.)

The names of the new planets around main sequence stars are:

* HD 2638 b
* HD 27894 b
* HD 63454 b
* HD 102117 b
* HD 93083 b
* HD 142022A b
* HD 45350 b
* HD 99492 b
* HD 117207 b
* HD 183263 b
* HD 188015 b

Original Source: NASA Astrobiology Report

Image credit: NASA
Some extrasolar planets may be made substantially from carbon compounds, including diamond, according to a report presented this week at the conference on extrasolar planets in Aspen, Colorado. Earth, Mars and Venus are “silicate planets” consisting mostly of silicon-oxygen compounds. Astrophysicists are proposing that some stars in our galaxy may host “carbon planets” instead.

“Carbon planets could form in much the same way as do certain meteorites in our solar system, the carbonaceous chondrites,” said Dr. Marc J. Kuchner of Princeton University, making the report in Aspen together with Dr. Sara Seager of the Carnegie Institute of Washington. “These meteorites contain large quantities of carbon compounds such as carbides, organics, and graphite, and even the occasional tiny diamond.” Imagine such a meteorite the size of a planet, and you are picturing a carbon planet.

Planets like the Earth are thought to condense from disks of gas orbiting young stars. In gas with extra carbon or too little oxygen, carbon compounds like carbides and graphite condense out instead of silicates, possibly explaining the origin of carbonaceous chondrites and suggesting the possibility of carbon planets. Any condensed graphite would change into diamond under the high pressures inside the carbon planets, potentially forming diamond layers inside the planets many miles thick.

Some of the already known low- and intermediate-mass extrasolar planets may be carbon planets, which should easily survive at high temperatures near a star if they have the mass of Neptune. Carbon planets would probably consist mostly of carbides, thought they may have iron cores and substanial atmospheres. Carbides are a kind of ceramic used to line the cylinders of motorcycle engines among other things.

The planets orbiting the pulsar PSR 1257+12 are good candidates for carbon planets; they may have formed from the disruption of a star that produced carbon as it aged. So are planets located near the center of the Galaxy, where stars are more carbon-rich than the sun, on average. Slowly, the galaxy as a whole is becoming more carbon-rich; in the future, all planets formed may be carbon planets.

“There’s no reason to think that extrasolar planets will be just like the planets in the solar system.” says Kuchner. “The possibilities are startling.”

Kuchner added, “NASA’s future Terrestrial Planet Finder (TPF) mission may be able to spot these planets.” The spectra of these planets should lack water, and instead reveal carbon monoxide, methane, and possibly long-chain carbon compounds synthesized photochemically in their atmospheres. The surfaces of carbon planets may be covered with a layer of long-chain carbon compounds–in other words, something like crude oil or tar.

The first TPF telescope, an optical telescope several times the size of the Hubble Space Telescope is scheduled to launch in 2015. The TPF missions are designed to search for planets like the Earth and determine whether they might be suitable for life.

Original Source: NASA Astrobiology Story

Penn State’s Alex Wolszczan, the discoverer in 1992 of the first planets ever found outside our solar system, now has discovered with Caltech’s Maciej Konacki the smallest planet yet detected,in that same far-away planetary system. Immersed in an extended cloud of ionized gas, the new planet orbits a rapidly spinning neutron star called a pulsar. The discovery, to be announced during a press conference at a meeting concerning planetary formation and detection in Aspen, Colorado, on 7 February, yields an astonishingly complete description of the pulsar planetary system and confirms that it is remarkably like a half-size version of our own solar system ? even though the star these planets orbit is quite different from our Sun.

“Despite the extreme conditions that must have existed at the time these planets were forming, Nature has managed to create a planetary system that looks like a scaled-down copy of our own inner solar system,” Wolszczan reports. The star at the center of this system is a pulsar named PSR B1257+12 ? the extremely dense and compact neutron star left over from a massive star that died in a violent explosion 1,500 light years away in the constellation Virgo.

Wolszczan and his colleagues earlier had discovered three terrestrial planets around the pulsar, with their orbits in an almost exact proportion to the spacings between Mercury, Venus, and Earth. The newly discovered fourth planet has an orbit approximately six times larger than that of the third planet in the system, which Konacki says is amazingly close to the average distance from our Sun to our solar system’s asteroid belt, located between the orbits of Mars and Jupiter.

“Because our observations practically rule out a possible presence of an even more distant, massive planet or planets around the pulsar, it is quite possible that the tiny fourth planet is the largest member of a cloud of interplanetary debris at the outer edge of the pulsar’s planetary system, a remnant of the original protoplanetary disk that created the three inner planets,” Wolszczan explains. The small planet, about one-fifth of the mass of Pluto, may occupy the same outer-boundary position in its planetary system as Pluto does in our solar system. “Surprisingly, the planetary system around this pulsar resembles our own solar system more than any extrasolar planetary system discovered around a Sun-like star,” Konacki says.

Fifteen years ago, before Wolszczan’s discovery of the first extrasolar planets, astronomers did not seriously entertain the idea that planets could survive around pulsars because they would have been blasted with the unimaginable force of the radiation and remnants of their exploding parent star. Since then, Wolszczan, Konacki, and colleagues have gradually been unraveling the mysteries of this system of pulsar planets, using the Arecibo radio telescope in Puerto Rico to collect and analyze pulsar-timing data. “We feel now, with this discovery, that the basic inventory of this planetary system has been completed,” Wolszczan says.

These discoveries have been possible because pulsars, especially those with the fastest spin, behave like very accurate clocks. “The stability of the repetition rate of the pulsar pulses compares favorably with the precision of the best atomic clocks constructed by humans,” Konacki explains. Measurements of the pulse arrival times, called pulsar timing, give astronomers an extremely precise method for studying the physics of pulsars and for detecting the phenomena that occur in a pulsar’s environment.

“A pulsar wobble due to orbiting planets manifests itself by variations in the pulse arrival times, just like a stellar wobble is detectable with the well-known Doppler effect so successfully used by optical astronomers to identify planets around nearby stars by the shifts of their spectral lines,” Wolszczan explains. “An important advantage of the fantastic stability of the pulsar clocks, which achieve precisions better than one millionth of a second, is that this method allows us to detect planets with masses all the way down to those of large asteroids.”

The very existence of the pulsar planets may represent convincing evidence that Earth-mass planets form just as easily as do the gas giants that are known to exist around more than 5 percent of the nearby Sun-like stars. However, Wolszczan says, “the message carried by the pulsar planets may equally well be that the formation of Earth-like planets requires special conditions, making such planets a rarity. For example, there is growing evidence that a nearby supernova explosion may have played an important role in our solar system’s formation.” Future space observatories, including the Kepler and the Space Interferometry Missions, and the Terrestrial Planet Finder, will play a decisive role in making a distinction between these fundamental alternatives.

Unique follow up observations carried out with NASA’s Hubble Space Telescope are providing important supporting evidence for the existence of a candidate planetary companion to a relatively bright young brown dwarf star located 225 light-years away in the southern constellation Hydra.

Astronomers at the European Southern Observatory’s Very Large Telescope (VLT) in Chile detected the planet candidate in April 2004 with infrared observations using adaptive optics to sharpen their view. The VLT astronomers spotted a faint companion object to the brown dwarf star 2MASSWJ 1207334-393254 (aka 2M1207). The object is a candidate planet because it is only one-seven-hundredth the brightness of the brown dwarf (at the longer-than-Hubble wavelengths observed with the VLT) and glimmers at barely 1800 degrees Fahrenheit, which is cooler than a light bulb filament.

Because an extrasolar planet has never been directly imaged before, this remarkable observation required Hubble’s unique abilities to do follow-up observations to test and validate if it is indeed a planet. Hubble’s Near Infrared Camera and Multi-Object Spectrometer (NICMOS) camera conducted complementary observations taken at shorter infrared wavelength observations unobtainable from the ground. This wavelength coverage is important because it is needed to characterize the object’s physical nature.

Very high precision measurements of the relative position between the dwarf and companion were obtained with NICMOS in August 2004. The Hubble images were compared to the earlier VLT observations to try and see if the two objects are really gravitationally bound and hence move across the sky together. Despite the four months between the VLT and NICMOS observations, astronomers say they can almost rule out the probability that the suspected planet is really a background object, because there was no noticeable change in its position relative to the dwarf.

If the two objects are indeed gravitationally bound together they are at least 5 billion miles apart, about 30 percent farther apart than Pluto is from the Sun. Given the mass of 2M1207, inferred from its spectrum, the companion object would take a sluggish 2,500 years to complete one orbit. Therefore, any relative motion seen between the two on much shorter time scales would reveal the candidate planet to be a background interloper and not a gravitationally bound planet.

“The NICMOS photometry supports the conjecture that the planet candidate is about five times the mass of Jupiter if it indeed orbits the brown dwarf,” says Glenn Schneider of the University of Arizona. “The NICMOS position measurements, relative to VLT’s, indicate the object is a true (and thus orbiting) companion at a 99 percent level of confidence — but further planned Hubble observations are required to eliminate the 1 percent chance that it is a coincidental background object which is not orbiting the dwarf.”

Schneider is presenting these latest Hubble observations today at the meeting of the American Astronomical Society in San Diego, Calif.

The candidate planet and dwarf are in the nearby TW Hydrae association of young stars that are estimated to be no older than 8 million years. The Hubble NICMOS observations found the object to be extremely red and relatively much brighter at longer wavelengths. The colors match theoretical expectations for an approximately 8 million-year-old object that is about five times as massive as Jupiter.

Further Hubble observations by the NICMOS team are planned in April 2005.

Original Source: Hubble News Release

Astronomers say a dusty disc swirling around the nearby star Vega is bigger than earlier thought. It was probably caused by collisions of objects, perhaps as big as the planet Pluto, up to 2,000 kilometers (about 1,200 miles) in diameter.

NASA’s Spitzer Space Telescope has seen the dusty aftermath of this “run-in.” Astronomers think embryonic planets smashed together, shattered into pieces and repeatedly crashed into other fragments to create ever-finer debris. Vega’s light heats the debris, and Spitzer’s infrared telescope detects the radiation.

Vega, located 25 light-years away in the constellation Lyra, is the fifth brightest star in the night sky. It is 60 times brighter than our sun. Observations of Vega in 1984, with the Infrared Astronomical Satellite, provided the first evidence for dust particles around a typical star. Because of Vega’s proximity and because its pole faces Earth, it provides a great opportunity for detailed study of the dust cloud around it.

“Vega’s debris disc is another piece of evidence demonstrating the evolution of planetary systems is a pretty chaotic process,” said lead author of the study, Dr. Kate Su of the University of Arizona, Tucson, Ariz. The findings were presented today at the 205th meeting of the American Astronomical Society in San Diego.

Like a drop of ink spreading out in a glass of water, the particles in Vega’s dust cloud don’t stay close to the star long. “The dust we are seeing in the Spitzer images is being blown out by intense light from the star,” Su said. “We are witnessing the aftermath of a relatively recent collision, probably within the last million years,” she explained.

Scientists say this disc event is short-lived. The majority of the detected material is only a few microns in size, 100 times smaller than a grain of Earth sand. These tiny dust grains leave the system and dissipate into interstellar space on a time scale less than 1,000 years. “But there are so many tiny grains,” Su said. “They add up to a total mass equal to one third of the weight of our moon,” she said.

The mass of these short-lived grains implies a high dust-production rate. The Vega disc would have to have an improbably massive reservoir of planet-building material and collisions to maintain this amount of dust production throughout the star’s life (350 million years, 13 times younger than our sun). “We think a transient disc phenomenon is more likely,” Su said.

Su and her colleagues were struck by other characteristics of Vega’s debris disc, including its physical size. It has a radius of at least 815 astronomical units, roughly 20 times larger than our solar system. One astronomical unit is the distance from Earth to the sun, which is 150 million kilometers (93 million miles). A study of the disc’s surface brightness indicates the presence of an inner hole at a radius of 86 astronomical units (twice the distance between Pluto and the sun). Large embryonic planets at the edge of this inner hole may have collided to make the rest of the debris around Vega.

“Spitzer has obtained the first high spatial-resolution infrared images of Vega’s disc,” said Dr. Michael Werner, co-author and project scientist for Spitzer at NASA’s Jet Propulsion Laboratory (JPL), Pasadena, Calif. “Its sensitive infrared detectors have allowed us to see that Vega is surrounded by an enormous disc of debris,” he said.

JPL manages the Spitzer Space Telescope mission for NASA’s Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology (Caltech) in Pasadena. JPL is a division of Caltech. The multi-band imaging photometer for Spitzer, which made the new disc observations, was built by Ball Aerospace Corporation, Boulder, Colo.; the University of Arizona; and Boeing North American, Canoga Park, Calif.

Imagery and additional information about the Spitzer Space Telescope is available on the Internet, at:

http://www.spitzer.caltech.edu/Media

Original Source: NASA News Release

Two of NASA’s Great Observatories, the Spitzer Space Telescope and the Hubble Space Telescope, have provided astronomers an unprecedented look at dusty planetary debris around stars the size of our sun.

Spitzer has discovered for the first time dusty discs around mature, sun-like stars known to have planets. Hubble captured the most detailed image ever of a brighter disc circling a much younger sun-like star. The findings offer “snapshots” of the process by which our own solar system evolved, from its dusty and chaotic beginnings to its more settled present-day state.

“Young stars have huge reservoirs of planet-building materials, while older ones have only leftover piles of rubble. Hubble saw the reservoirs and Spitzer, the rubble,” said Dr. Charles Beichman of NASA’s Jet Propulsion Laboratory (JPL), Pasadena, Calif. He is lead author of the Spitzer study. “This demonstrates how the two telescopes complement each other,” he added.

The young star observed by Hubble is 50 to 250 million years old. This is old enough to theoretically have gas planets, but young enough that rocky planets like Earth may still be forming. The six older stars studied by Spitzer average 4 billion years old, nearly the same age as the sun. They are known to have gas planets, and rocky planets may also be present. Prior to the findings, rings of planetary debris, or “debris discs,” around stars the size of the sun had rarely been observed, because they are fainter and more difficult to see than those around more massive stars.

“The new Hubble image gives us the best look so far at reflected light from a disc around a star the mass of the sun,” said Hubble study lead author, Dr. David Ardila of the Johns Hopkins University, Baltimore. “Basically, it shows one of the possible pasts of our own solar system,” he said.

Debris discs around older stars the same size and age as our sun, including those hosting known planets, are even harder to detect. These discs are 10 to 100 times thinner than the ones around young stars. Spitzer’s highly sensitive infrared detectors were able to sense their warm glow for the first time.

“Spitzer has established the first direct link between planets and discs,” Beichman said. “Now, we can study the relationship between the two.” These studies will help future planet-hunting missions, including NASA’s Terrestrial Planet Finder and the Space Interferometry Mission, predict which stars have planets. Finding and studying planets around other stars is a key goal of NASA’s exploration mission.

Rocky planets arise out of large clouds of dust that envelop young stars. Dust particles collide and stick together, until a planet eventually forms. Sometimes the accumulating bodies crash together and shatter. Debris from these collisions collects into giant doughnut-shaped discs, the centers of which may be carved out by orbiting planets. With time, the discs fade and a smaller, stable debris disc, like the comet-filled Kuiper Belt in our own solar system, is all that is left.

The debris disc imaged by Hubble surrounds the sun-like star called HD 107146, located 88 light-years away. John Krist, a JPL astronomer, also used Hubble to capture another disc around a smaller star, a red dwarf called AU Microscopii, located 32 light-years away and only 12 million years old. The Hubble view reveals a gap in the disc, where planets may have swept up dust and cleared a path. The disc around HD 107146 also has an inner gap.

Beichman and his colleagues at JPL and the University of Arizona, Tucson, used Spitzer to scan 26 older sun-like stars with known planets, and found six with Kuiper Belt-like debris discs. The stars range from 50 to 160 light-years away. Their discs are about 100 times fainter than those recently imaged by Hubble, and about 100 times brighter than the debris disc around the sun. These discs are also punctuated by holes at their centers.

Both Hubble images were taken with the advanced camera for surveys. They will be published in the Astronomical Journal and the Astrophysical Journal Letters. The Spitzer observations are from the multiband imaging photometer and will appear in the Astrophysical Journal.

The Space Telescope Science Institute (STScI) is operated by the Association of Universities for Research in Astronomy, Inc. (AURA), for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA).

Original Source: Hubble News Release

One of the currently hottest astrophysical topics – the hunt for Earth-like planets around other stars – has just received an important impetus from new spectral observations with the MIDI instrument at the ESO VLT Interferometer (VLTI).

An international team of astronomers [2] has obtained unique infrared spectra of the dust in the innermost regions of the proto-planetary discs around three young stars – now in a state possibly very similar to that of our solar system in the making, some 4,500 million years ago.

Reporting in this week’s issue of the science journal Nature, and thanks to the unequalled, sharp and penetrating view of interferometry, they show that in all three, the right ingredients are present in the right place to start formation of rocky planets at these stars.

“Sand” in the inner regions of stellar discs
The Sun was born about 4,500 million years ago from a cold and massive cloud of interstellar gas and dust that collapsed under its own gravitational pull. A dusty disc was present around the young star, in which the Earth and other planets, as well as comets and asteroids were later formed.

This epoch is long gone, but we may still witness that same process by observing the infrared emission from very young stars and the dusty protoplanetary discs around them. So far, however, the available instrumentation did not allow a study of the distribution of the different components of the dust in such discs; even the closest known are too far away for the best single telescopes to resolve them. But now, as Francesco Paresce, Project Scientist for the VLT Interferometer and a member of the team from ESO explains, “With the VLTI we can combine the light from two well-separated large telescopes to obtain unprecedented angular resolution. This has allowed us, for the first time, to peer directly into the innermost region of the discs around some nearby young stars, right in the place where we expect planets like our Earth are forming or will soon form”.

Specifically, new interferometric observations of three young stars by an international team [2], using the combined power of two 8.2-m VLT telescopes a hundred metres apart, has achieved sufficient image sharpness (about 0.02 arcsec) to measure the infrared emission from the inner region of the discs around three stars (corresponding approximately to the size of the Earth’s orbit around the Sun) and the emission from the outer part of those discs. The corresponding infrared spectra have provided crucial information about the chemical composition of the dust in the discs and also about the average grain size.

These trailblazing observations show that the inner part of the discs is very rich in crystalline silicate grains (“sand”) with an average diameter of about 0.001 mm. They are formed by coagulation of much smaller, amorphous dust grains that were omnipresent in the interstellar cloud that gave birth to the stars and their discs.

Model calculations show that crystalline grains should be abundantly present in the inner part of the disc at the time of formation of the Earth. In fact, the meteorites in our own solar system are mainly composed of this kind of silicate.

Dutch astronomer Rens Waters, a member of the team from the Astronomical Institute of University of Amsterdam, is enthusiastic: “With all the ingredients in place and the formation of larger grains from dust already started, the formation of bigger and bigger chunks of stone and, finally, Earth-like planets from these discs is almost unavoidable!”

Transforming the grains
It has been known for some time that most of the dust in discs around newborn stars is made up of silicates. In the natal cloud this dust is amorphous, i.e. the atoms and molecules that make up a dust grain are put together in a chaotic way, and the grains are fluffy and very small, typically about 0.0001 mm in size. However, near the young star where the temperature and density are highest, the dust particles in the circumstellar disc tend to stick together so that the grains become larger. Moreover, the dust is heated by stellar radiation and this causes the molecules in the grains to re-arrange themselves in geometric (crystalline) patterns.

Accordingly, the dust in the disc regions that are closest to the star is soon transformed from “pristine” (small and amorphous) to “processed” (larger and crystalline) grains.

Spectral observations of silicate grains in the mid-infrared wavelength region (around 10 ?m) will tell whether they are “pristine” or “processed”. Earlier observations of discs around young stars have shown a mixture of pristine and processed material to be present, but it was so far impossible to tell where the different grains resided in the disc.

Thanks to a hundred-fold increase in angular resolution with the VLTI and the highly sensitive MIDI instrument, detailed infrared spectra of the various regions of the protoplanetary discs around three newborn stars, only a few million years old, now show that the dust close to the star is much more processed than the dust in the outer disc regions. In two stars (HD 144432 and HD 163296) the dust in the inner disc is fairly processed whereas the dust in the outer disc is nearly pristine. In the third star (HD 142527) the dust is processed in the entire disc. In the central region of this disc, it is extremely processed, consistent with completely crystalline dust.

An important conclusion from the VLTI observations is therefore that the building blocks for Earth-like planets are present in circumstellar discs from the very start. This is of great importance as it indicates that planets of the terrestrial (rocky) type like the Earth are most probably quite common in planetary systems, also outside the solar system.

The pristine comets
The present observations also have implications for the study of comets. Some – perhaps all – comets in the solar system do contain both pristine (amorphous) and processed (crystalline) dust. Comets were definitely formed at large distances from the Sun, in the outer regions of the solar system where it has always been very cold. It is therefore not clear how processed dust grains may end up in comets.

In one theory, processed dust is transported outwards from the young Sun by turbulence in the rather dense circumsolar disc. Other theories claim that the processed dust in comets was produced locally in the cold regions over a much longer time, perhaps by shock waves or lightning bolts in the disc, or by frequent collisions between bigger fragments.

The present team of astronomers now conclude that the first theory is the most likely explanation for the presence of processed dust in comets. This also implies that the long-period comets that sometimes visit us from the outer reaches of our solar system are truly pristine bodies, dating back to an era when the Earth and the other planets had not yet been formed.

Studies of such comets, especially when performed in-situ, will therefore provide direct access to the original material from which the solar system was formed.

More information
The results reported in this ESO PR are presented in more detail in a research paper “The building blocks of planets within the “terrestrial” region of protoplanetary disks”, by Roy van Boekel and co-authors (Nature, November 25, 2004). The observations were made in the course of ESO’s early science demonstration programme.

Notes

[1]: This ESO press release is issued in collaboration with the Astronomical Institute of the University of Amsterdam, The Netherlands (NOVA PR) and the Max-Planck-Institut f?r Astronomie (Heidelberg, Germany (MPG PR).

[2]: The team consists of Roy van Boekel, Michiel Min, Rens Waters, Carsten Dominik and Alex de Koter (Astronomical Institute, University of Amsterdam, The Netherlands), Christoph Leinert, Olivier Chesneau, Uwe Graser, Thomas Henning, Rainer K?hler and Frank Przygodda (Max-Planck-Institut f?r Astronomie, Heidelberg, Germany), Andrea Richichi, Sebastien Morel, Francesco Paresce, Markus Sch?ller and Markus Wittkowski (ESO), Walter Jaffe and Jeroen de Jong (Leiden Observatory, The Netherlands), Anne Dutrey and Fabien Malbet (Observatoire de Bordeaux, France), Bruno Lopez (Observatoire de la Cote d’Azur, Nice, France), Guy Perrin (LESIA, Observatoire de Paris, France) and Thomas Preibisch (Max-Planck-Institut f?r Radioastronomie, Bonn, Germany).

[3]: The MIDI instrument is the result of a collaboration between German, Dutch and French institutes. See ESO PR 17/03 and ESO PR 25/02 for more information.

Original Source: ESO News Release

Image credit: NASA/JPL
In June, researchers from the University of Rochester announced they had located a potential planet around another star so young that it defied theorists’ explanations. Now a new team of Rochester planet-formation specialists are backing up the original conclusions, saying they’ve confirmed that the hole formed in the star’s dusty disk could very well have been formed by a new planet. The findings have implications for gaining insight into how our own solar system came to be, as well as finding other possibly habitable planetary systems throughout our galaxy.

“The data suggests there’s a young planet out there, but until now none of our theories made sense with the data for a planet so young,” says Adam Frank, professor of physics and astronomy at the University of Rochester. “On the one hand, it’s frustrating; but on the other, it’s very cool because Mother Nature has just handed us the planet and we’ve got to figure out how it must have been created.”

Intriguingly, working from the original team’s data, Frank, Alice Quillen, Eric Blackman, and Peggy Varniere revealed that the planet was likely smaller than most extra-solar planets discovered thus far – about the size of Neptune. The data also suggested that this planet is about the same distance from its parent star as our own Neptune is from the Sun. Most extra-solar planets discovered to date are much larger and orbit extremely close to their parent star.

The original Rochester team, led by Dan Watson, professor of physics and astronomy, used NASA’s new Spitzer Space Telescope to detect a gap in the dust surrounding a fledgling star. The critical infrared “eyes” of the infrared telescope were designed in part by physics and astronomy professors Judith Pipher, William Forrest, and Watson, a team that has been among the world leaders in opening the infrared window to the universe. It was Forrest and Pipher who were the first U.S. astronomers to turn an infrared array toward the skies: In 1983, they mounted a prototype infrared detector onto the University telescope in the small observatory on top of the Wilmot Building on campus, taking the first-ever telescopic pictures of the moon in the infrared, a wavelength range of light that is invisible to the naked eye as well as to most telescopes.

The discovered gap strongly signaled the presence of a planet. The dust in the disk is hotter in the center near the star and so radiates most of its light at shorter wavelengths than the cooler outer reaches of the disk. The research team found that there was an abrupt dearth of light radiating at all short infrared wavelengths, strongly suggesting that the central part of the disk was absent. Scientists know of only one phenomenon that can tunnel such a distinct “hole” in the disk during the short lifetime of the star – a planet at least 100,000 years old.

This possibility of a planet on the order of only 100,000 to half a million years old was met with skepticism by many astronomers because neither of the leading planetary formation models seemed to allow for a planet of this age. Two models represent the leading theories of planetary formation: core accretion and gravitational instability. Core accretion suggests that the dust from which the star and system form begins to clump together into granules, and those granules clump into rocks, asteroids, and planetoids until whole planets are formed. But the theory says it should take about 10 million years for a planet to evolve this way – far too long to account for the half-million-year-old planet found by Watson.

Conversely, the other leading theory of planetary formation, gravitational instability, suggests that whole planets could form essentially in one swoop as the original cloud of gas is pulled together by its own gravity and becomes a planet. But while this model suggests that planetary formation could happen much faster – on the order of centuries – the density of the dust disk surrounding the star seems to be too sparse to support this model either.

“Even though it doesn’t fit either model, we’ve crunched the numbers and shown that yes, in fact, that hole in that dust disk could have been formed by a planet,” says Frank. “Now we have to look at our models and figure out how that planet got there. At the end of it all, we hope we have a new model, and a new understanding of how planets come to be.”

This research was funded by the National Science Foundation.

Original Source: University of Rochester News Release