Does Tidal Evolution Cause Stars to Eat Planets?

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With the success of the Kepler mission, the viability of looking for planets via transits has reached maturity. However, Kepler is not the first intensive study. Previously, other observatories have employed transit searches. To increase the chances of discovery, studies often concentrated on large clusters in which thousands of stars could be observed simultaneously. Based on the percentage of stars with super Jovian planets in the Sun’s vicinity, a Hubble observation run on the globular cluster 47 Tuc expected to find roughly 17 “hot Jupiters”. Yet not a single one was found. Follow-up studies on other regions of 47 Tuc, published in 2005, also reported a similar lack of signals.

Could the subtle effect of tidal forces have caused the planets to be consumed by their parent stars?

Within our solar system, the effects of tidal influences are more subtle than planetary destruction. But on stars with massive planets in tight orbits, the effects can be very different. As a planet would orbit its parent star, its gravitational pull would pull the star’s photosphere towards it. In a frictionless environment, the raised bulge would remain directly under the planet. Since the real world has real friction, the bulge will be displaced.

If the star rotates slower than the planet orbits (a likely scenario for close in planets since stars slow themselves via magnetic breaking during formation), the bulge will trail behind the planet since the pull has to compete against the photospheric material through which its pulling. This is the same effect that happens between the Earth-Moon system and is why we don’t have tides whenever the moon is overhead, but rather, the tides occur some time later. This lagging bulge creates a component of the gravitational force opposed to the direction of motion of the planet, slowing it down. As time goes on, the planet gets dragged closer to the star by this torque which increases the gravitational force and accelerating the process until the planet eventually enters the star’s photosphere.

Since transit discoveries rely on the planets orbital plane being exactly in line with its parent star and our planet, this favors planets in a very tight orbit since planets further out are more likely to pass above or below their parent star when viewed from Earth. The result of this is that planets that could potentially be discovered by this method are especially prone to this tidal slowing and destruction. This effect with the combination of the old age of 47 Tuc, may explain the dearth of discoveries.

Using a Monte-Carlo simulation, a recent paper explores this possibility and finds that, with the tidal effects, the non-detection in 47 Tuc is completely accounted for without the need to include additional reasons (such as metal deficiency in the cluster). However, to go beyond simply explaining a null result, the team made several predictions that would serve to confirm the destruction of such planets. If a planet were wholly consumed, the heavier elements should be present in the atmospheres of their parent star and thus be detectable via their spectra in contrast with the overall chemical composition of the cluster. Planets that were tidally stripped of atmospheres by filling their Roche Lobes could still be detected as an excess of rocky, super Earths.

Another test could inolve comparison between several of the open clusters visible in the Kepler study. Should astronomers find a decrease in the probability of finding hot Jupiters corresponding with a decrease with cluster age, this would also confirm the hypothesis. Since several such clusters exist within the area planned for the Kepler survey, this option is the most readily accessible. Ultimately, this result make sit clear that, should astronomers rely on methods that are best suited for short period planets, they may need to expand their observation window sufficiently since planets with a sufficiently short period may be prone to being consumed.

Extrasolar Volcanoes May Soon be Detectable

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We’ve all seen pictures of erupting terrestrial volcanoes from space, and even eruptions on Jupiter’s moon Io in the outer solar system, but would it be possible to detect an erupting volcano on an exoplanet? Astronomers say the answer is yes! (with a few caveats)

It’s going to be decades before telescopes will be able to resolve even the crudest surface features of rocky extrasolar planets, so don’t hold your breath for stunning photos of alien volcanoes outside our solar system. But astronomers have already been able to use spectroscopy to detect the composition of exoplanet atmospheres, and a group of theorists at the Harvard-Smithsonian Center for Astrophysics think a similar technique could detect the atmospheric signature of exo-eruptions.

By collecting spectra right before and right after the planet goes behind its star, astronomers can subtract out the star’s spectrum and isolate the signal from the planet’s atmosphere. Once this is done, they can look for evidence of molecules common in volcanic eruptions. Models suggest that sulfur dioxide is the best candidate for detection because volcanoes produce it in huge quantities and it lasts in a planet’s atmosphere for a long time.

Still, it won’t be easy.

“You would need something truly earthshaking, an eruption that dumped a lot of gases into the atmosphere,” said Smithsonian astronomer Lisa Kaltenegger. “Using the James Webb Space Telescope, we could spot an eruption 10 to 100 times the size of Pinatubo for the closest stars,” she added.

To be detected, exoplanet eruptions would have to be 10 to 100 times larger than the 1991 eruption of Mt. Pinatubo shown here. Image source: USGS

In 1991 Mount Pinatubo in the Philippines belched 17 million tons of sulfur dioxide into the stratosphere. Volcanic eruptions are ranked using the Volcanic Explosivity Index (VEI). Pinatubo ranked ‘colossal’ (VEI of 6) and the largest eruption in recorded history was the ‘super-colossal’ Tambora event in 1815. With a VEI of 7 it was about 10 times as large as Pinatubo. Even larger eruptions (more than 100 times larger than Pinatubo) on Earth are not unheard of: geologic evidence suggests that there have been 47 such eruptions in the past 36 million years, including the eruption of the Yellowstone caldera about 600,000 years ago.

The best candidates for detecting extrasolar volcanoes are super-earths orbiting nearby, dim stars, but the Kaltenegger and her colleagues found that volcanic gases on any earth-like planet up to 30 light years away might be detectable. Now they just have to wait until the James Webb Space Telescope is launched 2014 to test their prediction.

The Origin of Exoplanets

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We truly live in an amazing time for exoplanet research. It was only 18 years ago the first planet outside our solar system was discovered. Fifteen since the first confirmation of one around a main sequence star. Even more recently, direct images have begun to sprout up, as well as the first spectra of the atmospheres of such planets. So much data is becoming available, astronomers have even begun to be able to make inferences as to how these extra solar planets could have formed.

In general, there are two methods by which planets can form. The first is via coaccretion in which the star and the planet would form from gravitational collapse independently of one another, but in close enough proximity that their mutual gravity binds them together in orbit. The second, the method through which our solar system formed, is the disk method. In this, material from a thin disk around a proto-star collapses to form a planet. Each of these processes has a different set of parameters that may leave traces which could allow astronomers to uncover which method is dominant. A new paper from Helmut Abt of Kitt Peak National Observatory, looks at these characteristics and determines that, from our current sampling of exoplanets, our solar system may be an oddity.

The first parameter that distinguishes the two formation methods is that of eccentricity. To establish a baseline for comparison, Abt first plotted the distribution of eccentricities for 188 main-sequence binary stars and compared that to the same type of plot for the only known system to have formed via the disk method (our Solar System). This revealed that, while the majority of stars have orbits with low eccentricity, this percentage falls off slowly as the eccentricity increases. In our solar system, in which only one planet (Mercury) has an eccentricity greater than 0.2, the distribution falls off much more steeply. When Abt constructed the distribution for the 379 planets with known eccentricity, it was nearly identical to that for binary stars.

A similar plot was created for the semi major axis of binary stars and our solar system. Again, when this was plotted for the known extra solar planets the distribution was similar to that of binary star systems.

Abt also inspected the configuration of the systems. Star systems containing three stars generally contained a pair of stars in a tight binary orbit with a third in a much larger orbit. By comparing the ratios of such orbits, Abt quantified the orbital spacing. However, instead of simply comparing to the solar system, he considered the analogous situation of formation of stars around the central mass of the galaxy and built a similar distribution in this manner. In this case, the results were ambiguous; Both modes of formation produced similar results.

Lastly, Abt considered the amount of heavy elements in the more massive body. It is widely known that most extra-solar planets are found around metal-rich stars. While there’s no reason planets forming in a disk couldn’t be formed around high mass stars, having a metal-rich cloud from which to form stars and planets is a requirement for the coaccretion model because it tends to accelerate the collapse process, allowing giant planets to fully form before the cloud was dissipated as the star became active. Thus, the fact that the vast majority of extra-solar planets exist around metal-rich stars favors the coaccretion hypothesis.

Taken together, this provides four tests for formation models. In every case, current observations suggest that the majority of planets discovered thus far formed from coaccretion and not in a disc. However, Abt notes that this is most likely due to statistical biases imposed by the sensitivity limits of current instruments. As he notes, astronomers “do not yet have the radial velocity sensitivity to detect disk systems like the solar system, except for single large planets, like Jupiter at 5 AU.” As such, this view will likely change as new generations of instruments become available. Indeed, as instruments improve to the point that three dimensional mapping becomes available, and orbital inclinations can be directly observed, astronomers will be able to add another test to determine the modes of formation.

EDIT: Following some confusion and discussion in the comments, I wanted to add one further note. Keep in mind this is only the average of all systems currently known that looks like coaccreted systems. While there are undoubtedly some in there that did form from disks, their rarity in the current data makes them not stand out. Certainly, we know of at least one system that fits a strong test for the disk method. This recent discovery by Kepler, in which three planets have been observed transiting their host star demonstrates that all of these planets must lie in a disk which does not conform to expectations of independent condensation. As more systems like this are discovered, we expect that the distributions of the tests described above will become bimodal, having components that match each formation hypothesis.

Did Kepler Scientist Leak Data? Um, Not Really

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Mainstream media (MSM) is funny. Well, maybe funny isn’t the right word, especially when they hose things up and create a story when there really isn’t one. Or when they miss the real story. MSM recently succeeded in spades on both accounts in regards to the Kepler mission. Just last month, the Kepler team announced they had found over 750 candidates for extrasolar planets, and 706 of these candidates potentially are planets from as small as Earth to around the size of Jupiter, with the majority having radii less than half that of Jupiter. This is such incredible news, especially when you factor in that the data was from just 43 days of observations! But MSM seemed to miss all this and instead focused on the fact that the Kepler team got approval from NASA to keep over half of their data for an additional six months to verify and confirm their findings, rather than releasing all of it, as per NASA’s standard policy which requires astronomers to release their data from publicly funded instruments in one year. Then over this past weekend, from a TED talk by Kepler co-investigator Dimitar Sasselov, MSM finally realized that Kepler has found a boat-load of potential Earth-sized exoplanets. Well, yes. That’s what they said in June.

But then MSM took things out of context and exaggerated just a tad.

Even though in his talk, Sasselov used the words “potential” and “candidates” and said the planets are “like Earth, that is, having a radius smaller than twice Earth’s radius,” MSM reported news that NASA has found rocky planets with land and water.

And now some people are saying that Sasselov “leaked” the proprietary Kepler data, and some say he is in trouble for doing so. Today, the Kepler team said via Twitter that they are “working hard to thoughtfully respond to the media flurry surrounding the TEDGlobal talk.”

Let me use one of my mother’s favorite admonitions: For Pete’s sakes!

Watch the TED talk. In my opinion, Sasselov does a good job of getting people excited about exoplanets and he doesn’t say we have actually found another Earth. He also does a good job of presenting what the Kepler team has found without revealing any really huge proprietary data, even though he used this graph:

Screenshot from Sasselov's TED talk.

But really, this is pretty much what the Kepler team said in June, that they expected half of the 750 planet candidates would turn out not to be planets, and a fair number of those might be Earth-sized. The graph takes into account the amount of potential planets that Kepler found, plus the planets found previously by other telescopes and missions.

While it is exciting to think about the potential of finding Earth-sized and maybe even Earth-like planets, we’re likely a long way off from actually finding and then actually confirming another Earth. Additionally, right now, we’re only capable of finding planets that orbit relatively close to their parent star, which most likely wouldn’t put them in the “Goldilocks Zone” of being habitable.

You can read our original article from June here, where the Kepler team announced their findings. There’s also an explanation there of why the team requested to keep part of their data for an extra six months.

UPDATE: 10 pm Tuesday: Sasselov has written an blog post at the Kepler website, bascially saying that there is a big difference between Earth-sized and Earth-like. You can read it here.

Dropping a Bomb About Exoplanets

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Not all exoplanets are created equal, and new discoveries about the orbits of newly found extra solar planets could challenge the current theories of planet formation. The discoveries also suggest that systems with exoplanets of the type known as hot Jupiters are unlikely to contain Earth-like planets. “This is a real bomb we are dropping into the field of exoplanets,” said Amaury Triaud, a PhD student at the Geneva Observatory who led an observational campaign from several observatories.

Six exoplanets out of twenty-seven were found to be orbiting in the opposite direction to the rotation of their host star — the exact reverse of what is seen in our own Solar System. The team announced the discovery of nine new planets orbiting other stars, and combined their results with earlier observations. Besides the surprising abundance of retrograde orbits, the astronomers also found that more than half of all the so-called “hot Jupiters” in their survey have orbits that are misaligned with the rotation axis of their parent stars.

Hot Jupiters are planets orbiting other stars that have masses similar to or greater than Jupiter, but which orbit their parent stars much more closely.

Planets are thought to form in the disc of gas and dust encircling a young star, and since this proto-planetary disc rotates in the same direction as the star itself, it was expected that planets that form from the disc would all orbit in more or less the same plane, and that they would move along their orbits in the same direction as the star’s rotation.

“The new results really challenge the conventional wisdom that planets should always orbit in the same direction as their stars spin,” said Andrew Cameron of the University of St Andrews, who presented the new results at the RAS National Astronomy Meeting (NAM2010) in Glasgow, Scotland this week.

Artist’s impression of an exoplanet in a retrograde orbit. Credit: ESO

At this writing, 454 planets have been found orbiting other stars, and in the 15 years since the first hot Jupiters were discovered, astronomers have been puzzled by their origin. The cores of giant planets are thought to form from a mix of rock and ice particles found only in the cold outer reaches of planetary systems. Hot Jupiters must therefore form far from their star and subsequently migrate inwards to orbits much closer to the parent star. Many astronomers believed this was due to gravitational interactions with the disc of dust from which they formed. This scenario takes place over a few million years and results in an orbit aligned with the rotation axis of the parent star. It would also allow Earth-like rocky planets to form subsequently, but unfortunately it cannot account for the new observations.

To account for the new retrograde exoplanets an alternative migration theory suggests that the proximity of hot Jupiters to their stars is not due to interactions with the dust disc at all, but to a slower evolution process involving a gravitational tug-of-war with more distant planetary or stellar companions over hundreds of millions of years. After these disturbances have bounced a giant exoplanet into a tilted and elongated orbit it would suffer tidal friction, losing energy every time it swung close to the star. It would eventually become parked in a near circular, but randomly tilted, orbit close to the star. “A dramatic side-effect of this process is that it would wipe out any other smaller Earth-like planet in these systems,” says Didier Queloz of Geneva Observatory.

The observatories for this survey included the Wide Angle Search for Planets (WASP), the HARPS spectrograph on the 3.6-metre ESO telescope at the La Silla observatory in Chile, and the Swiss Euler telescope, also at La Silla. Data from other telescopes to confirm the discoveries.

Source: ESO

Planet of Lava a Former Gas Giant

Matryoshka dolls are a popular novelty for tourists going to Russia to bring home for their children. These dolls, which are hollow wooden bowling pin-shaped representations of a Russian woman (or babushka), are nested inside of each other, each doll smaller than the one that encases it.

In a perfect model of planetary matryoshka dolls, the exoplanet Corot 7-b – which is currently one of the exoplanets that is closest in size and mass to the Earth – used to be nestled inside a much larger version of itself. Corot 7-b was formerly a gas giant with a mass of 100 Earths, which is about that of Saturn. Its mass now: 4.8 times that of our planet.

How this rocky, lava-covered world got to its current state was presented at the American Astronomical Society’s meeting last week in Washington, DC by Brian Jackson of NASA’s Goddard Space Flight Center. Corot 7-b was discovered in February of 2009 by the ESA’s planet-hunting satellite, Convection, Rotation and planetary Transits(CoRoT), and has since been the subject of intense study.

The planet is about 1.7 larger in diameter than the Earth, and a little shy of five times as massive. Its star is about 1.5 billion years old, a third that of our Sun. It orbits very close to its star, which is much like our own Sun, only taking 20.4 hours to circle the star. The system lies in the constellation Monoceros, and is about 480 light-years away.

This tight orbit makes the planet extremely hot, as in 3,600 degrees Fahrenheit (1,982 degrees Celsius). That’s hot enough that the crust of the planet facing the star is an ocean of lava. Since Corot 7-b is tidally locked to its star, only one side of the planet faces the star at all times (just like we only see one side of the Moon from the Earth). On the opposite side of Corot 7-b from its star, the surface temperature is estimated to be a chilly negative 350 degrees F (negative 210 degrees C).

It rains on Corot 7-b just like it does here, though you wouldn’t want to be caught out in it. The rain on Corot 7-b is made of rock, so even the heaviest umbrella wouldn’t do much for you, and the very thin atmosphere is composed of rock vapor. In other words, we aren’t looking to Corot 7-b for signs of life. What we are looking there for is signs of planetary formation and evolution.

Jackson et al. modeled the orbit of the planet backwards, and showed that the star blew off much of the material that made up the planet in its previous incarnation as a gas giant. It previously orbited about 50 percent further out than it currently is. The stellar wind – a constant flow of charge particles from the star – interacted with the gassy atmosphere of the planet, blowing away the atmosphere.

“There’s a complex interplay between the mass the planet loses and its gravitational pull, which raises tides on the star,” Jackson said.

As it was pulled in closer to the star due to the process of tidal migration, more and more of the gas evaporated, and the orbital change of the planet slowed to the distance at which it currently orbits. Once the planet got closer to the star, it also heated up, and this heating process contributed to the mass loss of Corot 7-b. This evaporative process left only the rocky core of the planet.

“CoRoT-7b may be the first in a new class of planet — evaporated remnant cores. Studying the coupled processes of mass loss and migration may be crucial to unraveling the origins of the hundreds of hot, earthlike planets space missions like CoRoT and NASA’s Kepler will soon uncover,” Jackson said.

Many of the extrasolar planets discovered early on were gas giants that orbited close to their stars, so-called “hot Jupiters”. It’s possible that many of them will experience the same or similar fate as Corot 7-b, as we wrote about in an article last April.

Corot 7-b will likely lose more mass because of the proximity to its star, though not at the rate seen previously. What the next planetary matryoshka of Corot 7-b will look like is anyone’s guess. My prediction: turtles all the way down.

Source: NASA press release

New Observations of TrES-2b May Reveal New Exoplanet

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For those know their solar system history, the discovery of Neptune is an especially exciting story. Before it was detected observationally, its gravitational effects on another planet (Uranus) were discovered. From this, astronomers were able to predict the position of the yet unobserved planet and in 1846 they discovered the predicted planet observationally from Berlin Observatory. (For a more complete retelling of the story, see my summary/review of the book The Neptune File). This discovery prompted searches for other planets from orbital discrepancies attributed to gravitational perturbations on Mercury. However, none were ever found and it was eventually that Mercury’s orbital irregularities were due to relativistic effects.

However, this technique of inferring planets from orbital oddities of a planet may have been used for the first time outside our solar system.

The exoplanet known as TrES-2b is one of the exceptional cases of known exoplanets for which the plane of the orbit lies almost directly in our line of sight. This circumstance means that the planet will appear to cross the disk of the star as it orbits. Although we cannot resolve that disk, it shows up as a characteristic dip in the brightness which can reveal additional information about the system such as “very accurate determinations of the radii of star and planet (relative to the semi-major axis) and the inclination of the orbital plane of the planet”. This additional information allows for excellent determinations of the orbital parameters in order to predict future transits.

A team of German astronomers observed the TrES-2 system in 2006 and 2008 in order to build their understanding of the orbit of the planet. However, when they continued in observation in 2009 they found significant changes in the inclination of the orbit and the period of the orbit. Although planetary migration could change these parameters, it is not expected that such an event could occur on such a short time scale. Additionally, a oddly shaped host star would explain the change, but the degree to which the star would have to be squished at the equator would be impossibly high given the slow rotation rate known for TrES-2.

Instead, the authors suggest “the existence of a third body in the form of an additional planet would provide a very natural explanation”. Although this explanation is anything but conclusive, it does pose an easily testable scenario. If the plane of the orbit of the system is very nearly along the line of sight, this provides the most ideal situation for attempting to detect planets using the radial-velocity of the parent star. The authors even go so far as to suggest a range of periods for a potential planet to have the observed effects. They state, “a planet of one Jovian mass with periods between 50 – 100 days would suffice to cause the observed inclination changes”.

Furthermore, the authors note that several similar systems are known to exist with a close in planet and a second massive planet in a longer orbit. “[I]n the system HIP 14810 there is a close-in planet with a 6.6 day period and a somewhat lighter planet with a period of 147 days, in the HD 160691 system the close-in planet has a period of 9.6 days and two outer planets with Jupiter masses are known with periods of 310 and 643 days.”

Astronomers Find Super-Earth With An Atmosphere

This artist’s conception shows the newly discovered super-Earth GJ 1214b, which orbits a red dwarf star 40 light-years from our Earth. Credit: Credit: David A. Aguilar, CfA

More exoplanets this week! Today astronomers announced the discovery of so called super-Earth around a nearby, low-mass star, GJ1214. The newly discovered planet has a mass about six times that of Earth and 2.7 times its radius, falling in between the size of Earth and the ice giants of the Solar System, Uranus and Neptune. But this latest exoplanet, GJ1214b, has something else, too: an atmosphere about 200 km thick. “This atmosphere is much thicker than that of the Earth, so the high pressure and absence of light would rule out life as we know it,” said David Charbonneau, lead author of a paper in Nature reporting the discovery, “but these conditions are still very interesting, as they could allow for some complex chemistry to take place.”

GJ1214b is also a very hot place to be. It orbits its star once every 38 hours at a distance of only two million kilometres — 70 times closer to its star than the Earth is to the Sun. “Being so close to its host star, the planet must have a surface temperature of about 200 degrees Celsius, too hot for water to be liquid,” said Charbonneau.

However, another member of the team said water ice could possibly be present on GJ1214b, deep inside the heart of the planet. “Despite its hot temperature, this appears to be a waterworld,” said graduate Zachory Berta who first spotted the hint of the planet among the data. “It is much smaller, cooler, and more Earth-like than any other known exoplanet.”

The star is a small, red type M star about one-fifth the size of our Sun. It has a surface temperature of only about 2,700 C (4,900 degrees F) and a luminosity only three-thousandths as bright as the Sun.

Artist impression of how the newly discovered super-Earth surrounding the nearby star GJ1214 may look.  Credit: ESO/L. Calçada
Artist impression of how the newly discovered super-Earth surrounding the nearby star GJ1214 may look. Credit: ESO/L. Calçada

Charbonneau compared this new exoplanet to Corot-7b, the first rocky super-Earth found using the transit method, when the planet’s orbit is takes it across the face of its parent star, from our vantage point. .
The astronomers were also able to obtain the mass and radius of GJ1214b, allowing them to determine the density and to infer the inner structure.

Although the mass of GJ1214b is similar to that of Corot-7b, its radius is much larger, suggesting that the composition of the two planets must be quite different. While Corot-7b probably has a rocky core and may be covered with lava, astronomers believe that three quarters of GJ1214b is composed of water ice, the rest being made of silicon and iron.

“The differences in composition between these two planets are relevant to the quest for habitable worlds,” said Charbonneau. “If super-Earth planets in general are surrounded by an atmosphere similar to that of GJ1214b, they may well be inhospitable to the development of life as we know it on our own planet.”

The atmosphere was detected when the astronomers compared the measured radius of GJ1214b with theoretical models of planets. They found that the observed radius exceeds the models’ predictions, and deduced that a thick atmosphere was blocking the star’s light.

“Because the planet is too hot to have kept an atmosphere for long, GJ1214b represents the first opportunity to study a newly formed atmosphere enshrouding a world orbiting another star,” said Xavier Bonfils, another member of the team. “Because the planet is so close to us, it will be possible to study its atmosphere even with current facilities.”
The MEarth (pronounced "mirth") Project is an array of eight identical 16-inch-diameter RC Optical Systems telescopes that monitor a pre-selected list of 2,000 red dwarf stars. Each telescope perches on a highly accurate Software Bisque Paramount and funnels light to an Apogee U42 charge-coupled device (CCD) chip, which many amateurs also use. Credit: Dan Brocious, CfA
The planet was first discovered as a transiting object within the MEarth project, which follows about 2000 low-mass stars to look for transits by exoplanets, and uses a fleet of eight small, (16-inch) amateur-sized ground-based telescopes.

To confirm the planetary nature of GJ1214b and to obtain its mass (using the so-called Doppler method), the astronomers needed the full precision of the HARPS spectrograph, attached to ESO’s 3.6-metre telescope at La Silla.

The next step for astronomers is to try to directly detect and characterize the atmosphere, which will require a space-based instrument like NASA’s Hubble Space Telescope. GJ1214b is only 40 light-years from Earth, within the reach of current observatories.

Source: ESO, CFA

Forming Planets Around Binary Stars

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Fanciful science fiction and space art frequently depict the lovely visage of a twin sunset where a pair of binary stars dips below the horizon (think Star Wars). While it has been established that planets could exist in such a system by orbiting in resonances, that only holds true for fully formed planets. Can forming star systems even support an accretion disk from which to form planets? This is the question a new paper by M. G. Petr-Gotzens and S. Daemgen of the European Southern Observatory with S Correia from the Astronomiches Institut Potsdam attempts to answer.

Observations of single young stars with disks have revealed that the main force causing the dispersion of the disk is the star itself. The stellar wind and radiation pressure blow the disk away within 6 to 10 million years. Predictably, more massive and hotter stars will disperse their disks more quickly. However, “many stars are members of a binary or multiple system, and for nearby solar-like stars the binary fraction is even as high as ~60%.” Could gravitational perturbations or the added intensity from two stars strip disks before planets could form?

To explore this, the team observed 22 young and forming binary star systems in the Orion Nebula to look for signs of disks. They used two primary methods: The first was to look for excess emission in the near infrared. This would trace accretion disks as they radiate away absorbed energy as heat. The second was to look spectroscopically for specific bromine emission that is excited as the magnetic field of the young star pulls this (and other) elements from the disk onto the stars surface.

When the results were analyzed they found that as much as 80% of the binary systems had an active accretion disk. Many only contained a disk around the primary star although nearly as many contained disks around both stars. Only one system had evidence of an accretion disk around only the secondary (lower mass) star. They authors note, “[t]he under-representation of active accretion
disks among secondaries hints at disk dissipation working faster on (potentially) lower mass secondaries, leading us to speculate that secondaries are possibly less likely to form planets.”

However, the average age of the stars observed was only ~1 million years. This means that, even though disks may be able to form, the study was not comprehensive enough to determine whether or not they would last. Yet a survey of the currently known extra-solar planets reveal that they must. The authors comment, “[a]lmost 40 of all the extra-solar planets discovered to date reside in wide binary systems where the component separation is larger than 100AU (large enough that planet formation around one star should not strongly be inuenced [sic] by the companion star).”

Strangely, this seems to stand at odds with a 2007 paper by Trilling et al. which studied other binary systems for the same IR excess indicative of debris disks. In their study, they determined “[a] very large fraction (nearly 60%) of observed binary systems
with small (<3 AU) separations have excess thermal emission.” This suggests that such close systems may indeed be able to retain disks for some time. It is unclear on whether or not it can be retained long enough to form planets although it seems unlikely since no exoplanets are known around close binaries.

Second Exoplanet with Retrograde Orbit Discovered

The exoplanet HAT-P-7b has been observed to have a very curious orbit. It either has a highly tilted orbit – passing almost over the poles of its parent star, HAT-P-7 –  or a retrograde orbit; that is, orbiting in the opposite direction of its parent star. Two teams of researchers, both using the Subaru Telescope in Japan, have published papers on the bizarre properties of this planet, the second exoplanet ever observed to have a retrograde orbit.

In our Solar System, the planets calmly rotate in the same direction as that of their parent star, in our case the Sun. This is called a prograde orbit, and the Earth has the most inclined orbit with regard to the equator of the Sun, of 7.15 degrees. The planet HAT-P-7b, however, has an orbit that is the opposite of the rotation of its parent star but in the same plane as the equator (effectively a 180 degree incline). This is called a retrograde orbit. It may also be the case that it is inclined to at least 86 degrees of the equator of its Sun, so as to have almost a polar orbit. The researchers have yet to determine the true rotation of the star HAT-P-7, and thus which scenario is true for the exoplanet.

“There is a large range of uncertainty because we have not measured the true angle between the orbit and the stellar equator. Instead we can only measure the angle that we see from our perspective on Earth,” said Winn in a MIT press release.

HAT-P-7b is about 1.4 times as wide and 1.8 times as massive as Jupiter, and lies approximately 1,000 light years from the Earth.

A Japanese collaboration led by Norio Narita of the National Astronomical Observatory of Japan, and a team led by MIT assistant professor of physics Joshua Winn both published papers detailing their studies of HAT-P-7b. These studies were published in the Publications of Astronomical Society of Japan Letters October 25, 2009 and the Astrophysical Journal Letters for October 1, 2009, respectively. The paper by the Japanese team is available for your perusal on Arxiv here.

Both research teams used the Subaru Telescope’s High Dispersion Spectrograph instrument to observe the star HAT-P-7. The spectrograph allowed the researchers to monitor the redshift or blueshift of light as the planet orbited the star. In planets with a prograde orbit, their transit in front of the star blocks the blue shifting of the light from the star first, then blocks the redshift of the light, making the star appear to move more that it actually is.

In the case of HAT-P-7b the effect was reversed – that is, the redshifted light appeared bluer, then the blueshifted light appeared redder, making it apparent that the orbit of the planet was not in the same direction of that of HAT-P-7. This effect is called the Rossiter-McLaughlin effect, illustrated below.

The Rossiter McLaughlin effect makes a star appear to have a greater radial velocity than it actually does because of a transiting planet. Image Credit: Nicholas Shanks, WikiMedia Commons
The Rossiter McLaughlin effect makes a star appear to have a greater radial velocity than it actually does because of a transiting planet. Image Credit: Nicholas Shanks, WikiMedia Commons

The odd orbit of HAT-P-7b could have been caused by a number of different factors, and theorists that model the formation of exoplanetary systems will not have to “go back to the drawing boards”. The general consensus is that planets form out of a large disk of material orbiting the star, and thus all orbit in the same direction as the disk out of which they formed.

Multiple planets could have formed in an unstable configuration around the star, and their proximity to each other could have caused a rather chaotic series of gravitational billiards to boot HAT-P-7b into its current orbit. Another explanation is the presence of a third object in the system, such as another massive planet or companion star, that is tilting the orbit of HAT-P-7b due to what’s known as the Kozai effect.

The announcement of the retrograde orbit of HAT-P-7b came only one day after the announcement on August 12th, 2009 that the planet WASP-17b orbits opposite its parent star. HAT-P-7b is also one of the first exoplanets to be studied by the Kepler mission, which studied the planet’s orbit over 10 days. Kepler will take further images of the star during its mission, and by observing the rotation of spots on the surface of the star, nail down the orbital direction, after which we’ll know whether HAT-P-7b is orbiting “backwards” or around the poles of the star.

Source: Subaru Telescope, MIT