Category: Extrasolar Planets

The brown dwarf 2M1207 and its planetary companion. Image credit: ESO/VLT/NACO. Click to enlarge.
An international team of astronomers reports today confirmation of the discovery of a giant planet, approximately five times the mass of Jupiter, that is gravitationally bound to a young brown dwarf. This puts an end to a year long discussion on the nature of this object, which started with the detection of a red object close to the brown dwarf.

In February and March of this year, the astronomers took new images of the young brown dwarf and its giant planet companion with the state-of-the-art NACO instrument on ESO’s Very Large Telescope in northern Chile. The planet is near the southern constellation of Hydra and approximately 200 light years from Earth.

“Our new images show convincingly that this really is a planet, the first planet that has ever been imaged outside of our solar system,” tells Gael Chauvin, astronomer at ESO and leader of the team of astronomers who conducted the study.

“The two objects – the giant planet and the young brown dwarf – are moving together; we have observed them for a year, and the new images essentially confirm our 2004 finding,” says Benjamin Zuckerman, UCLA professor of physics and astronomy, member of NASA’s Astrobiology Institute, and a member of the team. “I’m more than 99 percent confident.” The separation between the planet and the brown dwarf is 55 times the separation of the Earth and Sun.

Anne-Marie Lagrange, another member of the team from the Grenoble Observatory in France, looks towards the future: “Our discovery represents a first step towards one of the most important goals of modern astrophysics: to characterize the physical structure and chemical composition of giant and, eventually, terrestrial-like planets.”

Last September, the same team of astronomers reported a faint reddish speck of light in the close vicinity of a young brown dwarf (see ESO PR 23/04). The feeble object, now called 2M1207b, is more than 100 times fainter than the brown dwarf, 2M1207A. The spectrum of 2M1207b presents a strong signature of water molecules, thereby confirming that it must be cold. Based on the infrared colours and the spectral data, evolutionary model calculations led to the conclusion that 2M1207b is a 5 Jupiter-mass planet. Its mass can be estimated also by use of a different method of analysis, which focuses on the strength of its gravitational field; this technique suggests that the mass might be even less than 5 Jupiters.

At the time of its discovery in April 2004, it was impossible to prove that the faint source is not a background object (such as an unusual galaxy or a peculiar cool star with abnormal infrared colours), even though this appeared very unlikely. Observations with the Hubble Space Telescope, obtained in August 2004, corroborated the VLT/NACO observations, but were taken too soon after the NACO ones to conclusively demonstrate that the faint source is a planet.

The new observations show with high confidence that the two objects are moving together and hence are gravitationally bound.

“Given the rather unusual properties of the 2M1207 system, the giant planet most probably did not form like the planets in our solar system,” says Gael Chauvin. “Instead it must have formed the same way our Sun formed, by a one-step gravitational collapse of a cloud of gas and dust.”

The paper describing this research has been accepted for publication in Astronomy and Astrophysics.

The same European/American team has had another paper just accepted for publication in Astronomy & Astrophysics; this paper reports the imaging discovery with the same VLT/NACO instrumentation of a lightweight companion to AB Pictoris, a young star located about 150 light years from Earth. The estimated mass of the companion is between 13 and 14 times the mass of Jupiter, which places the companion right on the border line between massive planets and the lowest mass brown dwarfs.

Original Source: ESO News Release

Image credit: NWU
The peculiar orbits of three planets looping around a faraway star can be explained only if an unseen fourth planet blundered through and knocked them out of their circular orbits, according to a new study by researchers at the University of California, Berkeley, and Northwestern University.

The conclusion is based on computer extrapolations from 13 years of observations of planet motions around the star Upsilon Andromedae. It suggests that the non-circular and often highly elliptical orbits of many of the extrasolar planets discovered to date may be the result of planets scattering off one another. In such a scenario, the perturbing planet could be shot out of the system entirely or could be kicked into a far-off orbit, leaving the inner planets with eccentric orbits.

“This is probably one of the two or three extrasolar systems that have the best observations and tightest constraints, and it tells a unique story,” said Eric Ford, a Miller postdoctoral fellow at UC Berkeley. “Our explanation is that the outer planet’s original orbit was circular, but it got this sudden kick that permanently changed its orbit to being highly eccentric. To provide that kick, we’ve hypothesized that there was an additional planet that we don’t see now. We believe we now understand how this system works.”

If such a planet had caromed through our solar system early in its history, the researchers noted, the inner planets might not now have such nicely circular orbits, and, based on current assumptions about the origins of life, Earth’s climate might have fluctuated too much for life to have arisen.

“While the planets in our solar system remain stable for billions of years, that wasn’t the case for the planets orbiting Upsilon Andromedae,” Ford said. “While those planets might have formed similarly to Jupiter and Saturn, their current orbits were sculpted by a late phase of chaotic and violent interactions.”

According to Ford’s colleague, Frederic A. Rasio, associate professor of physics and astronomy at Northwestern, “Our results show that a simple mechanism, often called ‘planet-planet scattering’ – a sort of slingshot effect due to the sudden gravitational pull between two planets when they come very near each other – must be responsible for the highly eccentric orbits observed in the Upsilon Andromedae system. We believe planet-planet scattering occurred frequently in extrasolar planetary systems, not just this one, resulting from strong instabilities. So, while planetary systems around other stars may be common, the kinds of systems that could support life, which, like our solar system, presumably must remain stable over very long time scales, may not be so common.”

The computer simulations are reported in the April 14 issue of the journal Nature by Ford, Rasio and Verene Lystad, an undergraduate student majoring in physics at Northwestern. Ford was a student of Rasio’s at the Massachusetts Institute of Technology before pursuing graduate studies at Princeton University and arriving at UC Berkeley in 2004.

The planetary system around Upsilon Andromedae is one of the most studied of the 160-some systems with planets discovered so far outside our own solar system. The inner planet, a “hot Jupiter” so close to the star that its orbit is only a few days, was discovered in 1996 by UC Berkeley’s Geoff Marcy and his planet-hunting team. The two outer planets, with elongated orbits that perturb each other strongly, were discovered in 1999. These three, huge, Jupiter-like planets around Upsilon Andromedae comprised the first extrasolar multi-planet system discovered by Doppler spectroscopy.

Because of the unusual nature of the planetary orbits around Upsilon Andromedae, Marcy and his team have studied it intensely, making nearly 500 observations – 10 times more than for most other extrasolar planets that have been found. These observations, the wobbles in the star’s motion induced by the orbiting planets, allow a very precise charting of the planets’ motions around the star.

“The observations are so precise that we can watch and predict what will happen for tens of thousands of years in the future,” Ford said.

Today, while the innermost planet huddles close to the star, the two outer planets orbit in egg-shaped orbits. Computer simulations of past and future orbital changes showed, however, that the outer planets are engaged in a repetitive dance that, once every 7,000 years, brings the orbit of the middle planet to a circle.

“That property of returning to a very circular orbit is quite remarkable and generally doesn’t happen,” Ford said. “The natural explanation is that they were once both in circular orbits, and one got a big kick that caused it to become eccentric. Then, the subsequent evolution caused the other planet to grow its eccentricity, but because of the conservation of energy and angular momentum, it returns periodically to a very nearly circular orbit.”

Previously, astronomers had proposed two possible scenarios for the formation of Upsilon Andromedae’s planet system, but the observational data was not yet sufficient to distinguish the two models. Another astronomer, Renu Malhotra at the University of Arizona, had previously suggested that planet-planet scattering might have excited the eccentricities in Upsilon Andromedae. But an alternative explanation claimed that interactions among the planets and a gas disk surrounding the star could also have produced such eccentric orbits. By combining additional observational data with new computer models, Ford and his colleagues were able to show that interactions with a gas disk would not have produced the observed orbits, but that interactions with another planet would naturally produce them.

“The key distinguishing feature between those theories was that interactions with an outer disk would cause the orbits to change very slowly, and a strong interaction with a passing planet would cause the orbits to change very quickly compared to the 7,000-year time scale for the orbits to evolve,” Ford said. “Because the two hypotheses make different predictions for the evolution of the system, we can constrain the history of the system based on the current planetary orbits.”

Ford said that as the planets formed inside a disk of gas and dust, the drag on the planets would have kept their orbits circular. Once the dust and gas dissipated, however, only an interaction with a passing planet could have created the particular orbits of the two outer planets observed today. Perhaps, he noted, the perturbing planet was knocked into the inner planets by interactions with other planets far from the central star.

However it started, the resulting chaotic interactions would have created a very eccentric orbit for the third planet, which then also gradually perturbed the second planet’s orbit. Because the outer planet dominates the system, over time it perturbed the middle planet’s orbit enough to deform it slowly into an eccentric orbit as well, which is what is seen today, although every 7,000 years or so, the middle planet returns gradually to a circular orbit.

“This is what makes the system so peculiar,” said Rasio. “Ordinarily, the gravitational coupling between two elliptic orbits would never make one go back to a nearly perfect circle. A circle is very special.”

“Originally the main objective of our research was to simulate the Upsilon Andromedae planetary system, essentially in order to determine whether the outer two planets lie in the same plane like the planets in the solar system do,” said Lystad, who started working with Rasio when she was a sophomore and did many of the computer integrations as part of her senior thesis. “We were surprised to find that, for many of our simulations, it was difficult to tell whether the planets were in the same plane due to the fact that the middle planet’s orbit periodically became so very nearly circular. Once we noticed this strange behavior was present in all of our simulations, we recognized it as an earmark of a system that had undergone planet-planet scattering. We realized there was something much more interesting going on than anyone had found before.”

Understanding what happened during the formation and evolution of Upsilon Andromedae and other extrasolar planetary systems has major implications for our own solar system.

“Once you realize that most of the known extrasolar planets have highly eccentric orbits (like the planets in Upsilon Andromedae), you begin to wonder if there might be something special about our solar system,” Ford said. “Could violent planet-planet scattering be so common that few planetary systems remain calm and habitable? Fortunately, astronomers – led by Geoff Marcy, a professor of astronomy at UC Berkeley – are diligently making the observations that will eventually answer this exciting question.”

The research was supported by the National Science Foundation and UC Berkeley’s Miller Institute for Basic Research.

Original Source: Berkeley News Release

Illustration by: Jimmy Paillet
As of February 5, we know of 136 extrasolar planets. These have been discovered in four ways: The first – called pulsar timing – allowed us to detect Earth-sized and smaller planets by studying the variations in arrival time of radiation generated by a pulsar. The next – Doppler spectroscopy – allows ground-based telescopes to measure the “shift” in a star’s spectrum caused by the gravity of an orbiting planet. The third – astrometry – is used in much the same way – looking for the periodic “wobble” in position that a possible planet could cause on its parent star. And the last? Transit photometry allows for the study of the periodic dimming of a star as a body passes in front of it from a particular viewpoint – producing a light curve.

In April 2004, Luc F. A. Arnold, (Observatoire de Haute-Provence CNRS 04870 Saint-Michel – l’Observatoire, France) was working on a transit generated by a saturn-like planet when he had an idea. Could this same principle be applied to look for transiting bodies that were artificial in nature?

“I discussed the idea with several colleagues who found it interesting,” commented Arnold. A collection of artificial bodies would produce light curves easily distinguishable from natural ones. For example, a triangular object or something shaped like our own man-made satellites would show an entirely different signature. If multiple artificial objects were detected transiting – this could possibly be a form of signaling the presence of other intelligent life – one with an effectiveness equal to the range of the laser pulse method.

A cost-effective alternative to radio SETI or optical SETI is to look for artificial planet-size bodies which may exist around other stars. Since they would always pass in front of their parent star for a given remote observer, there is a strong possibility they can be detected and characterized using the transit photometry method. A planetary transit light curve contains fine features due to the object shape – such as planet oblateness, double planets or ringed planets. As Arnold explains, “The sphere is the equilibrium shape preferred for massive and planet-size bodies to adapt to their own gravity, (but) one can consider non-spherical bodies, especially if they are small and lightweight and orbit a dwarf star. Their transits in front of a star would produce a detectable signal.” Non-spherical artificial objects – like a triangle – would produce a specific transit light curve. If multiple objects should transit, a remarkable light curve would be created by their “on again – off again” nature of light. Such an observation would clearly claim an artificial nature. To visualize this, think of a flashlight moving behind a lowered window blind, and you’ll begin to get the idea!

The bulk of Luc Arnold’s work – just accepted for publication in the “Astrophysical Journal” – has been to prove through computer simulation the effects of different and multiples shapes and show these differing light curves. To help you better understand, the screen that you are now looking at is composed of pixels – a logical rather than a physical unit. If you were to place a triangle shape over your monitor’s screen, it would cover the pixels in a specific arrangement. During a simulation, the stellar flux is zeroed out in pixels and compared to the normal flux of the star. This simulated artificial body transit is then fitted against known planetary transit using a Powell algorithm.

“But most complex artificial objects’ light curve cannot be exactly superposed by a planetary transit, and the algorithm ends with non-zero residuals, i.e. a non-zero difference between the two light curves. This difference is the ‘personal’ signature of the artificial object. Should it rotate, the residual light curves will show additional modulation. When set against a gradient, such as the limb, an artificial object would also show sudden slope variations in the light curve during ingress or egress,” explains Arnold.

The equilateral triangle produces a transit light curve different than a sphere. In fact, its light curve resembles a ringed planet transit, so an ambiguity may remain in distinguishing these objects. But more complex objects, such as clusters of shapes, for example, create very specific signatures. For an artificial satellite-like object, its symmetrical structure would be apparent – as each area would impact the light curve at specific intervals. An elongated object, would produce undulation in its longer period of ingress and egress – in effect causing multiple “transits” making detection easier. The nature of these oscillations could very well be considered a sign of intelligent device. If several objects were spatially arranged in groups to ingress a star in a mathematically constant manner, these drops in the light curve could clearly represent a type of message – the language of science.

With the computer simulations perfected, Arnold knows what a natural or artificial transiting body should look like in a light curve – but has science observed a planetary transit? “Up to now, there is only one transit light curve obtained with a very good accuracy – the transit for HD 209 458b observed with the Hubble Space Telescope. T. Brown and colleagues found the light curve could be fitted with a spherical body to within the measurement accuracy.” This type of information provides Arnold with the model he needs. In June 2006, his vision may be realized. COROT (a space mission approved by the French Space Agency CNES, with a participation of Austria, Belgium, Brazil, Germany, Spain, ESA and ESTEC) will be dedicated to stellar seismology and the study of extrasolar planets – the first approved space mission solely devoted to these subjects. The spacecraft will consist of a ~ 30 cm telescope with an array of detectors to monitor the light curves of well chosen stars through CCD. The overall potential of COROT (COnvection, ROtation and planetary Transits) is to detect several tens of Earth sized planets and more upcoming programs such as the Terrestrial Planet Finder (TPF) and Space Interferometry Mission (SIM) will change the face of all we know about extrasolar planets.

What does this kind of new technology mean to researchers like Luc Arnold? “These space missions will give a (photometric) accuracy of down to 0.01% – but 1% could be sufficient if objects are big enough.” According to his research a single transit of an artificial body would require that kind of accuracy, but a multiple transit would be much more relaxed. “1% photometry is within the capability of thousands of amateur astronomers equipped with CCD.” Chances are far greater that a communicative civilization would favour a series of objects over a single non-spherical one for signaling their presence. Transits of opaque objects are achromatic, putting them within detectability of CCD over the entire spectrum.

As Luc points out, this type of research may well be within the realm of the contributing amateur astronomer. Currently the search for signs of extra-terrestrial intelligence are limited to radio and the search for laser pulse which demands specialized equipment. “For the moment, there is no project to apply this idea. If it the idea turns into a specific (SETI) observing program, a number of collaborations would be welcome!”

The search for planetary transits is already in operation, such as the Optical Gravitational Lensing Experiment (OGLE), “and the multiple transit case could be discovered within the course of these programs – maybe tomorrow!” While tomorrow might seem like an impossible dream, Arnold knows differently. His work has already been submitted to the SETI institute. For the rest of the citizens of planet Earth, we await the results. Will tomorrow show us a possible energy collection, communication or study device put into orbit by another sentient species? If we consider what we know of astronomy to be a basic “truth” throughout the Cosmos, then a discovery of this magnitude could be the biggest news of them all… “Assuming we are sure to have detected an alien artifact in a transit light curve, my opinion is that we should consider it as a clear ‘Hello world… We are here!’ addressed to the whole Galaxy!”

Written by Tammy Plotner

Since the discovery in 1995 of the first planet orbiting a normal star other than the Sun, there are now more than 150 candidates of these so-called exoplanets known. Most of them are detected by indirect methods, based either on variations of the radial velocity or the dimming of the star as the planet passes in front of it (see ESO PR 06/03, ESO PR 11/04 and ESO PR 22/04).

Astronomers would, however, prefer to obtain a direct image of an exoplanet, allowing them to better characterize the object’s physical nature. This is an exceedingly difficult task, as the planet is generally hidden in the “glare” of its host star.

To partly overcome this problem, astronomers study very young objects. Indeed, sub-stellar objects are much hotter and brighter when young and therefore can be more easily detected than older objects of similar mass.

Based on this approach, it might well be that last year’s detection of a feeble speck of light next to the young brown dwarf 2M1207 by an international team of astronomers using the ESO Very Large Telescope (ESO PR 23/04) is the long-sought bona-fide image of an exoplanet. A recent report based on data from the Hubble Space Telescope seems to confirm this result. The even more recent observations made with the Spitzer Space Telescope of the warm infrared glows of two previously detected “hot Jupiter” planets is another interesting result in this context. This wealth of new results, obtained in the time span of a few months, illustrates perfectly the dynamic of this field of research.

Tiny Companion
Now, a different team of astronomers [1] has possibly made another important breakthrough in this field by finding a tiny companion to a young star. Since several years these scientists have conducted a search for planets and low-mass objects, in particular around stars still in their formation process – so-called T-Tauri stars – using both the direct imaging and the radial velocity techniques. One of the objects on their list is GQ Lupi, a young T-Tauri star, located in the Lupus I (the Wolf) cloud, a region of star formation about 400 or 500 light-years away. The star GQ Lupi is apparently a very young object still surrounded by a disc, with an age between 100,000 and 2 million years.

The astronomers observed GQ Lupi on 25 June 2004 with the adaptive optics instrument NACO attached to Yepun, the fourth 8.2-m Unit Telescope of the Very Large Telescope located on top of Cerro Paranal (Chile). The instrument’s adaptive optics (AO) overcomes the distortion induced by atmospheric turbulence, producing extremely sharp near-infrared images.

As ESO PR Photo 10a/05 shows, the series of NACO exposures clearly reveal the presence of the tiny companion, located in the close vicinity of the star. This newly found object is only 0.7 arcsecond away, and would have been overlooked without the use of the adaptive optics capabilities of NACO.

At the distance of GQ Lupi, the separation between the star and its feeble companion is about 100 astronomical units (or 100 times the distance between the Sun and the Earth). This is roughly 2.5 times the distance between Pluto and the Sun.

The companion, called GQ Lupi B or GQ Lupi b [2], is roughly 250 times fainter than GQ Lupi A as seen in this series of image. Further images obtained with NACO in August and September confirmed the presence and the position of this companion.

Moving in the same direction
The astronomers then uncovered that the star had been previously observed by the Subaru telescope as well as by the Hubble Space Telescope. They retrieved the corresponding images from the data archives of these facilities for further analysis.

The older images, taken in July 2002 and April 1999, respectively, also showed the presence of the companion, giving the astronomers the possibility of precisely measuring the position of the two objects over a period of several years. This in turn allowed them to determine if the stars move together in the sky – as should be expected if they are gravitationally bound together – or if the smaller object is only a background object, just aligned by chance.

From their measurements, the astronomers found that the separation between the two objects did not change over the five-year period covered by the observations (see ESO PR Photo 10b/05). For the scientists this is a clear proof that both objects are moving in the same direction in the sky. “If the faint object would be a background object”, says Ralph Neuh?user of the University of Jena (Germany) and leader of the team, “we would see a change in separation as GQ Lup would be moving in the sky. From 1999 to 2004, the separation would have changed by 0.15 arcsec, while we are confident that the change is a least 20 times smaller.”

Exoplanet or brown dwarf?
To further probe the physical nature of the newly discovered object, the astronomers used NACO on the VLT to take a series of spectra. These showed the typical signature of a very cool object, in particular the presence of water and CO bands. Taking into account the infrared colours and the spectral data available, atmospheric model calculations point to a temperature between 1,600 and 2,500 degrees and a radius that is twice as large as Jupiter (see PR Photo 10c/05). According to this, GQ Lupi B is thus a cold and rather small object.

But what is the nature of this faint object? Is it a bona-fide exoplanet or is it a brown dwarf, those “failed” stars that are not massive enough to centrally produce major nuclear reactions? Although the borderline between the two is still a matter of debate, one way to distinguish between the two is by their mass (as this is also done between brown dwarfs and stars): (giant) planets are lighter than about 13 Jupiter-masses (the critical mass needed to ignite deuterium fusion), brown dwarfs are heavier.

What about GQ Lupi b?
Unfortunately, the new observations do not provide a direct estimate of the mass of the object. Thus the astronomers must rely on comparison with theoretical models of such objects. But this is not as easy as it sounds. If, as astronomers generally accept, GQ Lupi A and B formed simultaneously, the newly found object is very young. The problem is that for such very young objects, traditional theoretical models are probably not applicable. If they are used, however, they provide an estimate of the mass of the object that lies somewhere between 3 to 42 Jupiter-masses, i.e. encompassing both the planet and the brown dwarf domains.

These early phases in brown dwarf and planet formation are essentially unknown territory for models. It is very difficult to model the early collapse of the gas clouds given the conditions around the forming parent star. One set of models, specifically tailored to model the very young objects, provide masses as low as one to two Jupiter-masses. But as Ralph Neuh?user points out “these new models still need to be calibrated, before the mass of such companions can be determined confidently”.

The astronomers also stress that from the comparison between their VLT/NACO spectra and the theoretical models of co-author Peter Hauschildt from Hamburg University (Germany), they arrive at the conclusion that the best fit is obtained for an object having roughly 2 Jupiter radii and 2 Jupiter masses. If this result holds, GQ Lupi b would thus be the youngest and lightest exoplanet to have been imaged.

Further observations are still required to precisely determine the nature of GQ Lupi B. If the two objects are indeed bound, then the smallest object will need more than 1,000 years to complete an orbit around its host star. This is of course too long to wait but the effect of the orbital motion might possibly be detectable – as a tiny change in the separation between the two objects – in a few years. The team therefore plans to perform regular observations of this object using NACO on the VLT, in order to detect this motion. No doubt that in the mean time, further progress on the theoretical side will be achieved and that many sensational discoveries in this field will be made.

More information
The research presented in this ESO Press Release is published in a Letter to the Editor accepted for publication by Astronomy and Astrophysics (“Evidence for a co-moving sub-stellar companion of GQ Lup” by R. Neuh?user et al.) and available in PDF form at http://www.edpsciences.org/articles/aa/pdf/forthpdf/aagj061_forth.pdf.

Note
[1]: The team is composed of Ralph Neuh?user, G?nther Wuchterl, Markus Mugrauer, and Ana Bedalov (University of Jena, Germany), Eike Guenther (Th?ringer Landessternwarte Tautenburg, Germany), and Peter Hauschildt (Hamburger Sternwarte, Germany).

[2]: In the astronomical literature, the convention is to put capitals for stars member of multiple systems, but small letters for planets. If the companion to GQ Lupi A turns out to be a planet, it would be called GQ Lupi b, while if it is a brown dwarf, it would be identified as GQ lupi B. Given the present uncertainty, we have therefore used both denominations in this press release, as did the authors in the original scientific paper.

Original Source: ESO News Release

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