Category: Extrasolar Planets

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The European Space Agency’s COROT spacecraft has discovered an unusual exoplanet orbiting a star slightly more massive than our Sun. The planet, currently called COROT-exo-4b, is about the same size as Jupiter, and it takes 9.2 days to orbit its star. Most peculiar however, is that the planet and the star are in sync: the star rotates at the same pace as the planet’s period of revolution. Astronomers feel the planet is too low in mass and too distant from the star for the star to have any major influence on the planet’s rotation. But they are trying to understand the special interaction between this star and planet.

COROT stands for Convection, Rotation and planetary Transits. Launched in 2006, the mission has now observed more than 50,000 stars. The spacecraft is designed to detect rocky exoplanets almost as small as Earth. The satellite uses transits, the tiny dips in the light output from a star when a planet passes in front of it, to detect and study planets. This is followed up by extensive ground-based observations.

COROT-exo-4b is the fifth exoplanet found by the COROT spacecraft. Monitoring continuously over several months, the team tracked variations in its brightness between transits. They derived its period of rotation by monitoring dark spots on its surface that rotated in and out of view. It takes 9.2 days for the planet to orbit its star, which so far, is the longest period for any transiting exoplanet ever found.

It is not known whether COROT-exo-4b and its star have always been rotating in sync since their formation about 1000 million years ago, or if the star’s rotation synchronized later. Studying such systems with COROT will help scientists gain valuable insight into star-planet interactions.

This is the first transiting exoplanet found with such a peculiar combination of mass and period of rotation. Astronomers believe there must be something unique about how it formed and evolved.

Original News Source: ESA

HD 189733b is a Jupiter-sized extrasolar planet orbiting a yellow dwarf star. Due to its size and compact orbit, HD 189733b is one of the most studied extrasolar planets. HD 189733b shares many similar characteristics as HD 209458b (a.k.a. “Osiris,” as I reported in a UT article yesterday), and similar techniques have been used to analyse the spectral emissions from both parent stars. Although HD 189733b’s atmosphere isn’t thought to be evaporating like Osiris’, atmospheric gases extend far beyond the planetary “surface” allowing stellar light to pass through, giving astronomers a peek into what chemical compounds surround HD 189733b. From this analysis, scientists have deduced that water and methane is contained in the atmosphere; the Spitzer space telescope has even mapped the temperature distribution around the globe. Now, an Indian researcher has published work indicating a thin layer of particles exists in the upper atmosphere of HD 189733b. So what is the weather like on HD 189733b?

HD 189733b was discovered in 2005 and orbits a star in a binary system called HD 189733 in the constellation of Vulpecula. As the main star in the binary is a variable star (due to the transit of HD 189733b, periodically eclipsing the star), it has been designated with the variable name V452 Vulpeculae. The star system itself is located near the Dumbell Nebula, approximately 62 light years from Earth. As the star is relatively dim, as the exoplanet transits the star, there is an appreciable decrease in luminosity (of about 3%), creating the ideal conditions for the atmosphere of HD 189733b to be studied.

This exoplanet is approximately the same mass (1.15 ± 0.04 M_J) and radius (1.154 ± 0.032 R_J) as Jupiter, but it orbits very close to its parent star (~0.03 AU) so it is known as a “Hot Jupiter.” Due to the water/methane mix in the planet’s atmosphere, it is believed HD 189733b may have a blue hue, much like the colour of Uranus.

In 2007, the Spitzer Space Telescope observed HD 189733b and compiled a temperature map of the planet, showing that the equator was much hotter than the poles. Astronomers were also able to deduce that the atmosphere contains iron, silicate and aluminium oxide particulates. In new research by Sujan Sengupta from the Indian Institute of Astrophysics in Bangalore, it appears that these particles may collect in the upper atmosphere, forming a thin haze. This tentative conclusion was reached after careful examination of the polarization of emission from the star as HD 189733b transited. Preliminary results suggest there is a thin, reflective cloud in the exosphere.

So what is the weather like on HD 189733b? Hot and cloudy.

Source: arXiv Blog
Paper: arXiv:0807.1794v1 [astro-ph]

Observations of planets orbiting other stars are becoming increasingly common as astronomical techniques become more and more sophisticated. But some extrasolar planets have a stronger than normal spectroscopic signature, often stronger than their optical signature. What could be causing this? In a recent study, observations of the extrasolar planet HD 209458b (also unofficially known as “Osiris”, which orbits a star in the constellation of Pegasus) revealed the strongest ever spectroscopic signature for a giant extrasolar planet, indicating Osiris is producing a huge cloud of gas. This gas is being lost from the planet’s atmosphere; Osiris is evaporating…

Osiris orbits a star (imaginatively) called HD 209458, a yellow dwarf not too dissimilar to our Sun (with 1.1 solar masses, 1.2 solar radii and a surface temperature of 6000 K). This extrasolar planet is special in that it is readily observable during its transit period of 3.5 terrestrial days. This very short year is due to its small orbital radius of only 0.047 AU. Osiris could be called a “hot Jupiter” as it is a gas giant, approximately 60% the mass of Jupiter and it orbits within 0.05 AU of its parent star. Because of its close proximity to HD 209458, Osiris has a surface temperature of over 1000 K.

Osiris’ size and compact orbit causes HD 209458’s luminosity to vary by 2% as the planet passes in front of the star. It is for this reason that HD 209458 has been designated as a “variable star” with the name V376 Pegasi.

However, spectroscopic analysis of the star show that emissions from elements such as neutral hydrogen and a carbon ion are dimmed far more than the 2% optical luminosity dimming. What could be causing this increase in dimming for spectroscopic emission lines? As light is produced by HD 209458, it is blocked by the Osiris planetary disk, creating the 2% dimming observed by optical instrumentation. However, something is increasing the disk cross section area, absorbing certain spectral wavelengths of stellar emission. For example, there is a 5-15% dimming effect on neutral hydrogen (H I at 121.6 nm) and a 7-13% dimming effect on both atomic oxygen (O I at 130.5 nm) and singly ionized carbon (C II at around 133.5 nm). This led astronomers to realize there was a cloud of gas surrounding Osiris, allowing most of the optical wavelengths to pass through, but absorbing some spectroscopic lines.

As Osiris is orbiting so close to its star, the X-ray and EUV emissions are exciting gases in the exosphere (the uppermost reaches of the gas giant’s atmosphere), causing heating and expansion. As the planet is strongly influenced by its star’s gravitational pull, tides will play a strong part in amplifying the expansion of Osiris’ atmosphere. At a certain point, when the planet’s “exobase” (or the base of the exosphere) reaches the Roche Limit, atmospheric gases will begin to escape the gravitational pull of the planet and the interaction with HD 209458 causes a geometrical blow-off, ejecting huge amounts of atmospheric gases into space. The atmosphere of Osiris is therefore evaporating.

This is an intriguing subject, and more details can be found in the review recently published by David Ehrenreich from the Laboratoire d’astrophysique de Grenoble, Universite Joseph Fourier, France.

Source: arXiv:0807.1885v1 [astro-ph]

With yesterday’s announcement about finding a batch of so-called “super-Earths” – rocky alien worlds a few times more massive than our own – as well as another announcement back in May that 45 relatively low mass planets had been found, it’s obvious astronomers are constantly improving on their techniques to find new worlds. While the vast majority of the almost 300 previously discovered exoplanets are Jupiter-like gas giants, the new discoveries of large numbers of small planets – and especially that at least three of them orbit one star — suggests that they are abundant in our galaxy, and may outnumber Jupiter-sized giants by 3 to 1. But how much like Earth are these alien worlds?

Super-Earths are planets that have than ten times or less the mass of Earth. The three planets around the star HD 40307 have masses of 4.2, 6.7, and 9.4 times the mass of the Earth. They orbit their star with periods of 4.3, 9.6, and 20.4 days, respectively. That’s a short orbital period, meaning they are very close to the star. Since they are close to the star, astronomers believe its likely they are terrestrial, rocky-type planets rather than gas giants like Jupiter and Saturn. But also, being so close to the star means they are very warm – perhaps 1000 degrees Celsius. This would not be a pleasant or probable environment for life as we know it to take a foothold. But we don’t know for sure, and since we are curious creatures, we want to know more about these planets.

The observatory that made the discovery of the 3 planets around HD40307, as well as the 45 planets that were announced back in May is the High Accuracy Radial Velocity Planet Searcher (HARPS) survey based at the European Southern Observatory in La Silla, Chile. Astronomers spotted them by recording how each planet’s gravitational tug makes its parent star wobble.

But now astronomers know these planets are there, they can try other methods of studying the planets to glean some detailed information about what these planets are like. For years, astronomers have been waiting for a super-Earth to be found with an orbit that “transits” its parent star: in other words, it passes directly in front of the star as viewed from Earth. When exoplanets have short orbital periods, the likelihood of being able to observing transits increases. These new planets fit that category.

Being able to observe transits would give astronomers data to help figure out many of the planet’s characteristics, from measuring its radius to deducing its internal structure to “seeing” its atmosphere.

Getting information about the planet’s atmosphere would be especially exciting. By watching for changes in a star’s spectrum as it filters a fraction of the star’s light during a transit, the presence of methane and water vapor in the gaseous atmosphere could be revealed.

A few satellites are capable of watching for a transit, among them the Canadian MOST satellite. Another is the recycled Deep Impact spacecraft that is hosting the EPOCh (Extrasolar Planet Observation and Characterization) mission. So far, 4 new planets have been found with this spacecraft, using the transit method, and the goal of the mission is to find an exoplanet smaller than Earth. Also, EPOCh hopes to be able to identify features on an exoplanet, such as continents and oceans. Exciting prospect, indeed.

It’s only a matter of time until astronomers will be able to tell us how Earth-like these newly found Super Earths are.

Sources: New Scientist, Bad Astronomy, EPOCh

“Does every single star harbor planets and, if yes, how many?” wonders planet hunter Michel Mayor. “We may not yet know the answer but we are making huge progress towards it.” Mayor and his team of European astronomers have found a star which is orbited by at least three planets. Using the High Accuracy Radial velocity Planet Searcher (HARPS) instrument at the ESO La Silla Observatory, they have found a triple system of super-Earths around the star HD 40307. This is the first system known to have at least three “super-Earth” sized planets.

Back in 1995, Mayor, along with Didier Queloz, made the first discovery of an extrasolar planet around 51 Pegasi, and since then more than 270 exoplanets have been found, mostly around sun-like stars.

Most of these planets are giants, such as Jupiter or Saturn, and current statistics show that about 1 out of 14 stars harbors this kind of planet.

“With the advent of much more precise instruments such as the HARPS spectrograph on ESO’s 3.6-m telescope at La Silla, we can now discover smaller planets, with masses between 2 and 10 times the Earth’s mass,” says Stéphane Udry, one of Mayor’s colleagues. Such planets are called super-Earths, as they are more massive than the Earth but less massive than Uranus and Neptune (about 15 Earth masses).

HD 40307 is slightly less massive than our Sun, and is located 42 light-years away towards the southern Doradus and Pictor constellations.

“We have made very precise measurements of the velocity of the star HD 40307 over the last five years, which clearly reveal the presence of three planets,” says Mayor.

The planets, having 4.2, 6.7, and 9.4 times the mass of the Earth, orbit the star with periods of 4.3, 9.6, and 20.4 days, respectively.

The group made the announcement at a conference about extrasolar planets being held in France. The same team also announced the discovery of two other planetary systems, also with the HARPS spectrograph. In one, a super-Earth (7.5 Earth masses) orbits the star HD 181433 in 9.5 days. This star also hosts a Jupiter-like planet with a period close to 3 years. The second system contains a 22 Earth-mass planet having a period of 4 days and a Saturn-like planet with a 3-year period as well.

“Clearly these planets are only the tip of the iceberg,” says Mayor. “The analysis of all the stars studied with HARPS shows that about one third of all solar-like stars have either super-Earth or Neptune-like planets with orbital periods shorter than 50 days.”

A planet in a tight, short-period orbit is indeed easier to find than one in a wide, long-period orbit.
“It is most probable that there are many other planets present: not only super-Earth and Neptune-like planets with longer periods, but also Earth-like planets that we cannot detect yet. Add to it the Jupiter-like planets already known, and you may well arrive at the conclusion that planets are ubiquitous,” concludes Udry.

Calculations from the sample of stars studied with HARPS implies that one solar-like star out of three harbors planets with masses below 30 Earth masses and an orbital period shorter than 50 days.

News Source: ESO press release

How do you get the most out of one spacecraft and find exoplanets in the process? Re-use, recycle and share. The spacecraft bus that brought the Deep Impact “impactor” to comet Tempel 1 in July of 2005 is still out in its heliocentric orbit and has been put to work double time where two new missions are sharing the same spacecraft. The combined operation is called EPOXI, which is a combo-acronym of the two separate missions. The Deep Impact Extended Investigation (DIXI) of comets will observe comet 103P/Hartley 2 during a close flyby in October 2010. But of current interest is the other half of the dynamic duo, called the Extrasolar Planet Observation and Characterization (EPOCh) which is observing stars already known to have transiting giant planets. Since the orbital plane of the giant planet has been identified, EPOCh is looking in that same plane for planets closer to Earth size. So far, 4 new planets have been found with this spacecraft, using the transit method. But EPOCh is also looking back at our home planet, using Earth as a baseline to be able to identify features on an exoplanet, such as continents and oceans.

The EPOXI team has focused most of its attention on the star GJ436. This red dwarf star which is 32 light-years from Earth has a Neptune-sized planet that transits in front of the star. Spitzer observations have shown its orbit to be oval shaped, or eccentric. “That virtually guarantees there is a second planet in this system,” said Drake Deming, Deputy Principal Investigator for EPOXI . “We have three weeks of data on this system. The habitable zone corresponds with where we believe this planet to be, and we hope to be below the Earth as far as the size.”

Earth observations will help to calibrate future observations of Earth-like exoplanets. EPOXI obtained a particularly interesting view of the Earth on May 29, when the Moon passed in front of the Earth as viewed from the spacecraft. This “transit” of the Moon is an event that may also be observed to occur for Earth-like exoplanets, and it may help us to deduce the nature of their surface features.

Deming and Deep Impact team leader Michael A’Hearn both said that sharing the spacecraft has gone smoothly. The EPOCh mission will continue until August 30 of this year, with the option of doing more planet searching if the team is able to preserve the margin of hydrazine fuel on board. “But,” said Deming, “when the hydrazine runs out we’re done for sure.”

Source: AAS press conference

Most of the planets found to date have been massive and orbiting their parent stars at a fraction the orbit of Mercury – the hot jupiters. They’re interesting to astronomers, but the big goal is going to be finding Earth-mass planets orbiting other stars. To do this, astronomers are looking for less massive stars, where the effects of gravity from a smaller, Earth-sized planet will be easier to spot. Today, an international team of astronomers announced they have found a planet with only 3 times the mass of the Earth orbiting a tiny star that can barely support nuclear reactions.

The announcement of this new planet, known as MOA-2007-BLG-192Lb, was made at the 212th meeting of the American Astronomical Society held in St. Louis from June 1-5, 2008. Researchers from several universities, including the University of Notre Dame presented their findings.

The star is known as MOA-2007-BLG-192L, and it’s located about 3,000 light-years away. It’s probably not actually a star, with only 6% the mass of our own Sun. These objects are classified as brown dwarfs, because they don’t have enough mass to sustain nuclear reactions in the core. I say “probably” because the uncertainty of the observations might put it into the very low end of a hydrogen-burning star.

Researchers found the planet and star using the gravitational microlensing technique. This is where two stars line up perfectly from our point of view here on Earth. As the two stars begin to line up, the foreground star acts as a lens to magnify and distort the light from the more distant star. By watching how this brightening happens, astronomers can learn a tremendous amount about the nature of both the foreground and background star.

In this case, there was an additional gravitational distortion from the planet orbiting the foreground star MOA-2007-BLG-192L, which astronomers were able to tease out in their data.

This technique demonstrates the gravitational microlensing might be one of the best ways to find Earth-mass planets. In fact, the researchers think the technique will turn up the first one. Here’s David Bennett, from the University of Notre Dame: “I’ll hazard a prediction that the first extra-solar Earth-mass planet will be found by microlensing. But we’ll have to be very quick to beat the radial velocity programs and NASA’s Kepler mission, which will be launched in early 2009.”

Unfortunately, the lensing events can only happen one time. The foreground star will probably never be seen again since it was only revealed by the two stars lining up. Astronomers have to work fast to get all their data collected.

Original Source: University of Notre Dame News Release

With several space- and ground-based telescopes, as well as dedicated space missions searching for exoplanets, or planets orbiting other stars, the count of new discoveries keeps rising. The current total now stands at 287 planets. The newest spacecraft dedicated to this search, the COROT Mission (Convection, Rotation and planetary Transits), announced the finding of two new exoplanets as well as an unknown celestial object. This discovery may be a “missing link” between stars and planets astronomers have been searching for.

The two new planets are gas giants of the hot Jupiter type, which orbit very close to their parent star and tend to have extensive atmospheres because heat from the nearby star gives them energy to expand. Most of the exoplanets found so far are the gas giant variety because of the limits of current technology.

In addition, an oddity dubbed “COROT-exo-3b” has raised particular interest among astronomers. It appears to be something between a brown dwarf, a sub-stellar object without nuclear fusion at its core but with some stellar characteristics, and a planet. Its radius is too small for it to be a super-planet.

If it is a star, it would be among the smallest ever detected. Follow-up observations from the ground have determined it to be at 20 Jupiter massses. This makes it twice as dense as the metal Platinum.

COROT has also detected extremely faint signals that, if confirmed, could indicate the existence of another exoplanet, as small as 1.7 times Earth’s radius.

This is an encouraging sign in the delicate and difficult search for small, rocky exoplanets that COROT has been designed for.

COROT launched in December 2006, with operations beginning in February of 2007. So far the mission has found four exoplanets. The mission started observations of its sixth star field at the beginning of May this year. During this observation phase, which will last 5 months, the spacecraft will simultaneously observe 12,000 stars.

More about COROT.

Original News Source: ESA

If you want to know what a watery world might look like orbiting another star, just observe our own planet… from afar. The Blue Marble image of Earth, taken by the Apollo 17 astronauts on December 1972, shows how our planet is 70% water. Another world like ours, orbiting a distant star should be obvious – assuming you have a much more powerful telescope, and use the right techniques to analyze the light bouncing off the watery world.

Researchers from Penn State and the University of Hawaii have developed a technique that they think will help identify these watery worlds; potential homes for life around other stars. This technique is detailed in the most recent edition of the journal Icarus.

“We are looking for Earth-like planets in the habitable zone of their star, a band not too hot nor too cold for life to exist,” says Darren M. Williams, associate professor of physics and astronomy, Penn State Erie. “We also want to know if there is water on these planets.”

Here’s how you might tell the difference between a hellish planet like Venus, and a more comfortable watery world like Earth. A planet like Venus has a very dense atmosphere that scatters sunlight in all directions. From our vantage point, we would see the amount of light coming from the planet change depending on its position to its parent star. Just like Venus, we would see this extrasolar planet go through phases, changing in brightness in a very predictable way.

A watery world, like the Earth, would actually appear much darker when the whole disk is illuminated, since water is darker than dirt. But when the planet is in crescent, sunlight would glance off the surface of the water, and it would actually appear brighter.

The astronomers want to monitor the light curve of a distant planet as it spins on its axis and orbits its star. By watching the brightness of the light coming from the planet, they should be able to determine if it has a dense Venusian atmosphere, or is a better match for a watery world.

The equipment isn’t ready yet, but in the next 10 to 20 years, an observatory will probably be built with enough sensitivity to collect light from Earth-sized planets orbiting other stars. And this method should help determine if they’ll watery worlds, capable of supporting life.

Original Source: Penn State News Release