Rogue Planets Can Find Homes Around Other Stars

In this artist's conception, a rogue planet drifts through space. Credit: Christine Pulliam (CfA)

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As crazy as it sounds, free-floating rogue planets have been predicted to exist for quite some time and just last year, in May 2011, several orphan worlds were finally detected. Then, earlier this year, astronomers estimated that there could be 100,000 times more rogue planets in the Milky Way than stars. Now, the latest research suggests that sometimes, these rogue, nomadic worlds can find a new home by being captured into orbit around other stars. Scientists say this finding could explain the existence of some planets that orbit surprisingly far from their stars, and even the existence of a double-planet system.

“Stars trade planets just like baseball teams trade players,” said Hagai Perets of the Harvard-Smithsonian Center for Astrophysics.

Astronomers now understand that rogue planets are a natural consequence of both star and planetary formation. Newborn star systems often contain multiple planets, and if two planets interact, one can be ejected in a form of planetary billiards, kicked out of the star system to become an interstellar traveler.

But later, if a rogue planet encounters a different star moving in the same direction at the same speed, be captured into orbit around that star, say Perets and Thijs Kouwenhoven of Peking University, China, the authors of a new paper in The Astrophysical Journal.

A captured planet tends to end up hundreds or thousands of times farther from its star than Earth is from the Sun. It’s also likely to have a, orbit that’s tilted relative to any native planets, and may even revolve around its star backward.

Perets and Kouwenhoven simulated young star clusters containing free-floating planets. They found that if the number of rogue planets equaled the number of stars, then 3 to 6 percent of the stars would grab a planet over time. The more massive a star, the more likely it is to snag a planet drifting by.

While there haven’t actually been planets found yet that are definitely a ‘captured’ world, the best bet would perhaps be a planet in a distant orbit around a low-mass star. The star’s disk wouldn’t contain enough material to form a planet that distant, Perets and Kouwenhoven said.

The best evidence of a captured planet comes from the European Southern Observatory, which announced in 2006 the discovery of two planets (weighing 14 and 7 times Jupiter) orbiting each other without a star.

“The rogue double-planet system is the closest thing we have to a ‘smoking gun’ right now,” said Perets. “To get more proof, we’ll have to build up statistics by studying a lot of planetary systems.”

As for our own solar system, there’s no evidence at this time that our Sun could have captured an alien world, which would lie far beyond Pluto.

“There’s no evidence that the Sun captured a planet,” said Perets. “We can rule out large planets. But there’s a non-zero chance that a small world might lurk on the fringes of our solar system.”

Read the team’s paper.

Source: CfA

‘Nomad’ Planets Could Outnumber Stars 100,000 to 1

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Could the number of wandering planets in our galaxy – planets not orbiting a sun — be more than the amount of stars in the Milky Way? Free-floating planets have been predicted to exist for quite some time and just last year, in May 2011, several orphan worlds were finally detected. But now, the latest research concludes there could be 100,000 times more free-floating planets in the Milky Way than stars. Even though the author of the study, Louis Strigari from the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), called the amount “an astronomical number,” he said the math is sound.

“Even though this is a large number, it is actually consistent with the amount of mass and heavy elements in our galaxy,” Strigari told Universe Today. “So even though it sounds like a big number, it puts into perspective that there could be a lot more planets and other ‘junk’ out in our galaxy than we know of at this stage.”

And by the way, these latest findings certainly do not lend any credence to the theory of a wandering planet named Nibiru.

Several studies have suggested that our galaxy could perhaps be swarming with billions of these wandering “nomad” planets, and the research that actually found a dozen or so of these objects in 2011 used microlensing to identify Jupiter-sized orphan worlds between 10,000 and 20,000 light-years away. That research concluded that based on the number of planets identified and the area studied, they estimated that there could literally be hundreds of billions of these lone planets roaming our galaxy….literally twice as many planets as there are stars.

But the new study from Kavli estimates that lost, homeless worlds may be up to 50,000 times more common than that.

Using mathematical extrapolations and relying on theoretical variables, Strigari and his team took into account the known gravitational pull of the Milky Way galaxy, the amount of matter available to make such objects and how that matter might be distributed into objects ranging from the size of Pluto to larger than Jupiter.

“What we did was we put together the observations of what the galaxy is made of, what kind of elements it has, as well as how much mass there could possibly be that has been deduced from the gravitational pull from the stars we observed,” Stigari said via phone. “There are a couple of general bounds we used: you can’t have more nomads in the galaxy than the matter we observe, as well as you probably can’t have more than the amount of so called heavy elements than we observe in the galaxy (anything greater that helium on the periodic table).”

But any study of this type is limited by the lack of understanding of planetary formation.

“We don’t at this stage have a good theory that tells us how planets form,” Strigari said, “so it is difficult to predict from a straight theoretical model how many of these objects might be wandering around the galaxy.”

Strigari said their approach was largely empirical. “We asked how many could there possibly be, consistent with the broad constraints, that gives us a limit to how many these objects could possibly exist.”

So, in absence of any theory that really predicts how many of these things should exist, the estimate of 100,000 times the amount of stars in the Milky Way is an upper limit.

“A lot of times in science and astronomy, in order to learn what the galaxy and universe is made of, we first have to ask questions, what is it not made of, and so you start from an upper bound of how many of these planets there could be,”Strigari said. “Maybe when our data gets better we will start reducing this limit and then we can start learning from empirical observations and start having more constrained observations that go into your theoretical models.”

In other words, Strigari said, it doesn’t mean this is the final answer, but this is the state of our knowledge right now. “It kind of quantifies our ignorance, you could say,” he said.

A good count, especially of the smaller objects, will have to wait for the next generation of big survey telescopes, especially the space-based Wide-Field Infrared Survey Telescope and the ground-based Large Synoptic Survey Telescope, both set to begin operation in the early 2020s.

So, where did all these potential free range planets come from? One option is that they formed like stars, directly from the collapse of interstellar gas clouds. According to Strigari some were probably ejected from solar systems. Some research has indicated that ejected planets could be rather common, as planets tend to migrate over time towards the star, and as they plow through the material left over from the solar system’s formation, any other planet between them and their star will be affected. Phil Plait explained it as, “some will shift orbit, dropping toward the star themselves, others will get flung into wide orbits, and others still will be tossed out of the system entirely.”

Don’t worry – our own solar system is stable now, but it could have happened in the past, and some research has suggested we originally started out with more planets in our solar system, but some may have been ejected.

Of course, when discussing planets, the first thing to pop into many people’s minds is if a wandering planet could be habitable.

“If any of these nomad planets are big enough to have a thick atmosphere, they could have trapped enough heat for bacterial life to exist,” Strigari said. Although nomad planets don’t bask in the warmth of a star, they may generate heat through internal radioactive decay and tectonic activity.

As far as a Nibiru-type wandering world in our solar system right now the answer is no. There is no evidence or scientific basis whatsoever for such a planet. If it was out there and heading towards Earth for a December 21, 2012 meetup, we would have seen it or its effects by now.

Sources: Stanford University, conversation with Louis Strigari

Scientists Find New Clues About the Interiors of ‘Super-Earth’ Exoplanets

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As we learned in science class in school, the Earth has a molten interior (the outer core) deep beneath its mantle and crust. The temperatures and pressures are increasingly extreme, the farther down you go. The liquid magmas can “melt” into different types, a process referred to as pressure-induced liquid-liquid phase separation. Graphite can turn into diamond under similar extreme pressures. Now, new research is showing that a similar process could take place inside “Super-Earth” exoplanets, rocky worlds larger than Earth, where a molten magnesium silicate interior would likely be transformed into a denser state as well.

Simply put, the magnesium silicate undergoes what’s called a phase change while in the liquid state. The scientists were able to replicate the extreme temperatures and pressures that would be found inside those exoplanets by using the Janus laser at the Lawrence Livermore National Laboratory and OMEGA at the University of Rochester. A powerful laser pulse generated a shock wave as it passed through the samples. Changes in the velocity of the shock and the temperature of the sample indicated when a phase change was detected.

Interestingly, the different liquid states of the silicate magma in the experiments showed different physical properties under high pressures and temperatures, even though they were still of the same composition. Due to varying densities, the different liquid states tended to want to separate, much like oil and water.

The findings should help to better understand the interiors of terrestrial-type exoplanets, whether they are “Super-Earths” or smaller, like Earth or Mars.

Lead scientist Dylan Spaulding, at the University of California, Berkeley, states: “Phase changes between different types of melts have not been taken into account in planetary evolution models. But they could have played an important role during Earth’s formation and may indicate that extra-solar ‘Super-Earth’ planets are structured differently from Earth.”

The paper was published in the February 10, 2012 edition of the journal Physical Review Letters.

Carbon “Super Earths” – Diamond Planets

[/caption]During a laboratory experiment at Ohio State University, researchers were simulating the pressures and conditions necessary to form diamonds in the Earth’s mantle when they came across a surprise… A carbon “Super Earth” could exist. While endeavoring to understand how carbon might behave in other solar systems, they wondered if planets high in this element could be pressurized to the point of producing this valuable gemstone. Their findings point to the possibility that the Milky Way could indeed be home to stars where planets might consist of up to 50% diamond.

The research team is headed by Wendy Panero, associate professor in the School of Earth Sciences at Ohio State, and doctoral student Cayman Unterborn. As part of their investigation they incorporated their findings from earlier experiments into a computer modeling simulation. This was then used to create scenarios where planets existed with a higher carbon content than Earth..

The result: “It’s possible for planets that are as big as fifteen times the mass of the Earth to be half made of diamond,” Unterborn said. He presented the study Tuesday at the American Geophysical Union meeting in San Francisco.

“Our results are striking, in that they suggest carbon-rich planets can form with a core and a mantle, just as Earth did,” Panero added. “However, the cores would likely be very carbon-rich – much like steel – and the mantle would also be dominated by carbon, much in the form of diamond.”

At the center of our planet is an assumed molten iron core, overlaid with a mantle of silica-based minerals. This basic building block of Earth is what condensed from the materials in our solar cloud. In an alternate situation, a planet could form in a carbon-rich environment, thereby having a different planet structure – and a different potential for life. (Fortunately for us, our molten interior provides geothermal energy!) On a diamond planet, the heat would dissipate quickly – leading to a frozen core. On this basis, a diamond planet would have no geothermal resources, lack plate tectonics and wouldn’t be able to support either an atmosphere or a magnetic field.

“We think a diamond planet must be a very cold, dark place,” Panero said.

How did they come up with their findings? Panero and former graduate student Jason Kabbes took a miniature sample of iron, carbon, and oxygen and subjected it to pressures of 65 gigapascals and temperatures of 2,400 Kelvin (close to 9.5 million pounds per square inch and 3,800 degrees Fahrenheit – conditions similar to the Earth’s deep interior). As they observed the experiment microscopically, they saw oxygen bonding with iron to create rust… but what was left turned to pure carbon and eventually formed diamond. This led them to wonder about planetary formation implications.

“To date, more than five hundred planets have been discovered outside of our solar system, yet we know very little about their internal compositions,” said Unterborn, who is an astronomer by training.

“We’re looking at how volatile elements like hydrogen and carbon interact inside the Earth, because when they bond with oxygen, you get atmospheres, you get oceans – you get life,” Panero said. “The ultimate goal is to compile a suite of conditions that are necessary for an ocean to form on a planet.”

But don’t confuse their findings with recent, unrelated studies which involves the remnants of an expired star from a binary system. The OSU team’s finding simply suggest this type of planet could form in our galaxy, but how many or where they might be is still very open to interpretation. It’s a question that’s being investigated by Unterborn and Ohio State astronomer Jennifer Johnson.

Because diamonds are forever…

Original Story Source: Ohio State Research News.

Asteroid Lutetia… A Piece Of Earth?

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According to data received from ESA’s Rosetta spacecraft, ESO’s New Technology Telescope, and NASA telescopes, strange asteroid Lutetia could be a real piece of the rock… the original material that formed the Earth, Venus and Mercury! By examining precious meteors which may have formed at the time of the inner Solar System, scientists have found matching properties which indicate a relationship. Independent Lutetia must have just moved its way out to join in the main asteroid belt…

A team of astronomers from French and North American universities have been hard at work studying asteroid Lutetia spectroscopically. Data sets from the OSIRIS camera on ESA’s Rosetta spacecraft, ESO’s New Technology Telescope (NTT) at the La Silla Observatory in Chile, and NASA’s Infrared Telescope Facility in Hawaii and Spitzer Space Telescope have been combined to give us a multi-wavelength look at this very different space rock. What they found was a very specific type of meteorite called an enstatite chondrite displayed similar content which matched Lutetia… and what is theorized as the material which dates back to the early Solar System. Chances are very good that enstatite chondrites are the same “stuff” which formed the rocky planets – Earth, Mars and Venus.

“But how did Lutetia escape from the inner Solar System and reach the main asteroid belt?” asks Pierre Vernazza (ESO), the lead author of the paper.

It’s a very good question considering that an estimated less than 2% of the material which formed in the same region of Earth migrated to the main asteroid belt. Within a few million years of formation, this type of “debris” had either been incorporated into the gelling planets or else larger pieces had escaped to a safer, more distant orbit from the Sun. At about 100 kilometers across, Lutetia may have been gravitationally influenced by a close pass to the rocky planets and then further affected by a young Jupiter.

“We think that such an ejection must have happened to Lutetia. It ended up as an interloper in the main asteroid belt and it has been preserved there for four billion years,” continues Pierre Vernazza.

Asteroid Lutetia is a “real looker” and has long been a source of speculation due to its unusual color and surface properties. Only 1% of the asteroids located in the main belt share its rare characteristics.

“Lutetia seems to be the largest, and one of the very few, remnants of such material in the main asteroid belt. For this reason, asteroids like Lutetia represent ideal targets for future sample return missions. We could then study in detail the origin of the rocky planets, including our Earth,” concludes Pierre Vernazza.

Original Story Source: ESO News Release.

Planetary Pinball – Uranus Gets The “Tilt”

Between 3 to 4 billion years ago, a body twice the size of Earth impacted Uranus, knocking the ice giant onto its side. Image Credit: Jacob A. Kegerreis/Durham University

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Popular theory on how Uranus ended up with a highly eccentric axis has always been pretty standard – one giant blow. However, at today’s (October 6) EPSC-DPS Joint Meeting in Nantes, astronomers are thinking things may have occurred slightly differently. Instead of a singular impact, the glowing blue-green gas giant may have been the victim of a series of smaller punches.

At a 98 degree inclination, Uranus and its satellites have always been somewhat of a mystery to planetary scientists. While many of the Solar Systems planets have an inclined axis, none can compare with nearly being on its side. It has always been popular conjecture that Uranus was plastered that way at some point in its evolution by a body a few times larger than Earth. While this seems plausible, only one hole remains in the theory. Why did its moons take on the same inclination instead of staying in their original position?

This long-standing puzzle may have been solved by an international team of scientists led by Alessandro Morbidelli (Observatoire de la Cote d’Azur in Nice, France). Their theory relies on computer modeling – and the thought the impact might have occurred while Uranus was still forming. If the simulations are correct and the strike happened when the planet was still surrounded by a protoplanetary disk, ” the disk would have reformed into a fat doughnut shape around the new, highly-tilted equatorial plane. Collisions within the disk would have flattened the doughnut, which would then go onto form the moons in the positions we see today.”

But that’s not a neat answer. Just like throwing a tilt into pinball, the game changes. In this new scheme, the moons displayed retrograde motion – precisely the opposite of the way things are now. So what’s a player to do? Change the game again by re-arranging the parameters. By adding multiple strikes to Uranus – instead of just one large – the satellites now behave as we observe them.

Of course, when you “tilt” the game is over, and the new research doesn’t jive with current theories of planetary formation. This may mean re-writing the rules again. Morbidelli elaborates: “The standard planet formation theory assumes that Uranus, Neptune and the cores of Jupiter and Saturn formed by accreting only small objects in the protoplanetary disk. They should have suffered no giant collisions. The fact that Uranus was hit at least twice suggests that significant impacts were typical in the formation of giant planets. So, the standard theory has to be revised.”

That deaf, dumb and blind kid… Sure plays a mean pinball!

Original Story Source: Europlanet News Release.

Zooming in on Proto-Planetary Disks

On the road to planetary formation, the first step is an accretion disk around a proto-star. Such disks, known as proplyds, are frequently detected in star forming regions like the Orion nebula providing an understanding of the early life of planetary systems. The telltale hint that they exist is the warm infrared glow of the forming (or perhaps nearly formed) star heating the gas and dust, but although many have been detected this way, few have been observed with resolution that makes out any details on the disk itself. A new study aims to help add to the understanding of these systems with spatially resolved observations of two proplyds, including one already known to be host to a multiple planet system.

The two new systems under study are HD 107146 and HR 8799. The latter of these two systems is notable for having four known planets which have been directly imaged previously. HD 107146 is relatively close to our solar system, being only 28.5 pc away. This young star is similar to the Sun in mass and composition and is estimated to be somewhere between 80 and 200 million years young. Previous studies have examined this system’s disk and revealed that it is composed of nearly as much dust as there is gas, which means that much of the gas has likely been either accreted or stripped. Although not directly detected, the earlier studies have also suggested that the system may be hiding young planets. The evidence for this comes from possible banding in the disk. This is interpreted as similar to the rings and gaps in Saturn’s system, caused by shepherding moons, except in this case, the moon’s role would be fulfilled by planets creating resonances.

The new research, led by Meredith Hughes from the University of California, Berkeley, confirmed the presence of the disk around the star and found its brightness peaked at a distance of about 100 AU from the parent star (more than twice the average orbital distance of Pluto). Overall, their observations match models with a “broad ring extending from 50 to 170 AU”.

When looking at HR 8799’s disk, the team was given four nights, but due to poor weather, only one night’s worth of data from the Submillimeter Array atop Mauna Kea. The reduced amount of data left high uncertainties in the subsequent analysis. While the team attempted to search for banding that could induced by planets, the team was unable to find any. A study published earlier this year by a team at the University of Exeter also examined the HR 8799 disk and reported a slightly brighter clump on one side. The new study finds a similar clump but cautions that, due to the still poor observations of this system, the result may be suspect. A similar case happened when astronomers studied Vega’s dust disk and reported finding clumpy structure when it was, in reality, it was nothing but statistical noise.

These results, as well as the previous ones from the Exeter team and observations from Spitzer have suggested that the dust ring extends out to as far as 250 AU, and as far inwards as 80, but it is likely the inner radius is closer to 150 AU. If the inner radius is the correct value, this places it at roughly the limit that it could be shaped by the outermost planet HR 8799b which lies at just under 70 AU.

The Hunt for Young Exoplanets

While there is a great deal of excitement and effort in the hopes of finding small, terrestrial sized exoplanets, another realm of exoplanet discovery that is often overlooked is that of ones of differing ages to explore how planetary systems can evolve. The first discovered exoplanet orbited a pulsar, showing that planets can be hardy enough to survive the potential violent deaths of their parent stars. On the other end, young planets can help astronomers constrain how planets form and a potential new discovery may help in those regards.


Historically, astronomers have often avoided looking at stars younger than about 100 million years. Their young nature tends to make them unruly. They are prone to flares and other eccentric behaviors that often make observations messy. Additionally, many young stars often retain debris disks or are still embedded in the nebula in which they formed which also obscures observations.

Despite this, some astronomers have begun developing targeted searches for young exoplanets. The age of the exoplanet is not independently derived, but instead, taken from the age of the host star. This too can be difficult to determine. For isolated stars, there are precious few methods (such as gyrochronology) and they generally have large errors associated with them. Thus, instead of looking for isolated stars, astronomers searching for young exoplanets have tended to focus on clusters which can be dated more easily using the main sequence turn off method.

Through this methodology, astronomers have searched clusters and other groups, such as Beta Pictoris which turned up a planet earlier this year. The Beta Pic moving group boasts an age of ~12 million years making it one of the youngest associations currently known.

Trumpler 37 (also known as IC 1396 and the Elephant Trunk Nebula) is one of the few clusters with an even younger age of 1-5 million years. This was one of several young clusters observed by a team of German astronomers led by Gracjan Maciejewski of Jena University. The group utilized an array of telescopes across the world to continuously monitor Trumpler 37 for several weeks. During that time, they discovered numerous flares and variable stars, as well as a star with a dip in its brightness that could be a planet.

The team cautions that the detection may not be a planet. Several objects can mimic planetary transit lightcurves such as “the central transit of a low-mass star in front of a large main-sequence star or red giant, grazing eclipses in systems consisting of two main-sequence stars and a contamination of a fainter eclipsing binary along the same line of sight.” Due to the physics of small objects, the size of brown dwarfs and many Jovian type planets are similar leading difficulty in distinguishing from the light curve alone. Spectroscopic results will have to be undertaken to confirm the object truly is a planet.

However, assuming it is, based on the size of the dip in brightness, the team predicts the planet is about twice the radius of Jupiter, and about 15 times the mass. If so, this would be in good agreement with models of planetary formation for the expected age. Ultimately, planets of such age will help test our understanding of how planets form, whether it be from a single gravitational collapse early on, or slow accretion over time.

Tight Binaries are ‘Death Stars’ for Planets

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Astronomers studying double star systems where the two stars are extremely close have found a pattern of destruction. While there probably isn’t a Star Wars-like Death Star roaming the Universe, tight binary systems might provide the equivalent of Darth Vader’s favorite weapon. “This is real-life science fiction,” said Jeremy Drake of the Harvard-Smithsonian Center for Astrophysics. “Our data tell us that planets in these systems might not be so lucky — collisions could be common. It’s theoretically possible that habitable planets could exist around these types of stars, so if there happened to be any life there, it could be doomed.”

Using the Spitzer Space Telescope, Drake and his team spotted a surprisingly large amount of dust around three mature, close-orbiting star pairs, that might be the aftermath of tremendous planetary collisions.

Drake is the principal investigator of the research, published in the Aug.19 issue of the Astrophysical Journal Letters.

The particular class of binary stars in the study are extremely close together. Named RS Canum Venaticorums, or RS CVns for short, they are separated by only about 3.2-million kilometers (two-million miles ), or two percent of the distance between Earth and our sun. The binaries orbit around each other every few days, with one face on each star perpetually locked and pointed toward the other.

These stars are familiarly like our own Sun – about the same size and probably about a billion to a few billion years old — roughly the age of our sun when life first evolved on Earth. But these stars spin much faster, and, as a result, have powerful magnetic fields, and giant, dark spots. The magnetic activity drives strong stellar winds — gale-force versions of the solar wind — that slow the stars down, pulling the twirling duos closer over time.

This is not a good scenario for planetary survival.

As the stars cozy up to each other, their gravitational influences change, and this could cause disturbances to planetary bodies orbiting around both stars. Comets and any planets that may exist in the systems would start jostling about and banging into each other, sometimes in powerful collisions. This includes planets that could theoretically be circling in the double stars’ habitable zone, a region where temperatures would allow liquid water to exist. Though no habitable planets have been discovered around any stars beyond our sun at this point in time, tight double-star systems are known to host planets; for example, one system not in the study, called HW Vir, has two gas-giant planets.

“These kinds of systems paint a picture of the late stages in the lives of planetary systems,” said Marc Kuchner, a co-author from NASA Goddard Space Flight Center. “And it’s a future that’s messy and violent.”

The temperatures around these systems measured by Spitzer are about the same as molten lava. The astronomers says that dust normally would have dissipated and blown away from the stars by this mature stage in their lives. They conclude that something — most likely planetary collisions — must therefore be kicking up the fresh dust. In addition, because dusty disks have now been found around four, older binary systems, the scientists know that the observations are not a fluke. Something chaotic is very likely going on.

If any life forms did exist in these star systems, and they could look up at the sky, they would have quite a view. Marco Matranga, lead author of the paper, also from Harvard-Smithsonian said, “The skies there would have two huge suns, like the ones above the planet Tatooine in ‘Star Wars.'”

The research was published in the Aug.19 issue of the Astrophysical Journal Letters.

Source: JPL

How Water Protected Our Molecules

One would think that crafting a shield out of water wouldn’t do much good (not in medieval combat re-enactments, anyways). But that’s precisely what the molecules in the early Solar System – some of the same ones that you are made out of today, perhaps – may have done. In their case, protection from broadswords wasn’t as much of a concern as the effects of ultraviolet radiation from the Sun.

UV light is pretty hard on molecules because it readily breaks them up into their constituent parts. Larger organic molecules that coalesced in the dusty disk out of which our planets formed billions of years ago would have been broken apart by the Sun’s rays, but calculations by two astronomers at the University of Michigan show that thousands of oceans worth of water present in a protoplanetary disk can shield other molecules from being broken up.

Edwin (Ted) Bergin and Thomas Bethell, both of the Department of Astronomy at the University of Michigan, calculated that in Sun-like systems the abundance of water early on can absorb much of the ultraviolet light from the central star. By shielding other molecules from being broken up, they continue to persist in the later stages of the disk’s development. In other words, these molecules hang around until the formation of planetesimals and planets, and this mechanism could have been guarded the constituents of life from the ravages of the Sun in our own Solar System.

Circumstellar disks modeled by Bergin and Bethell in their paper include DR Tau, AS 205A and AA Tau.

Bergin told Universe Today, “At present there have been upwards of 4 systems with water vapor observed.  All are consistent with our model. I understand that there are numerous other detections of water vapor by Spitzer but these have yet to be published. The water vapor that we see is continually replenished by high temperature chemistry in these systems, so you would not see any degradation.”

In systems like the Solar System, planets form out of a disk of dust and gas that surrounds the young star. This large, flat disk later solidifies into planets, comets and asteroids. Near the center of the disk, between 1 and 5 astronomical units, warm water vapor in the disk could “protect” molecules inside this layer from being broken apart by UV light.

H2O breaks down when exposed to UV light into hydrogen and hydroxide. The hydroxide can be further broken down into oxygen and hydrogen atoms. But water, unlike other molecules, reforms at a quick pace, replenishing the shield of water vapor.

Smaller dust grains within the disk capture some of the UV radiation in the early formation periods of a protoplanetary disk. Once these dust grains start to snowball into bigger pieces, though, the UV light filters through and breaks apart molecules in the inner portions of the disk, where planets are in their early stages of formation.

The previous model for how organic molecules persisted past this point suggested that comets from the outer portion of the disk somehow fall into the center, releasing water to absorb the harmful radiation. But this model didn’t explain the hydroxide measurements for the disks so far observed.

If enough water is present, which seems to be the case in a handful of disks observed by the Spitzer Space Telescope, these other molecules remain intact, and as a bonus the water present in the interior portions of the disk also sticks around.

Bergin told Universe Today, “There are other molecules that can shield themselves – CO and H2 – but these cannot shield other molecules as well (because they capture only a fraction of the spectrum of light). Water is the only one with a strong formation that can compensate for destruction. It then provides the full shielding for other species. It is unlikely that another molecule will do this.”

This mechanism would only protect water vapor and other molecules in the inner part of the disk, closest to the star.

“This will likely be active in the inner few AU — at some point say between 5-10 AU it will become inactive and things will be inhospitable for various species [of molecule],” Bergin said.

So, where does all of the water go once the planets form? The vapor closest to the star – within about 1 AU – eventually gets broken down by the starlight into hydrogen and oxygen. At about 3 AU from the star, the water could constitute part of the planets and asteroids that form in that region. It may have been such asteroids that carried water to the surface of the Earth during its early formation, filling up our oceans. Outside of this region, H2O is broken down into hydrogen and oxygen and blown into space, said Bergin.

When asked whether this protective shield of water was present in our own Solar System, Bergin answered, “When we say that there were thousands of oceans of water vapor in the habitable zone, we mean around Sun-like stars.  Presumably this was present around our Sun as well.”

Source: Physorg, Science, email interview with Ted Bergin