The theorised evolution of the circumbinary planet PSR B1620?26 b. Credit: NASA.
[/caption]
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.
A huge impact may have formed the Moon, but other large impacts could have determined the makeup of Earth and other planetary bodies. Image Credit: Joe Tucciarone
[/caption]
One of the fundamental problems in planetary science is trying to determine how planetary bodies in the inner solar system formed and evolved. A new computer model suggests that huge objects – some as big as large Kuiper Belt Objects like Pluto and Eris — likely pummeled the Earth, Moon and Mars during the late stages of planetary formation, bringing heavy metals to the planetary surfaces. This model – created by various researchers from across the NASA Lunar Science Institute — surprisingly addresses many different puzzles across the Solar System, such as how Earth could retain metal-loving, elements like gold and platinum found in its mantle, how the interior of the Moon could actually be wet, and the strange distribution in the sizes of asteroids.
“Most of the evidence of what happened during the late stages of planetary formation has been erased over time,” said Bill Bottke from the Southwest Research Institute, who led the research team. “The trail we’ve been tracking on these worlds is pretty cold and to be able to dig more information out of what we have and be able answer some long standing problems is pretty exciting.”
Bottke told Universe Today that the story this new model tells “is not as complicated as it looks at first glance,” he said. “It includes a lot of concepts together, and some of the concepts have actually been around for awhile.”
Bottke and his team have published their results in the journal Science.
The researchers started with the widely accepted theory of how our Moon was created by a giant impact between the early Earth and another Mars-sized planetary body. “This was the most traumatic event the Earth probably ever went through, and that was the time when presumably the Earth and Moon both formed their cores,” Bottke said.
The heavy iron fell to the center of the two bodies, and so-called highly siderophile, or metal-loving, elements such as rhenium, osmium platinum, palladium, and gold should have followed the iron and other metals to the core in the aftermath of the Moon-forming event, leaving the rocky crusts and mantles of these bodies void of these elements.
“These elements love to follow the metal,” Bottke said, “so if the metal is draining to the core, these elements would want to drain with them. So if this is right, what we would expect that rocks derived from our mantle should have almost no highly siderophile elements, maybe 10 to the minus 5th level or so. But surprisingly, that is not what we see. They are only less abundant by a factor of less than 200, compared to what we would expect, a factor of 100,000 or so.”
Bottke said this problem has been argued about since the 1970’s, with various suggestions on how to answer the problem.
“The most viable answer is that after the Moon forming impact took place, there were also other things that hit the Earth during the late stages of planet formation, objects that were smaller, and these smaller objects replenished these elements and gave us the abundance we see today. This is what we refer to as late accretion,” he said.
On the Moon, the same thing was happening. But there was a problem with this scenario. The ratio of these elements on the Earth compared with rocks on the Moon is about 1000 to 1.
“The gravitational cross section of the Earth is about 20 times that of the Moon,” Bottke said, “So for every object that hit the Moon, about twenty should have hit the Earth. And if late accretion delivered these elements, you should have about a 20 to 1 ratio. But that is not what we see—we see a 1000 to 1 ratio.”
Bottke – a planetary dynamacist — discussed this with colleague David Nesvorny, also from SWRI, as well as geophysical-geochemical modelers, such as Richard Walker from the University of Maryland, James Day from the University of Maryland, and Linda Elkins-Tanton from the Massachusetts Institute of Technology.
They came up with a computer model that seemed to provide an answer.
“By playing roulette with these objects, I found that very often the Earth was getting hit by huge impactors that the Moon would never see,” Bottke said. “This result suggests that the things hitting the Earth and Moon at the end of the planet formation period was dominated by very large objects.”
The model predicted that the largest of the late impactors on Earth, at 2,400 – 3,200 km (1,500-2,000 miles) in diameter, while those for the Moon, at approximately 240 – 320 km.
Bottke called that a “cute” result – but they needed more supporting evidence. So, they took a look at the last surviving population of the things that built the planets, the inner asteroid belt. “You find large asteroids like Ceres, Vesta and Pallas” Bottke said, so there are the large ones at 500 to 900 km, but then your next largest asteroids are only about 250 km. This matched up with the sizes that our model came up with,” in which no asteroids with “in-between” sizes are observed in this region.
Maps of Mars' global topography. Credit: NASA
Next, they looked at Mars, which has some very large impact basins which are probably left over from the days of when the planet formed, including the Borealis Basin, which is so large it likely accounts for the differences in the northern and southern hemispheres on the Red Planet.
“We looked and projected the size of the impactors that would have created those impact basins and we saw the distribution of sizes was very much like what was predicted for the Earth and Moon, and also what is found in the inner asteroid belt.
So all those things together — the theoretical basis, the observational evidence from elements on the Earth and Moon, and impacts on Mars collectively says something about the distribution of sizes of objects towards the end of planetary formation.
And what are the implications?
“We could make predictions for what was hitting the Earth, Moon and Mars at that time, and they line up with what we see on the surfaces,” Bottke said. “On Mars we can play a game of what is the biggest projectiles that should have hit Mars, and it matches up well with the size that big basin that formed on Mars, and also produced the abundances of elements we see there.”
“For the Moon, the biggest impactors would be 250-300 km, which is about the size of the south pole Aiken basin,” Bottke continued. “For the Earth, these big impactors explain why some of these impacts managed to hit the Earth and not all the elements went to the core of the Earth.”
Bottke said that adding to the complications, some of the biggest impacts actually may have plowed through the Earth and actually came out the other side — in a very fragmented state — and rained back down on Earth. “If this is true, this provides a way to spread fragments all the way across the Earth,” he said, “but how the debris gets redistributed around the planetary body is a really interesting question. That part needs a lot more work and is simply at the edge right now of what we can do numerically.”
When it comes to water on the Moon’s interior – which was once thought to be dry, but recent sample measurements, however, suggest that the water content in the lunar mantle is between 200 and several thousand parts per billion — Bottke’s model could also address this issue.
“If true,” the team writes in their paper, “it is possible that the same projectile that delivered most of the Moon’s HSEs may have also have provided it with water….Late accretion provides an alternative explanation in case lunar mantle water cannot migrate from the post–giant impact Earth to a growing Moon through a hot and largely vaporized protolunar disk.”
As to why smaller projectiles hit the Moon as compared to Earth, Bottke said it is just a numbers game. “We start with a population which has a certain number of big things, middle sized things and small things,” he said. “And we randomly choose projectiles from that population and for every one big guy that hits the Moon, 20 hit the Earth. And we play that game, and if the number of projectiles is limited, if the Moon only gets hit once or twice from this population, that means the Earth gets hit 20-30 times, that is enough to give us – on most occasions – what we see.”
Bottke said this research gave him a chance to work with geochemists, “who have all sorts of interesting things to say which help constrain the processes that brought about planet formation. The problem is that sometimes they have great information but they don’t have a dynamical process that can work. So by working together I think we were able to come up with some interesting results.”
“The most exciting thing for me is that we should be able to use these abundances that we have on the Earth, Moon and Mars to really tell the story about planet formation,” Bottke said.
Well, not only may up to 25% of Sun-like stars have Earth-like planets – but if they are in the right temperature zone, apparently they are almost certain to have oceans. Current thinking is that Earth’s oceans formed from the accreted material that built the planet, rather than being delivered by comets at a later time. From this understanding, we can start to model the likelihood of a similar outcome occurring on rocky exoplanets around other stars.
Assuming terrestrial-like planets are indeed common – with a silicate mantle surrounding a metallic core – then we can expect that water may be exuded onto their surface during the final stages of magma cooling – or otherwise out-gassed as steam which then cools to fall back to the surface as rain. From there, if the planet is big enough to gravitationally retain a thick atmosphere and is in the temperature zone where water can remain fluid, then you’ve got yourself an exo-ocean.
We can assume that the dust cloud that became the Solar System had lots of water in it, given how much persists in the left-over ingredients of comets, asteroids and the like. When the Sun ignited some of this water may have been photodissociated – or otherwise blown out of the inner solar system. However, cool rocky materials seem to have a strong propensity to hold water – and in this manner, could have kept water available for planet formation.
Meteorites from differentiated objects (i.e. planets or smaller bodies that have differentiated such that, while in a molten state, their heavy elements have sunk to a core displacing lighter elements upwards) have around 3% water content – while some undifferentiated objects (like carbonaceous asteroids) may have more than 20% water content.
Mush these materials together in a planet formation scenario and materials compressed at the centre become hot, causing outgassing of volatiles like carbon dioxide and water. In the early stages of planet formation much of this outgassing may have been lost to space – but as the object approaches planet size, its gravity can hold the outgassed material in place as an atmosphere. And despite the outgassing, hot magma can still retain water content – only exuding it in the final stages of cooling and solidification to form a planet’s crust.
Mathematical modelling suggests that if planets accrete from materials with 1 to 3% water content, liquid water probably exudes onto their surface in the final stages of planet formation – having progressively moved upwards as the planet’s crust solidified from the bottom up.
Otherwise, and even starting with a water content as low as 0.01%, Earth-like planets would still generate an outgassed steam atmosphere that would later rain down as fluid water upon cooling.
As the Earth formed, water contained in rocky materials either 'outgassed' or just exuded onto the surface - as magma solidified, from the bottom up, to form the Earth's crust. And OK, this is just a nice image of a deep sea volcanic vent - but you get the idea. Credit: Woods Hole Oceanographic Institution.
If this ocean formation model is correct, it can be expected that rocky exoplanets from 0.5 to 5 Earth masses, which form from a roughly equivalent set of ingredients, would be likely to form oceans within 100 millions years of primary accretion.
This model fits well with the finding of zircon crystals in Western Australia – which are dated at 4.4 billion years and are suggestive that liquid water was present that long ago – although this preceded the Late Heavy Bombardment (4.1 to 3.8 billion years ago) which may have sent all that water back into a steam atmosphere again.
Currently it’s not thought that ices from the outer solar system – that might have been transported to Earth as comets – could have contributed more than around 10% of Earth’s current water content – as measurements to date suggest that ices in the outer solar system have significantly higher levels of deuterium (i.e. heavy water) than we see on Earth.
The spiral galaxy NGC 4565, considered a close analogue of the Milky Way and with distinctly dusty outer regions. Credit: ESO.
[/caption]
A Japanese team of astronomers have reported a strong correlation between the metallicity of dusty protoplanetary disks and their longevity. From this finding they propose that low metallicity stars are much less likely to have planets, including gas giants, due to the shorter lifetime of their protoplanetary disks.
As you are probably aware, ‘metal’ is astronomy-speak for anything higher up the periodic table than hydrogen and helium. The Milky Way has a metallicity gradient – where metallicity drops markedly the further out you go. In the extreme outer galaxy, about 18 kiloparsecs out from the centre, the metallicity of stars is only 10% that of the Sun (which is about 8 kiloparsecs – or around 25,000 light years – out from the centre).
This study compared young star clusters within stellar nurseries with relatively high metallicity (like the Orion nebula) against more distant clusters in the outer galaxy within low metallicity nurseries (like Digel Cloud 2).
The study’s conclusions are based on the assumption that the radiation output of stars with dense protoplanetary disks will have an excess of near and mid-infra red wavelengths. This is largely because the star heats its surrounding protoplanetary disk, making the disk radiate in infra-red.
The research team used the 8.2 metre Subaru Telescope and a procedure called JHK photometry to identify a measure they called ‘disk fraction’, representing the density of the protoplanetary disk (as determined by the excess of infra red radiation). They also used another established mass-luminosity relation measure to determine the age of the clusters.
Graphing disk fraction over age for populations of Sun-equivalent metallicity stars versus populations of low metallicity stars in the outer galaxy suggests that the protoplanetary disks of those low metallicity stars disperse much quicker.
Left image - The Subaru Telescope in Hawaii. Credit: NAOJ. Right image - the relationship between disk persistence for low metallicity stars (O/H = -0.7, red line) and stars with Sun-equivalent metallicity (O/H = 0, black line). The protoplanetary disks of low metal stars seem to disperse quickly, reducing the likelihood of planet formation. Credit: Yasui et al.
The authors suggest that the process of photoevaporation may underlie the shorter lifespan of low metal disks – where the impact of photons is sufficient to quickly disperse low atomic mass hydrogen and helium, while the presence of higher atomic weight metals may deflect those photons and hence sustain a protoplanetary disk over a longer period.
As the authors point out, the lower lifetime of low metallicity disks reduces the likelihood of planet formation. Although the authors steer clear of much more speculation, the implications of this relationship seem to be that, as well as expecting to find less planets around stars towards the outer edge of the galaxy – we might also expect to find less planets around any old Population II stars that would have also formed in environments of low metallicity.
Indeed, these findings suggest that planets, even gas giants, may have been exceedingly rare in the early universe – and have only become commonplace later in the universe’s evolution – after stellar nucleosynthesis processes had adequately seeded the cosmos with metals.
Planets form by accreting material from a protoplanetary disk. New research suggests it can happen quickly, and that Earth may have formed in only a few million years. Credit: NASA/NASA/JPL-Caltech
[/caption]
Wanna build celestial objects? I mean it sounds easy – you just start with a big cloud of dust and give it a nudge so that it starts to spin and accrete and you end up with a star with a few wisps of dust left in orbit that continue to accrete to form planets.
Trouble is, this process doesn’t seem to be physically possible – or at least nothing like it can be replicated in standard theoretical models and laboratory simulations. There’s a problem with the initial small scale accretion steps.
Dust particles seem to stick readily together when they are very small – through van der Waals and electrostatic forces – steadily building up to form millimeter and even centimeter sized aggregates. But once they get to this size those sticky forces become less influential – and the objects are still too small to generate a meaningful amount of gravitational attraction. What interaction they do have is more in the nature of bouncing collisions – which most often result in pieces being chipped off the bouncing objects, so that they start getting smaller again.
This is an astrophysics problem known as the meter barrier.
But increasingly, theorists are coming up with ways to get around the meter barrier. Firstly, it may be a mistake to assume that you start with a uniform dust cloud, in which spontaneous accretion happens everywhere throughout the cloud.
Current thinking is that it may take a nearby supernova or a closely migrating star to trigger the evolution of a dust cloud into a stellar nursery. It’s possible that turbulence in a dust cloud creates whirlpools and eddies that favor the local aggregation of small particles into larger particles. So rather than going from a uniform dust cloud to a uniform collection of very small rocks – there is just a chance formation of accreted objects here and there.
Or we can just assume a certain stochastic inevitability about anything that has the faintest chance of happening – eventually happening. Over several million years, within a huge dust cloud that might be several hundred astronomical units in diameter, a huge variety of interactions becomes possible – and even with a 99.99% likelihood that no object can ever aggregate to a size bigger than a meter, it’s still entirely likely that this is going to happen somewhere in that vast area.
Either way, once you have a few seed objects, it’s hypothesised that the snowball process takes over. Once an aggregated object achieves a certain mass, its inertia will mean it becomes less engaged in turbulent flow. In other words, the object will begin to move through, rather than move with, the turbulent dust. Under these circumstances, it will behave like a snowball rolling down a snow covered hill, collecting a covering of dust as it plows through the dust cloud – increasing its diameter as it goes.
An artist's impression of HD 98800. The snowball process works even faster in protoplanetary disks around binary stars (at least on paper). Well, Tatooine must have formed somehow... Credit: JPL, NASA.
The time span required to build such snowballed planetesimals from a radius (Rsnow) of 100 meters up to 1000 kilometers is long. The modelling used suggests a time span (Tsnow) of between 1 and 10 million years is required.
It’s also possible to model planet formation around binary stars. Using orbital parameters equivalent to those of the binary system Alpha Centauri A and B, the snowball process is calculated to work more efficiently so that Tsnow is probably no more than 1 million years.
Once hundred kilometer-sized planetesimals have formed, they would still engage in collisions. But at this size, the objects generate substantial self-gravity and collisions are more likely to be constructive – eventually resulting in planets with their own orbiting debris, which then forms rings and moons.
There is evidence that some stars can form planets (at least gas giants) within 1 million years – such as GM Aurigae – while our solar system may have taken a more leisurely 100 million years from the Sun’s birth until the current collection of rocky, gassy and icy planets fully accreted out of the dust.
So, there’s more than a snowball’s chance in hell that that this theory may contribute to a better understanding of planet formation.
Artist's impression of the planet OGLE-TR-L9b. Credit: ESO/H. Zodet
[/caption]
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.
This plot of data from NASA's Spitzer Space Telescope tells astronomers that a dusty planetary smashup probably occurred around a pair of tight twin, or binary, stars. Image credit: NASA/JPL-Caltech/Harvard-Smithsonian CfA
[/caption]
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.
Young stars have a disk of gas and dust around them called a protoplanetary disk. Credit: NASA/JPL-Caltech
[/caption]
For the first time, astronomers have observed in unprecedented detail the processes giving rise to stars and planets in nascent solar systems. Using both Keck telescopes on Mauna Kea in Hawaii outfitted with a specifically engineered instrument named ASTRA (ASTrometric and phase-Referenced Astronomy), Joshua Eisner from the University of Arizona and his colleagues were able to peer deeply into protoplanetary disks – swirling clouds of gas and dust that feed the growing star in its center and eventually coalesce into planets and asteroids to form a solar system. What they saw is providing insight into the way hydrogen gas from the protoplanetary disk is incorporated into the star.
In order to obtain the extremely fine resolution necessary to observe the processes that happen at the boundary between the star and its surrounding disk 500 light years from Earth, the team combined the light from the two Keck telescopes, which provides an angular resolution finer than Hubble’s. Eisner and his team also used a technique called spectro-astrometry to boost resolution even more. By measuring the light emanating from the protoplanetary disks at different wavelengths with both Keck telescope mirrors and manipulating it further with ASTRA, the researchers achieved the resolution needed to observe processes in the centers of the nascent solar systems.
“The angular resolution you can achieve with the Hubble Space Telescope is about 100 times too coarse to be able to see what is going on just outside of a nascent star not much bigger than our sun,” said Eisner. In other words, even a protoplanetary disk close enough to be considered in the neighborhood of our solar system would appear as a featureless blob.
With this new technique, the team was able to distinguish between the distributions of gas, mostly made up of hydrogen, and dust, thereby resolving the disk’s features.
“We were able to get really, really close to the star and look right at the interface between the gas-rich protoplanetary disk and the star,” said Eisner.
Protoplanetary disks form in stellar nurseries when clouds of gas molecules and dust particles begin to collapse under the influence of gravity.
Initially rotating slowly, the cloud’s growing mass and gravity cause it to become more dense and more compact. The preservation of rotational momentum speeds up the cloud as it shrinks, much like a figure skater spins faster as she tugs in her arms. The centrifugal force flattens the cloud into a spinning disk of swirling gas and dust, eventually giving rise to planets orbiting their star in roughly the same plane.
Astronomers know that stars acquire mass by incorporating some of the hydrogen gas in the disk that surrounds them, in a process called accretion, which can happen in one of two ways.
In one scenario, gas is swallowed as it washes up right to the fiery surface of the star.
In the second, much more violent scenario, the magnetic fields sweeping from the star push back the approaching gas and cause it to bunch up, creating a gap between the star and its surrounding disk. Rather than lapping at the star’s surface, the hydrogen atoms travel along the magnetic field lines as if on a highway, becoming super-heated and ionized in this process.
“Once trapped in the star’s magnetic field, the gas is being funneled along the field lines arching out high above and below the disk’s plane,” Eisner explained. “The material then crashes into the star’s polar regions at high velocities.”
In this inferno, which releases the energy of millions of Hiroshima-sized atomic bombs every second, some of the arching gas flow is ejected from the disk and spews out far into space as interstellar wind.
“We want to understand how material accretes onto the star,” Eisner said. “This process has never been measured directly.”
Eisner’s team pointed the telescopes at 15 protoplanetary disks with young stars varying in mass between one half and 10 times that of our sun.
“We could successfully discern that in most cases, the gas converts some of its kinetic energy into light very close to the stars” he said, a tell-tale sign of the more violent accretion scenario.
“In other cases, we saw evidence of winds launched into space together with material accreting on the star,” Eisner added. “We even found an example – around a very high-mass star – in which the disk may reach all the way to the stellar surface.”
The solar systems the astronomers chose for this study are still young, probably a few million years old.
“These disks will be around for a few million years more,” Eisner said. “By that time, the first planets, gas giants similar to Jupiter and Saturn, may form, using up a lot of the disk material.”
More solid, rocky planets like the Earth, Venus or Mars, won’t be around until much later.
“But the building blocks for those could be forming now,” he said, which is why this research is important for our understanding of how solar systems form, including those with potentially habitable planets like Earth.
“We are going to see if we can make similar measurements of organic molecules and water in protoplanetary disks,” he said. “Those would be the ones potentially giving rise to planets with the conditions to harbor life.”
The team’s paper was published in the Astrophysical Journal
Epsilon Andromedae. Illustration Credit: NASA, ESA, and A. Feild (STScI) Science Credit: NASA, ESA, and B. McArthur, University of Texas at Austin, McDonald Observatory.
[/caption]
Astronomers are finding that not only are there a wide range of different extrasolar planets, but there are different types of planetary systems, as well. “We’re not in Kansas anymore as far as solar systems go,” said Barbara McDonald from the University of Texas’ McDonald Observatory, at the American Astronomical Society meeting in Miami, Florida today. “The exciting thing is, we found another multi-planet system that is not at all like our own.”
A close look at the Upsilon Andromedae system with the Hubble Space Telescope, the Hobby-Eberly Telescope and other ground-based telescopes shows a whacky system where planets are out of tilt and have highly inclined orbits. The astronomers also found another planet, and also another star – this is likely a binary star system.
Even with Pluto’s inclined orbit, our solar system looks like an ocean of calm compared to Upsilon Andromedae. Comparison of solar systems. Credit: HubbleSite
McDonald said these surprising findings will impact theories of how multi-planet systems evolve, and it shows that some violent events can happen to disrupt planets’ orbits after a planetary system forms.
“The findings mean that future studies of exoplanetary systems will be more complicated,” she said. “Astronomers can no longer assume all planets orbit their parent star in a single plane.” says Barbara McArthur of The University of Texas at Austin’s McDonald Observatory.
Similar to our Sun in its properties, Upsilon Andromedae lies about 44 light-years away. It’s a little younger, more massive, and brighter than the Sun. For just over a decade, astronomers have known that three Jupiter-type planets orbit the yellow-white star Upsilon Andromedae.
But after over a thousand combined observations, McDonald and her team uncovered hints that a fourth planet, e, orbits the star much farther out. They were also able to determine the exact masses of two of the three previously known planets, Upsilon Andromedae c and d. Much more startling, though, is that not all planets orbit this star in the same plane. The orbits of planets c and d are inclined by 30 degrees with respect to each other. This research marks the first time that the “mutual inclination” of two planets orbiting another star has been measured.
“Most probably Upsilon Andromedae had the same formation process as our own solar system, although there could have been differences in the late formation that seeded this divergent evolution,” McArthur said. “The premise of planetary evolution so far has been that planetary systems form in the disk and remain relatively co-planar, like our own system, but now we have measured a significant angle between these planets that indicates this isn’t always the case.”
Until now the conventional wisdom has been that a big cloud of gas collapses down to form a star, and planets are a natural byproduct of leftover material that forms a disk. In our solar system, there’s a fossil of that creation event because all of the eight major planets orbit in nearly the same plane. The outermost dwarf planets like Pluto are in inclined orbits, but these have been modified by Neptune’s gravity and are not embedded deep inside the Sun’s gravitational field.
So what knocked the Upsilon Andromedae system around?
“Possibilities include interactions occurring from the inward migration of planets, the ejection of other planets from the system through planet-planet scattering, or disruption from the parent star’s binary companion star, Upsilon Andromedae B,” McArthur said.
Or, the companion star – a red dwarf less massive and much dimmer than the Sun — could be the culprit. is.
“We don’t have any idea what its orbit is,” said team member Fritz Benedict. “It could be very eccentric. Maybe it comes in very close every once in a while. It may take 10,000 years.” Such a close pass by the secondary star could gravitationally perturb the orbits of the planets.”
The two different types of data combined in this research were astrometry from the Hubble Space Telescope and radial velocity from ground-based telescopes.
Astrometry is the measurement of the positions and motions of celestial bodies. McArthur’s group used one of the Fine Guidance Sensors (FGSs) on the Hubble telescope for the task. The FGSs are so precise that they can measure the width of a quarter in Denver from the vantage point of Miami. It was this precision that was used to trace the star’s motion on the sky caused by its surrounding — and unseen — planets.
Radial velocity makes measurements of the star’s motion on the sky toward and away from Earth. These measurements were made over a period of 14 years using ground-based telescopes, including two at McDonald Observatory and others at Lick, Haute-Provence, and Whipple Observatories. The radial velocity provides a long baseline of foundation observations, which enabled the shorter duration, but more precise and complete, Hubble observations to better define the orbital motions.
The fact that the team determined the orbital inclinations of planets c and d allowed them to calculate the exact masses of the two planets. The new information told us that our view as to which planet is heavier has to be changed. Previous minimum masses for the planets given by radial velocity studies put the minimum mass for planet c at 2 Jupiters and for planet d at 4 Jupiters. The new, exact masses, found by astrometry are 14 Jupiters for planet c and 10 Jupiters for planet d.
“The Hubble data show that radial velocity isn’t the whole story,” Benedict said. “The fact that the planets actually flipped in mass was really cute.”
The fourth planet is so far out, that its signal does not reveal the curvature of its orbit.
The 14 years of radial velocity information compiled by the team uncovered hints that a fourth, long-period planet may orbit beyond the three now known. There are only hints about that planet because it’s so far out that the signal it creates does not yet reveal the curvature of an orbit. Another missing piece of the puzzle is the inclination of the innermost planet, b, which would require precision astrometry 1,000 times greater than Hubble’s, a goal attainable by a future space mission optimized for interferometry.
When considering how the solar system formed, there are a number of problems with the idea of planets just blobbing together out of a rotating accretion disk. The Nice model (and OK, it’s pronounced ‘niece’ – as in the French city) offers a better solution.
In the traditional Kant/Laplace solar nebula model you have a rotating protoplanetary disk within which loosely associated objects build up into planetesimals, which then become gravitationally powerful centres of mass capable of clearing their orbit and voila planet!
It’s generally agreed now that this just can’t work since a growing planetesimal, in the process of constantly interacting with protoplanetary disk material, will have its orbit progressively decayed so that it will spiral inwards, potentially crashing into the Sun unless it can clear an orbit before it has lost too much angular momentum.
The Nice solution is to accept that most planets probably did form in different regions to where they orbit now. It’s likely that the current rocky planets of our solar system formed somewhat further out and have moved inwards due to interactions with protoplanetary disk material in the very early stages of the solar system’s formation.
It is likely that within 100 million years of the Sun’s ignition, a large number of rocky protoplanets, in eccentric and chaotic orbits, engaged in collisions – followed by the inward migration of the last four planets left standing as they lost angular momentum to the persisting gas and dust of the inner disk. This last phase may have stabilised them into the almost circular, and only marginally eccentric, orbits we see today.
The hypothesized collision between 'Earth Mk 1' and Theia may have occurred late in rocky planet formation creating the Earth as we know it with its huge Moon of accreted impact debris
Meanwhile, the gas giants were forming out beyond the ‘frost line’ where it was cool enough for ices to form. Since water, methane and CO2 were a lot more abundant than iron, nickel or silicon – icy planetary cores grew fast and grew big, reaching a scale where their gravity was powerful enough to hold onto the hydrogen and helium that was also present in abundance in the protoplanetary disk. This allowed these planets to grow to an enormous size.
Jupiter probably began forming within only 3 million years of solar ignition, rapidly clearing its orbit, which stopped it from migrating further inward. Saturn’s ice core grabbed whatever gases Jupiter didn’t – and Uranus and Neptune soaked up the dregs. Uranus and Neptune are thought to have formed much closer to the Sun than they are now – and in reverse order, with Neptune closer in than Uranus.
And then, around 500 million years after solar ignition, something remarkable happened. Jupiter and Saturn settled into a 2:1 orbital resonance – meaning that they lined up at the same points twice for every orbit of Saturn. This created a gravitational pulse that kicked Neptune out past Uranus, so that it ploughed in to what was then a closer and denser Kuiper Belt.
The result was a chaotic flurry of Kuiper Belt Objects, many being either flung outwards towards the Oort cloud or flung inwards towards the inner solar system. These, along with a rain of asteroids from a gravitationally disrupted asteroid belt, delivered the Late Heavy Bombardment which pummelled the inner solar system for several hundred million years – the devastation of which is still apparent on the surfaces of the Moon and Mercury today.
Then, as the dust finally settled around 3.8 billion years ago and as a new day dawned on the third rock from the Sun – voila life!
