Posted on by Ken Kremer

Russia Fuels Phobos-Grunt and sets Mars Launch for November 9

The Phobos-Grunt spacecraft is scheduled blastoff on November 9, 2011 from Baikonur Cosmodrome. It will reach Mars orbit in 2012 and eventually land on Phobos and return the first ever soil samples back to Earth in 2014. Credit Roscosmos

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Russia’s Space Agency, Roscosmos, has set November 9 as the launch date for the Phobos-Grunt mission to Mars and its tiny moon Phobos. Roscosmos has officially announced that the audacious mission to retrieve the first ever soil samples from the surface of Phobos will blastoff from the Baikonur Cosmodrome in Kazakhstan atop a Zenit-2SB rocket at 00:16 a.m. Moscow time.

Roscosmos said that engineers have finished loading all the propellants into the Phobos-Grunt main propulsion module (cruise stage), Phobos lander and Earth return module at Facility 31 at Baikonur.

Phobos-Grunt is Russia’s first mission to Mars in almost two decades and a prelude to an ambitious program of even more interplanetary Russian science flights.

Russian Phobos-Grunt spacecraft is set to launch to Mars on November 9, 2011.
L-shaped soil sample transfer tube extends from Earth return module ( top -yellow) and solar panel to bottom (left) of lander module. 2 landing legs, communications antenna, sampling arm, propulsion tanks and more are visible. Credit Roscosmos

Technicians also fueled the companion Yinghou-1 mini-satellite, provided by China, that will ride along inside a truss segment between the MDU propulsion module and the Phobos-Grunt lander.

The 12,000 kg Phobos-Grunt interplanetary spacecraft is being moved to an integration and test area at Facility 31 for integration with the departure segments of the Zenit rocket.

The next step is to enclose Phobos-Grunt inside the protective payload fairing and transport it to Facility 42 for mating atop the upper stage of the stacked Zenit-2SB booster rocket.

After about an 11 month journey, the spaceship will enter Mars orbit and spend several months searching for a suitable landing site on Phobos. The goal of the bold mission is to retrieve up to 200 grams of soil and rock from Phobos and return them to Earth in August 2014. The samples will help unlock the mysteries of the origin and evolution of Phobos, Mars and the Solar System.

Scientists hope that bits of Martian soil will be mixed in with Phobos soil.

Phobos-Grunt is equipped with a powerful 50 kg payload of some 20 international science instruments.

The 110 kg Yinghou-1, which translates as Firefly-1, is China’s first spaceship to voyage to Mars. It will be jettisoned by Phobos-Grunt into a separate orbit about Mars. The probe will photograph the Red planet with two cameras and study it with a magnetometer to explore Mars’ magnetic field and science instruments to explore its upper atmosphere.

Earth’s other mission to Mars in 2011, NASA’s Curiosity rover, is set to blast off for Mars on Nov. 25

Labeled Schematic of Phobos-Grunt and Yinghou-1 (YH-1) orbiter

Read Ken’s continuing features about Russia’s Phobos-Grunt Mars mission here::
Phobos-Grunt and Yinghou-1 Arrive at Baikonur Launch Site to tight Mars Deadline
Phobos-Grunt: The Mission Poster
Daring Russian Sample Return mission to Martian Moon Phobos aims for November Liftoff

Read Ken’s continuing features about Curiosity starting here:
Curiosity Buttoned Up for Martian Voyage in Search of Life’s Ingredients
Assembling Curiosity’s Rocket to Mars
Encapsulating Curiosity for Martian Flight Test
Dramatic New NASA Animation Depicts Next Mars Rover in Action

Posted on by Nancy Atkinson

Herschel Observatory Detects ‘Oceans’ of Water Around Distant Star

Detection of water vapour in the spectrum of TW Hydrae's protoplanetary disc. Credits: ESA/NASA/JPL-Caltech/M. Hogerheijde (Leiden Observatory)

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There’s enough water in a planet-forming disk around a distant star to fill several thousand Earth oceans, according to new observations with the Herschel space observatory. Astronomers have found evidence of water vapor originating from ice on dust grains in the disc around a young star, TW Hydrae. The star is between 5-10 million years old, so is in its final stages of formation.

“The detection of water sticking to dust grains throughout the disc would be similar to events in our own Solar System’s evolution, where over millions of years, similar dust grains then coalesced to form comets,” said Michiel Hogerheijde of Leiden University in the Netherlands, who led the study. “These comets we believe became a contributing source of water for the planets.”

Herschel has found water around other stars, such as an old red giant star CW Leonis, and other telescopes like Spitzer have also observed abundant water in nascent planet forming regions around other stars.

But scientists say this latest research from Herschel breaks new ground in understanding water’s role in planet-forming discs and gives scientists a new testing ground for looking at how water came to our own planet.

“With Herschel we can follow the trail of water through all the steps of star and planet formation,” said Göran Pilbratt, Herschel Project Scientist at ESA.

Scientists think the water vapor signature is produced when the ice coated dust grains are warmed by interstellar UV radiation.

Read more on this discovery at the ESA Herschel website.

Posted on by Ray Sanders

How Did Jupiter Shape Our Solar System?

Shortly after forming, Jupiter was slowly pulled toward the sun. Saturn was also pulled in and eventually, their fates became linked. When Jupiter was about where Mars is now, the pair turned and moved away from the sun. Scientists have referred to this as the "Grand Tack," a reference to the sailing maneuver. Credit: NASA/GSFC

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Jupiter hasn’t always been in the same place in our solar system. Early in the history of our solar system, Jupiter moved inward towards the sun, almost to where Mars currently orbits now, and then back out to its current position.

The migration through our solar system of Jupiter had some major effects on our solar system. Some of the effects of Jupiter’s wanderings include effects on the asteroid belt and the stunted growth of Mars.

What other effects did Jupiter’s migration have on the early solar system and how did scientists make this discovery?

In a research paper published in the July 14th issue of Nature, First author Kevin Walsh and his team created a model of the early solar system which helps explain Jupiter’s migration. The team’s model shows that Jupiter formed at a distance of around 3.5 A.U (Jupiter is currently just over 5 A.U from the sun) and was pulled inward by currents in the gas clouds that still surrounded the sun at the time. Over time, Jupiter moved inward slowly, nearly reaching the same distance from the sun as the current orbit of Mars, which hadn’t formed yet.

“We theorize that Jupiter stopped migrating toward the sun because of Saturn,” said Avi Mandell, one of the paper’s co-authors. The team’s data showed that Jupiter and Saturn both migrated inward and then outward. In the case of Jupiter, the gas giant settled into its current orbit at just over 5 a.u. Saturn ended its initial outward movement at around 7 A.U, but later moved even further to its current position around 9.5 A.U.

Astronomers have had long-standing questions regarding the mixed composition of the asteroid belt, which includes rocky and icy bodies. One other puzzle of our solar system’s evolution is what caused Mars to not develop to a size comparable to Earth or Venus.

Artist's conception of early planetary formation from gas and dust around a young star. Image Credit: NASA/JPL-Caltech

Regarding the asteroid belt, Mandell explained, “Jupiter’s migration process was slow, so when it neared the asteroid belt, it was not a violent collision but more of a do-si-do, with Jupiter deflecting the objects and essentially switching places with the asteroid belt.”

Jupiter’s slow movement caused more of a gentle “nudging” of the asteroid belt when it passed through on its inward movement. When Jupiter moved back outward, the planet moved past the location it originally formed. One side-effect of caused by Jupiter moving further out from its original formation area is that it entered the region of our early solar system where icy objects were. Jupiter pushed many of the icy objects inward towards the sun, causing them to end up in the asteroid belt.

“With the Grand Tack model, we actually set out to explain the formation of a small Mars, and in doing so, we had to account for the asteroid belt,” said Walsh. “To our surprise, the model’s explanation of the asteroid belt became one of the nicest results and helps us understand that region better than we did before.”

With regards to Mars, in theory Mars should have had a larger supply gas and dust, having formed further from the sun than Earth. If the model Walsh and his team developed is correct, Jupiter foray into the inner solar system would have scattered the material around 1.5 A.U.

Mandell added, “Why Mars is so small has been the unsolvable problem in the formation of our solar system. It was the team’s initial motivation for developing a new model of the formation of the solar system.”

An interesting scenario unfolds with Jupiter scattering material between 1 and 1.5 AU. Instead of the higher concentration of planet-building materials being further out, the high concentration led to Earth and Venus forming in a material-rich region.

The model Walsh and his team developed brings new insight into the relationship between the inner planets, our asteroid belt and Jupiter. The knowledge learned not only will allow scientists to better understand our solar system, but helps explain the formation of planets in other star systems. Walsh also mentioned, “Knowing that our own planets moved around a lot in the past makes our solar system much more like our neighbors than we previously thought. We’re not an outlier anymore.”

If you’d like to access the paper (subscription or paid/university access required), you can do so at: http://www.nature.com/nature/journal/v475/n7355/full/nature10201.html

Source: NASA Solar System News, Nature

Posted on by Tammy Plotner

Cosmic Collisions Could Eject Habitable Planets

One of 42 new proplyds discovered in the Orion Nebula, 177-341E is one of the bright proplyds that lies relatively close to the nebula’s brightest star, Theta 1 Orionis C. The tadpole-shaped tail is actually a jet of matter flowing away from the excited cusp. Credit:NASA/ESA and L. Ricci (ESO)

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When it comes to solar systems, chances are good that we’re a lot more special than we thought. According to a German-British team led by Professor Pavel Kroupa of the University of Bonn, our orderly neighborhood of varied planet sizes quietly orbiting in a nearly circular path isn’t a standard affair. Their new models show that habitable planets might just get ejected in a violent scenario where forming solar systems mean highly inclined orbits where hot Jupiters rule.

Some 4600 million years ago, our local planetary system was surmised to have evolved from a blanket of dust surrounding a rather ordinary star. Its planets orbited the same direction as the solar spin and lined up neatly on a plane fairly close to the solar equator. We were good little children… But maybe other systems aren’t so hospitable. There could be systems where the planets cruise around in the opposite direction of their host star’s spin – and have highly inclined orbits. What could cause one protoplanetary disk to take on quiet properties while another is more radical? Try a cosmic crash.

This new study focuses on the theory of a protoplanetary disk colliding with another cloud of material… not unrealistic thinking since most stars form within a cluster. The results could mean the inclusion of up to thirty times the mass of Jupiter. This added “weight” of extra gas and dust could add a tilt to a forming system. Team member Dr Ingo Thies, also of the University of Bonn, has carried out computer simulations to test the new idea. What he has found is that adding extra material can not only incline a forming disk, but cause a reverse spin as well. It may even speed up the planetary formation, leaving the rogues in retrograde orbits. This inhospitable scenario means that smaller planets get ejected systematically, leaving only hot Jupiters to hug in close to the parent star. Thankfully our path was a bit less disturbing.

Says Dr Thies: “Like most stars, the Sun formed in a cluster, so probably did encounter another cloud of gas and dust soon after it formed. Fortunately for us, this was a gentle collision, so the effect on the disk that eventually became the planets was relatively benign. If things had been different, an unstable planetary system may have formed around the Sun, the Earth might have been ejected from the Solar System and none of us would be here to talk about it.”

Professor Kroupa sees the model as a big step forward. “We may be on the cusp of solving the mystery of why some planetary systems are tilted so much and lack places where life could thrive. The model helps to explain why our Solar System looks the way it does, with the Earth in a stable orbit and larger planets further out. Our work should help other scientists refine their search for life elsewhere in the Universe.”

Original News Source: Royal Astronomical Society News.

Posted on by Ray Sanders

Titan’s Giant Cloud Explained

This image from the Cassini spacecraft, shows a huge arrow-shaped storm measuring 1,500km in length. Image Credit: NASA/JPL/SSI

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Titan is making news again, this time with Cassini images from 2010 showing a storm nearly as big as Texas.  Jonathan Mitchell from UCLA and his research team have published their findings which help answer the question:

What could cause such large storms to develop on a freezing cold world?

For starters, the huge arrow isn’t a cosmic detour sign reminding us to “Attempt No Landings” on Jupiter’s moon Europa.

In the study by Mitchell and his team,  a model of Titan’s global weather was created to understand how atmospheric waves affect weather patterns on Titan.  During their research, the team discovered a “stenciling” effect that creates distinct cloud shapes, such as the arrow-shaped cloud shown in the Cassini image above.

“These atmospheric waves are somewhat like the natural, resonant vibration of a wine glass,” Mitchell said. “Individual clouds might ‘ring the bell,’ so to speak, and once the ringing starts, the clouds have to respond to that vibration.”

Titan is the only other body in the solar system (aside from Earth) known to have an active “liquid cycle”.  Much like Titan’s warmer cousin Earth, the small moon has an atmosphere primarily composed of Nitrogen.  Interestingly enough Titan’s atmosphere is roughly the same mass as Earth’s and has about 1.5 times the surface pressure.  At the extremely low temperatures on Titan, hydrocarbons such as methane appear in liquid form, rather than the gaseous form found on Earth.

With an active liquid both on the surface and in the atmosphere of Titan, clouds form and create rain. In the case of Titan, the rain on the plain is mainly methane.  Water on Titan is rock-hard, due to temperatures hovering around -200 c.

Studies of Titan show evidence of liquid runoff, rivers and lakes, further emphasizing Titan’s parallels to Earth. Researchers believe better understanding of Titan may offer clues to understanding Earth’s early atmosphere.  In another parallel to earth, the weather patterns on Titan created by the atmospheric waves can create intense rainstorms, sometimes with more than 20 times Titan’s average seasonal rainfall. These intense storms may cause erosion patterns that help form the rivers seen on Titan’s surface.  Mitchell described Titan’s climate as “all-tropics”,  basically comparing the weather to what is usually found near Earth’s equator.  Could these storms be Titan’s equivalent of  monsoon season?

Mitchell stated “Titan is like Earth’s strange sibling — the only other rocky body in the solar system that currently experiences rain”.  Mitchell also added, “In future work, we plan to extend our analysis to other Titan observations and make predictions of what clouds might be observed during the upcoming season”.

The research was published Aug. 14 in the online edition of the journal Nature Geoscience .

If you’d like to learn more about the Cassini mission, visit: http://saturn.jpl.nasa.gov/index.cfm

Posted on by Tammy Plotner

Genesis Sheds Light On Sun And Solar System Formation

Artist Concept of Genesis Courtesy of JPL/NASA

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For 886 days between 2001 and 2004, a tiny spacecraft named Genesis sat parked at Lagrange Point L1 quietly collecting solar wind samples. On Sept. 8, 2004, the spacecraft released a sample return capsule which bashed its way onto the Utah desert carrying its little payload. Despite the disastrous crash, solar-wind ions were found buried beneath the surface of the collectors and what they have to tell us about the possible formation of our solar system is pretty amazing.

In March 2005 the international scientific community was given the collectors to study – and one of their prime targets was the evolution of our solar system. How could these tiny particles give us clues as to our origin? According the bulk of evidence, it is surmised the outer layer of the Sun hasn’t changed in several billion years. If we are to agree this is a good basis for modeling our solar nebula, we could begin to understand the chemical processes which formed our solar system. For most rock-forming elements, there appears to be little fractionation of either elements or isotopes between the sun and the solar wind. Or is there?

“The implication is that we did not form out of the same solar nebula materials that created the sun — just how and why remains to be discovered,” said Kevin McKeegan, a Genesis co-investigator from the University of California, Los Angeles and the lead author of one of two Science papers published this week.

Using the deposits found on the collector plates, scientists found a higher rate of common oxygen isotopes and a lowered rate of rare ones – different from Earth’s ratios. The same held true of nitrogen composition.

“These findings show that all solar system objects, including the terrestrial planets, meteorites and comets, are anomalous compared to the initial composition of the nebula from which the solar system formed,” said Bernard Marty, a Genesis co-investigator from Centre de Recherches Petrographiques et Geochimiques in Nancy, France and the lead author of the second new Science paper. “Understanding the cause of such a heterogeneity will impact our view on the formation of the solar system.”

While more studies are in the making, this new evidence provides vital information which may correct how we initially perceived our beginnings. While these elements are the most copious of all, even slight differences make them as distinctive as salt and pepper.

“The sun houses more than 99 percent of the material currently in our solar system so it’s a good idea to get to know it better,” said Genesis principal investigator Don Burnett of the California Institute of Technology in Pasadena, Calif. “While it was more challenging than expected we have answered some important questions, and like all successful missions, generated plenty more.”

Original Story Source: JPL Genesis Mission News.

Posted on by Jason Major

The Flip Side of Exoplanet Orbits

New research reveals the possible cause of retrograde "hot Jupiters"

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It was once thought that our planet was part of a “typical” solar system. Inner rocky worlds, outlying gas giants, some asteroids and comets sprinkled in for good measure. All rotating around a central star in more or less the same direction. Typical.

But after seeing what’s actually out there, it turns out ours may not be so typical after all…

Astronomers researching exoplanetary systems – many discovered with NASA’s Kepler Observatory – have found quite a few containing “hot Jupiters” that orbit their parent star very closely. (A hot Jupiter is the term used for a gas giant – like Jupiter – that resides in an orbit very close to its star, is usually tidally locked, and thus gets very, very hot.) These worlds are like nothing seen in our own solar system…and it’s now known that some actually have retrograde orbits – that is, orbiting their star in the opposite direction.

“That’s really weird, and it’s even weirder because the planet is so close to the star. How can one be spinning one way and the other orbiting exactly the other way? It’s crazy. It so obviously violates our most basic picture of planet and star formation.”

– Frederic A. Rasio, theoretical astrophysicist, Northwestern University

Now retrograde movement does exist in our solar system. Venus rotates in a retrograde direction, so the Sun rises in the west and sets in the east, and a few moons of the outer planets orbit “backwards” relative to the other moons. But none of the planets in our system have retrograde orbits; they all move around the Sun in the same direction that the Sun rotates. This is due to the principle of conservation of angular momentum, whereby the initial motion of the disk of gas that condensed to form our Sun and afterwards the planets is reflected in the current direction of orbital motions. Bottom line: the direction they moved when they were formed is (generally) the direction they move today, 4.6 billion years later. Newtonian physics is okay with this, and so are we. So why are we now finding planets that blatantly flaunt these rules?

The answer may be: peer pressure.

Or, more accurately, powerful tidal forces created by neighboring massive planets and the star itself.

By fine-tuning existing orbital mechanics calculations and creating computer simulations out of them, researchers have been able to show that large gas planets can be affected by a neighboring massive planet in such a way as to have their orbits drastically elongated, sending them spiraling closer in toward their star, making them very hot and, eventually, even flip them around. It’s just basic physics where energy is transferred between objects over time.

It just so happens that the objects in question are huge planets and the time scale is billions of years. Eventually something has to give. In this case it’s orbital direction.

“We had thought our solar system was typical in the universe, but from day one everything has looked weird in the extrasolar planetary systems. That makes us the oddball really. Learning about these other systems provides a context for how special our system is. We certainly seem to live in a special place.”

– Frederic A. Rasio

Yes, it certainly does seem that way.

The research was funded by the National Science Foundation. Details of the discovery are published in the May 12th issue of the journal Nature.

Read the press release here.

Main image credit: Jason Major. Created from SDO (AIA 304) image of the Sun from October 17, 2010 (NASA/SDO and the AIA science team) and an image of Jupiter taken by the Cassini-Huygens spacecraft on October 23, 2000 (NASA/JPL/SSI).

Posted on by Anne Minard

Meteorites Illuminate Mystery of Chromium in Earth’s Core

It’s generally assumed that the Earth’s overall composition is similar to that of chondritic meteorites, the primitive, undifferentiated building blocks of the solar system. But a new study in Science Express led by Frederic Moynier, of the University of California at Davis, seems to suggest that Earth is a bit of an oddball.

 

 

Thin section of a chondritic meteorite. Credit: NASA

Moynier and his colleagues analyzed the isotope signature of chromium in a variety of meteorites, and found that it differed from chromium’s signature in the mantle.

“We show through high-precision measurements of Cr stable isotopes in a range of meteorites, which deviate by up to ~0.4‰ from the bulk silicate Earth, that Cr depletion resulted from its partitioning into Earth’s core with a preferential enrichment in light isotopes,” the authors write. “Ab-initio calculations suggest that the isotopic signature was established at mid-mantle magma ocean depth as Earth accreted planetary embryos and progressively became more oxidized.”

Chromium’s origins. New evidence suggests that, in the early solar nebula (A), chromium isotopes were divided into two components, one containing light isotopes, the other heavy isotopes. In the early Earth (B), these components formed a homogeneous mixture. During core partitioning (C), the core became enriched with lighter chromium isotopes, and the mantle with heavier isotopes. Courtesy of Science/AAAS

The results point to a process known as “core partitioning,” rather than an alternative process involving the volatilization of certain chromium isotopes so that they would have escaped from the Earth’s mantle. Core partitioning took place early on Earth at high temperatures, when the core separated from the silicate earth, leaving the core with a distinct composition that is enriched with lighter chromium isotopes, notes William McDonough, from the University of Maryland at College Park, in an accompanying Perspective piece.

McDonough writes that chromium, Earth’s 10th most abundant element, is named for the Greek word for color and “adds green to emeralds, red to rubies, brilliance to plated metals, and corrosion-proof quality to stainless steels.” It is distributed roughly equally throughout the planet.

He says the new result “adds another investigative tool for understanding and documenting past and present planetary processes. For the cosmochemistry and meteoritics communities, the findings further bolster the view that the solar nebula was a heterogeneous mixture of different components.”

Source: Science. The McDonough paper will be published online today by the journal Science, at the Science Express website.

Posted on by Anne Minard

First-Time Views of Solar System Births

SUBARU Telescope image of the protoplanetary disk around the young star LkCa 15. Credit: MPIA (C. Thalmann) & NAOJ

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Chalk up a sizzling success for the HiCIAO planet-hunter camera on the Subaru Telescope in Hawaii: it’s captured this unprecedented image of a stellar disk similar in size to our own solar system, featuring rings and gaps that are associated with the formation of giant planets.

The lead image shows a bright arc of scattered light, in white, from the protoplanetary disk around the young star LkCa 15.  LkCa 15 is in the center of the image, blacked out. The arc’s sharp inner edge traces the outline of a wide gap in the disk. The gap is decidedly lopsided – it is markedly wider on the left side – and has most likely been carved out of the disk by one or more newborn planets that orbit the star.

The disk gap is large enough to house the orbits of all the planets in our own Solar System. “We haven’t detected the planets themselves yet,” said Christian Thalmann, who led the LkCa 15 study while on staff at the Max Planck Institute for Astronomy.”But that may change soon.”

LkCa 15, aged a few million years, is in the Taurus constellation about 450 light years away.

The observations are part of a systematic survey called SEEDS, or the Strategic Explorations of Exoplanets and Disks with Subaru Project, with a goal to search for planets and disks around young stars using HiCIAO, a state-of-the-art high-contrast camera designed specifically for this purpose. The lead investigator on the project is Motohide Tamura at the National Astronomical Observatory of Japan, but it’s a collaborative effort with international participation. Their first significant discovery — an exoplanet candidate around a sun-like star — was announced in December.

Besides LkCa 15, the researchers have also captured a sharp images of the protoplanetary disk around the very young star AB Aur in the constellation Auriga, “the Charioteer.” Lead researcher Jun Hashimoto, of the National Observatory of Japan, and his team report nested rings of material that are tilted with respect to the disk’s equatorial plane, and whose material, intriguingly, is not distributed symmetrically around the star – irregular features that indicate the presence of at least one very massive planet.

Recent images of AB Aur taken by HiCIAO (top left), compared with an image taken in 2004 by its predecessor instrument CIAO (top right). The new images give a much more detailed view of the inner regions (bottom left; with explanations bottom right): Intricate bright and dark patterns indicate the presence of different rings of matter. The fact that their centers do not coincide with the position of the star and the other irregularities point to the existence of a massive giant planet which is sweeping up the material between the rings. Credit: NAOJ/J. Hashimoto

The researchers point out that no other telescopes, whether ground-based or in space, have ever penetrated so close to a central star, showing the details of its disk.

Planetary systems like our own share a humble origin as mere by-products of star formation. A newborn star’s gravity gathers leftover gas and dust in a dense, flattened disk of matter orbiting the star. Clumps in the disk sweep up more and more material, until their own gravity becomes sufficiently strong to compress them into the dense bodies we know as planets.

Sources: Max Planck Institute For Astronomy, National Observatory of Japan.

Links to the published results:

Thalmann, C. et al., Imaging of a Transitional Disk Gap in Reflected Light: Indications of Planet Formation Around the Young Solar Analog LkCa 15 in Astrophysical Journal Letters 718, p. L87-L91

Hashimoto, J. et al., accepted for publication in Astrophysical Journal Letters in January 2011.

Posted on by Steve Nerlich

Astronomy Without A Telescope – Forbidden Planets

The theorised evolution of the circumbinary planet PSR B1620?26 b. Credit: NASA.

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Binary star systems can have planets – although these are generally assumed to be circumbinary (where the orbit encircles both stars). As well as the fictional examples of Tatooine and Gallifrey, there are real examples of PSR B1620-26 b and HW Virginis b and c – thought to be cool gas giants with several times the mass of Jupiter, orbiting several astronomical units out from their binary suns.

Planets in circumstellar orbits around a single star within a binary system are traditionally considered to be unlikely due to the mathematical implausibility of maintaining a stable orbit through the ‘forbidden’ zones – which result from gravitational resonances generated by the motion of the binary stars. The orbital dynamics involved should either fling a planet out of the system or send it crashing to its doom into one or other of the stars. However, there may be a number of windows of opportunity available for ‘next generation’ planets to form at later stages in the evolving life of a binary system.

A binary stellar evolution scenario might go something like this:

1) You start with two main sequence stars orbiting their common centre of mass. Circumstellar planets may only achieve stable orbits very close in to either star. If present at all, it’s unlikely these planets would be very large as neither star could sustain a large protoplanetary disk given their close proximity.

2) The more massive of the binaries evolves further to become an Asymptotic Giant Branch star (i.e. red giant) – potentially destroying any planets it may have had. Some mass is lost from the system as the red giant blows off its outer layers – which is likely to increase the separation of the two stars. But this also provides material for a protoplanetary disk to form around the red giant’s binary companion star.

3) The red giant evolves into white dwarf, while the other star (still in main sequence and now with extra fuel and a protoplanetary disk) can develop a system of orbiting ‘second generation’ planets. This new stellar system could remain stable for a billion years or more.

4) The remaining main sequence star eventually goes red giant, potentially destroying its planets and further widening the separation of the two stars – but it also may contribute material to form a protoplanetary disk around the distant white dwarf star, providing the opportunity for third generation planets to form there.

How a binary system might give birth to generations of planets: a) First generation planets - small and close-in - might be possible while both stars are on the main sequence (MS) and in close proximity to each other; b) Eventually one star evolves from the main sequence to the Asymptotic Giant Branch (AGB) - in other words, it goes red giant. c) The two stars spread further apart while stellar material blown off from the red giant builds a protoplanetary disk around the other star and second generation planets form; d) the second star eventually goes red giant giving the first star (now a white dwarf - WD) a protoplanetary disk which could create a third generation of planets. Credit: Perets, H.B.

The development of the third generation planetary system depends on the white dwarf star sustaining a mass below its Chandrasekhar limit (being about 1.4 solar masses – depending on its rate of spin) despite it having received more material from the red giant. If it doesn’t stay below that limit, it will become a Type 1a supernova – potentially lobbing a small proportion of its mass back to the other star again, although by this stage that other star would be a very distant companion.

An interesting feature of this evolutionary story is that each generation of planets is built from stellar material with a sequentially increasing proportion of ‘metals’ (elements heavier than hydrogen and helium) as the material is cooked and re-cooked within each stars’ fusion processes. Under this scenario, it becomes feasible for old stars, even those which formed as low metal binaries, to develop rocky planets later in their lifetimes.

Further reading: Perets, H.B. Planets in evolved binary systems.