Artist's impression of Kepler-10c (foreground planet)
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Today at the American Astronomical Society conference in Boston, the Kepler team announced the confirmation of a new rocky planet in orbit around Kepler-10. Dubbed Kepler-10c, this planet is described as a “scorched, molten Earth.”
2.2 times the radius of Earth, Kepler-10c orbits its star every 45 days. Both it and its smaller, previously-discovered sibling 10b are located too close to their star for liquid water to exist.
Kepler-10c was validated using a new computer simulation technique called “Blender” as well as additional infrared data from NASA’s Spitzer Space Telescope. This method can be used to locate Earth-sized planets within Kepler’s field of view and could also potentially help find Earth-sized planets within other stars’ habitable zones.
This is the first time the team feels sure that it has exhaustively ruled out alternative explanations for dips in the brightness of a star… basically, they are 99.998% sure that Kepler-10c exists.
The Kepler-10 star system is located about 560 light-years away near the Cygnus and Lyra constellations.
Artist's concept of a free-floating Jupiter-like planet. (NASA / JPL-Caltech)
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We happen to live in a solar system where everything seems to be tucked neatly in place. Sun, planets, moons, asteroids, comets… all turning and traveling through space in relatively neat and orderly fashions. But that may not always be the case; sometimes planets can get kicked out of their solar systems entirely, banished to roam interstellar space without a sun of their own. And these “orphan planets” may be much more numerous than once thought.
Researchers in a joint Japan-New Zealand study surveyed microlensing events near the central part of our galaxy during 2006 and 2007 and identified up to 10 Jupiter-sized orphan worlds between 10,000 and 20,000 light-years away. Based on the number of planets identified and the area studied they estimate that there could literally be hundreds of billions of these lone planets roaming our galaxy….literally twice as many planets as there are stars.
“Although free-floating planets have been predicted, they finally have been detected, holding major implications for planetary formation and evolution models.”
– Mario Perez, exoplanet program scientist at NASA Headquarters in Washington.
From the NASA release:
Previous observations spotted a handful of free-floating, planet-like objects within star-forming clusters, with masses three times that of Jupiter. But scientists suspect the gaseous bodies form more like stars than planets. These small, dim orbs, called brown dwarfs, grow from collapsing balls of gas and dust, but lack the mass to ignite their nuclear fuel and shine with starlight. It is thought the smallest brown dwarfs are approximately the size of large planets.
On the other hand, it is likely that some planets are ejected from their early, turbulent solar systems, due to close gravitational encounters with other planets or stars. Without a star to circle, these planets would move through the galaxy as our sun and other stars do, in stable orbits around the galaxy’s center. The discovery of 10 free-floating Jupiters supports the ejection scenario, though it’s possible both mechanisms are at play.
“If free-floating planets formed like stars, then we would have expected to see only one or two of them in our survey instead of 10. Our results suggest that planetary systems often become unstable, with planets being kicked out from their places of birth.”
– David Bennett, a NASA and National Science Foundation-funded co-author of the study from the University of Notre Dame.
The study wasn’t able to resolve planets smaller than Saturn but it’s believed there are likely many more smaller, Earth-sized worlds than large Jupiter-sized ones.
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.
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).
The Extrasolar Planets Encyclopedia counted 548 confirmed extrasolar planets at 6 May 2011, while the NASA Star and Exoplanet Database (updated weekly) was today reporting 535. These are confirmed findings and the counts will significantly increase as more candidate exoplanets are assessed. For example, there were the 1,235 candidates announced by the Kepler mission in February, including 54 that may be in a habitable zone.
So what techniques are brought to bear to come up with these findings?
Pulsar timing – A pulsar is a neutron star with a polar jet roughly aligned with Earth. As the star spins and a jet comes into the line of sight of Earth, we detect an extremely regular pulse of light. Indeed, it is so regular that a slight wobble in the star’s motion, due to it possessing planets, is detectable.
The first extrasolar planets (i.e. exoplanets) were found in this way, actually three of them, around the pulsar PSR B1257+12 in 1992. Of course, this technique is only useful for finding planets around pulsars, none of which could be considered habitable – at least by current definitions – and, in all, only 4 such pulsar planets have been confirmed to date.
To look for planets around main sequence stars, we have…
The radial velocity method – This is similar in principle to detection via pulsar timing anomalies, where a planet or planets shift their star back and forth as they orbit, causing tiny changes in the star’s velocity relative to the Earth. These changes are generally measured as shifts in a star’s spectral lines, detectable via Doppler spectrometry, although detection through astrometry (direct detection of minute shifts in a star’s position in the sky) is also possible.
To date, the radial velocity method has been the most productive method for exoplanet detection (finding 500 of the 548), although it most frequently picks up massive planets in close stellar orbits (i.e. hot Jupiters) – and as a consequence these planets are over-represented in the current confirmed exoplanet population. Also, in isolation, the method is only effective up to about 160 light years from Earth – and only gives you the minimum mass, not the size, of the exoplanet.
To determine a planet’s size, you can use…
The transit method – The transit method is effective at both detecting exoplanets and determining their diameter – although it has a high rate of false positives. A star with a transiting planet, which partially blocks its light, is by definition a variable star. However, there are many different reasons why a star may be variable – many of which do not involve a transiting planet.
For this reason, the radial velocity method is often used to confirm a transit method finding. Thus, although 128 planets are attributed to the transit method – these are also part of the 500 counted for the radial velocity method. The radial velocity method gives you the planet’s mass – and the transit method gives you its size (diameter) – and with both these measures you can get the planet’s density. The planet’s orbital period (by either method) also gives you the distance of the exoplanet from its star, by Kepler’s (that is Johannes’) Third Law. And this is how we can determine whether a planet is in a star’s habitable zone.
It is also possible, from consideration of tiny variations in transit periodicity (i.e regularity) and the duration of transit, to identify additional smaller planets (in fact 8 have been found via this method, or 12 if you include pulsar timing detections). With increased sensitivity in the future, it may also be possible to identify exomoons in this way.
The transit method can also allow a spectroscopic analysis of a planet’s atmosphere. So, a key goal here is to find an Earth analogue in a habitable zone, then examine its atmosphere and monitor its electromagnetic broadcasts – in other words, scan for life signs.
Direct imaging of exoplanet Beta Pictoris b - assisted by nulling interferometry which removes Beta Pictoris' starlight from the image. The red flares are a circumstellar debris disk heated by the star. Credit: ESO.
To find planets in wider orbits, you could try…
Direct imaging – This is challenging since a planet is a faint light source near a very bright light source (the star). Nonetheless, 24 have been found this way so far. Nulling interferometry, where the starlight from two observations is effectively cancelled out through destructive interference, is an effective way to detect any fainter light sources normally hidden by the star’s light.
Gravitational lensing – A star can create a narrow gravitational lens and hence magnify a distant light source – and if a planet around that star is in just the right position to slightly skew this lensing effect, it can make its presence known. Such an event is relatively rare – and then has to be confirmed through repeated observations. Nonetheless, this method has detected 12 so far, which include smaller planets in wide orbits such as OGLE-2005-BLG-390Lb.
These current techniques are not expected to deliver a complete census of all planets within current observational boundaries, but do offer us an impression of how many there may be out there. It has been speculatively estimated from the scant data available so far, that there may be 50 billion planets within our galaxy. However, a number of definitional issues remain to be fully thought through, such as where you draw the line between a planet versus a brown dwarf. The Extrasolar Planets Encyclopedia currently set the limit at 20 Jupiter masses.
Anyhow, 548 confirmed exoplanets for only 19 years of planet spotting is not bad going. And the search continues.
An artist’s impression of Gliese 581d, an exoplanet about 20.3 light-years away from Earth, in the constellation Libra. Credit: NASA
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When last we checked in on Gliese 581d, a team from the University of Paris had suggested that the popular exoplanet, Gliese 581d may be habitable. This super-Earth found itself just on the edge of the Goldilocks zone which could make liquid water present on the surface under the right atmospheric conditions. However, the team’s work was based on one dimensional simulations of a column of hypothetical atmospheres on the day side of the planet. To have a better understanding of what Gliese 581d might be like, a three dimensional simulation was in order. Fortunately, a new study from the same team has investigated the possibility with just such an investigation.
The new investigation was called for because Gliese 581d is suspected to be tidally locked, much like Mercury is in our own solar system. If so, this would create a permanent night side on the planet. On this side, the temperatures would be significantly lower and gasses such as CO2 and H2O may find themselves in a region where they could no longer remain gaseous, freezing into ice crystals on the surface. Since that surface would never see the light of day, they could not be heated and released back into the atmosphere, thereby depleting the planet of greenhouse gasses necessary to warm the planet, causing what astronomers call an “atmospheric collapse.”
To conduct their simulation the team assumed that the climate was dominated by the greenhouse effects of CO2 and H2O since this is true for all rocky planets with significant atmospheres in our solar system. As with their previous study, they performed several iterations, each with varying atmospheric pressures and compositions. For atmospheres less than 10 bars, the simulations suggested that the atmosphere would collapse, either on the dark side of the planet, or near the poles. Past this, the effects of greenhouse gasses prevented the freezing of the atmosphere and it became stable. Some ice formation still occurred in the stable models where some of the CO2 would freeze in the upper atmosphere, forming clouds in much the same way it does on Mars. However, this had a net warming effect of ~12°C.
In other simulations, the team added in oceans of liquid water which would help to moderate the climate. Another effect of this was that the vaporization of water from these oceans also produced warming as it can serve as a greenhouse gas, but the formation of clouds could decrease the global temperature since water clouds increase the albedo of the planet, especially in the red region of the spectra which is the most prevalent form of light from the parent star, a red dwarf. However, as with models without oceans, the tipping point for stable atmospheres tended to be around 10 bars of pressure. Under that, “cooling effects dominated and runaway glaciation occurred, followed by atmospheric collapse.” Above 20 bars, the additional trapping of heat from the water vapor significantly increased temperatures compared to an entirely rocky planet.
The conclusion is that Gliese 581d is potentially habitable. The potential for surface water exists for a “wide range of plausible cases”. Ultimately, they all depend on the precise thickness and composition of any atmosphere. Since the planet does not transit the star, spectral analysis through transmission of starlight through the atmosphere will not be possible. Yet the team suggests that, since the Gliese 581 system is relatively close to Earth (only 20 lightyears), it may be possible to observe the spectra directly in the infrared portion of the spectra using future generations of instruments. Should the observations match the synthetic spectra predicted for the various habitable planets, this would be taken as strong evidence for the habitability of the planet.
One of the first known stars to host an extrasolar planet, was that of 55 Cancri. The first planet in this system was reported in 1997 and today the system is known to host at least five planets, the inner most of which, 55 Cnc e, was recently discovered to transit the star, giving new information about this planet.
55 Cnc is an interesting system in many respects. Being a mere 41 lightyears from the Earth, the system is composed of a primary, yellow dwarf star in a wide binary orbit (1,000 AU) with a red dwarf. The planetary system lies within this orbit. The primary star is just brighter than 6th magnitude meaning it is visible to the naked eye under good viewing conditions.
One of these planets, 55 Cnc e, was discovered in this system via radial velocity measurements in 2004. At that point, the planet was reported to have a period of 2.8 days, and a minimum mass of 14.2 times the mass of the Earth. However, in 2010, Rebekah Dawson and Daniel Fabrycky from the Harvard-Smithsonian Center for Astrophysics argued that gaps in the observational period skewed the statistics and the true period the planet should be a short 0.7365 days.
One of the results of this was that the planet would have to orbit closer to the parent star. In turn, this increased the likelihood that the planet could transit the star from 13% to 33%. A team led by Joshua Winn from the Massachusetts Institute of Technology went searching for this faint transit and report its detection in a recent paper. But while the star itself is one of the brightest stars in our sky to harbor known extrasolar planets, the eclipse is far from visible without precise observations, changing by only 0.0002%, one of the smallest changes known. The timing of the eclipses confirms that correction by Dawson and Fabrycky and adds new information about the body.
Given the radius determined as well as the mass, the team was able to estimate the structure of the planet and report that the mass is 8.57 ± 0.64 Earth masses. The reported radius is 1.63 ± 0.16 times that of Earth, and the density is 10.9 ± 3.1 g cm-3 (the average density of Earth is 5.515 g cm-3). This places the planet firmly into the categories of a rocky super-Earth.
The team also explores whether or not the planet could retain an atmosphere in such a close orbit (only three times the radius of the star itself). At this close range, the planet would likely be tidally locked and with an albedo typical of rocky planets, the planet would likely have an average temperature of nearly 2970 K (5,000° F). If the planet were able to redistribute the heat, it may be as low as 2100 K (3,300° F). Either way, a planet of such mass would have difficulty retaining any primordial, gaseous atmosphere. However, the team reports that it may be possible for volcanic activity to create a thin atmosphere of high molecular weight components.
While this new report adds precious little in the grand scheme of the rapidly growing body of knowledge of exoplanets, the authors close with the note that, “there is some pleasure in being able to point to a naked-eye star and know the mass and radius of one of its planets.”
An artist’s impression of WASP 12-b being slowly consumed as a result of its ridiculously tight orbit around its star. More recent observations suggests the exoplanet has a magnetosphere which may be partially protecting it from stellar wind erosion. Credit: NASA.
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New observations of one of the biggest and hottest known exoplanets in the galaxy, WASP 12b, suggest that it is generating a powerful magnetic field sufficient to divert much of its star’s stellar wind into a bow shock wave.
Like exoplanets themselves, the discovery of an exo-magnetosphere isn’t that much of a surprise – indeed it would be a surprise if Jovian-type gas giants didn’t have magnetic fields, since the gas giants in our own backyard have quite powerful ones. But, assuming the data for this finding remains valid on further scrutiny, it is a first – even if it is just a confirming-what-everyone-had-suspected-all-along first.
WASP-12 is a Sun-like G type yellow star about 870 light years away from Earth. The exoplanet WASP-12b orbits it at a distance of only 3.4 million km out, with an orbital period of only 26 hours. Compare this to Mercury’s orbital period of 88 days at a 46 million kilometer distance from the Sun at orbital perihelion.
So habitable zone, this ain’t – but a giant among gas giants ploughing through a dense stellar wind of charged particles sounds like an ideal set of circumstances to look for an exo-magnetosphere.
The bow shock was detected by an initial dip of the star’s ultraviolet light output ahead of the more comprehensive dip which was produced by the transiting planet itself. Given the rapid orbital speed of the planet, some bow wave effect might be expected regardless whether or not the planet generates a strong magnetic field. But apparently, the data from WASP 12-b best fits a model where the bow shock is produced by a magnetic, rather than just a dynamic physical, effect.
The finding is based on data from the SuperWASP (Wide Angle Search for Planets) project as well as Hubble Space Telescope data. Team leader Dr. Aline Vidotto of the University of St. Andrews said of the new finding. “The location of this bow shock provides us with an exciting new tool to measure the strength of planetary magnetic fields. This is something that presently cannot be done in any other way.”
Although WASP 12b’s magnetic field may be prolonging its life somewhat, by offering some protection from its star’s stellar wind – which might otherwise being blowing away its outer layers – WASP 12-b is still doomed due to the gravitational effects of the close-by WASP 12 star which has already been observed to be drawing material from the planet. Current estimates are that WASP 12-b will be completely consumed in about 10 million years.
WASP 12-b is not only one of the hottest hot Jupiters we've found, but also one of the biggest (although this may be largely a result of expansion due to heating).
There is at least one puzzle here, not really testable from such a distance. Presuming that a planet so close to its star is probably tidally-locked, it would not be spinning on its axis – which is generally thought to be a key feature of planets generating strong magnetic fields – at least the ones in our Solar System. This may need something like an OverwhelminglySuperWASP to investigate further.
A disk of debris around a star is a likely indicator of planets. A disk of debris with a wildly atypical chemistry could mean aliens.
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Recently, some researchers speculated on what types of observational data from distant planetary systems might indicate the presence of an alien civilization, determined that asteroid mining was likely to be worth looking for – but ended up concluding that most of the effects of such activity would be difficult to distinguish from natural phenomena.
And in any case, aren’t we just anthropomorphizing by assuming that intelligent alien activity will be anything like human activity?
Currently – apart from a radio, or other wavelength, transmission carrying artificial and presumably intelligent content – it’s thought that indicators of the presence of an alien civilization might include:
• Atmospheric pollutants, like chlorofluorocarbons – which, unlike methane or molecular oxygen, are clearly manufactured rather than just biogenically produced
• Propulsion signatures – remember how the Vulcans detected humanity in First Contact (or at least they decided we were worth visiting after all, despite all the I Love Lucy re-runs)
• Stellar engineering – where a star’s lifetime is artificially extended to maintain the habitable zone of its planetary system
• Dyson spheres – or at least their more plausible off-shoots, such as Dyson swarms.
And perhaps add to this list – asteroid mining, which would potentially create a lot of dust and debris around a star on a scale that might be detectable from Earth.
There is a lot of current interest in debris disks around other stars, which are detectable when they are heated up by the star they surround and then radiate that heat in the infra-red and sub-millimeter wavelengths. For mainstream science, debris disk observations may offer another way to detect exoplanets, which might produce clumping patterns in the dust through gravitational resonance. Indeed it may turn out that the presence of a debris disk strongly correlates with the existence of rocky terrestrial planets in that system.
But now going off the mainstream… presuming that we can eventually build up a representative database of debris disk characteristics, including their density, granularity and chemistry derived from photometric and spectroscopic analysis, it might become possible to identify anomalous debris disks that could indicate alien mining activities.
Some recent astronomy pareidolia. Not an alien mining operation on Mercury, but a chunk of solidified ejecta commonly found in the center of many impact craters. Credit: NASA.
For example, we might see a significant deficiency in a characteristic element (say, iron or platinum) because the aliens had extracted these elements – or we might see an unusually fine granularity in the disk because the aliens had ground everything down to fine particles before extracting what they wanted.
But surely it’s equally plausible to propose that if the aliens are technologically advanced enough to undertake asteroid mining, they would also do it with efficient techniques that would not leave any debris behind.
The gravity of Earth makes it easy enough to just blow up big chunks of rock to get at what you want since all the debris just falls back to the ground and you can sort through it later for secondary extraction.
Following this approach with an asteroid would produce a floating debris field that might represent a risk to spacecraft, as well as leaving you without any secondary extraction opportunities. Better to mine under a protective canopy or just send in some self-replicating nanobots, which can separate out an enriched chunk of the desired material and leave the remainder intact.
If you’re going to play the alien card, you might as well go all in.
The grass may definitely not be greener on some alien worlds, suggests a new study from the UK. For example, planets in double-star systems could have grey or black vegetation.
Researcher Jack O’Malley-James of the University of St Andrews in Scotland worked out how photosynthesis in plants is affected by the color of the light they receive. On Earth, most plants have evolved to be green in order to take advantage of the yellowish color of the sunlight that’s received on the surface of our planet. (Our Sun, classified as a “Population I yellow dwarf star”, would look bright white from space but our atmosphere makes it appear yellow.) There are lots of other stars like our Sun in the Universe, and many of them are in multiple systems sharing orbits with other types of stars…red dwarfs, blue stars, red giants, white dwarfs…stars come in many different colors depending on their composition, age, size and temperature. We may be used to yellow but nature really has no preference! (Although red dwarfs happen to be the garden variety star in our own galaxy.)
Terrestrial examples of dark-colored plants
Planets that orbit within these multiple systems and exist within the habitable “Goldilocks” zone (and we are finding more and more candidates every day!) could evolve plants that depend on suns with different colors than ours. Green does a good job powering photosynthesis here, but on a planet orbiting a red dwarf and Sun-like star plants could very well be grey or black to absorb more light energy, according to O’Malley-James.
“Our simulations suggest that planets in multi-star systems may host exotic forms of the more familiar plants we see on Earth. Plants with dim red dwarf suns for example, may appear black to our eyes, absorbing across the entire visible wavelength range in order to use as much of the available light as possible.”
– Jack O’Malley-James, School of Physics and Astronomy, University of St Andrews
The study takes into consideration many different combinations of star varieties and how any potential life-sustaining planets could orbit them.
In some instances different portions of a planet may be illuminated by a differently-colored star in a pair…what sorts of variations in plant (and subsequently, animal) evolution could arise then?
And it’s not just the colors of plants that could evolve differently. “For planets orbiting two stars like our own, harmful radiation from intense stellar flares could lead to plants that develop their own UV-blocking sunscreens, or photosynthesizing microorganisms that can move in response to a sudden flare,” said O’Malley-James.
Kermit may have been right all along…being green might really not be easy!
A circumstellar disk of debris around a matured stellar system may indicate that Earth-like planets lie within. LUVOIR will be able to see inside the disk to watch planets forming.
Credit: NASA
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Recent modeling of Sun-like stars with planetary systems, found that a system with four rocky planets and four gas giants in stable orbits – and only a sparsely populated outer belt of planetesimals – has only a 15 to 25% likelihood of developing. While you might be skeptical about the validity of a model that puts our best known planetary system in the unlikely basket, there may be some truth in this finding.
This modeling has been informed by the current database of known exoplanets and otherwise based on some prima facie reasonable assumptions. Firstly, it is assumed that gas giants are unable to form within the frost line of a system – a line beyond which hydrogen compounds, like water, methane and ammonia would exist as ice. For our Solar System, this line is about 2.7 astronomical units from the Sun – which is roughly in the middle of the asteroid belt.
Gas giants are thought to only be able to form this far out as their formation requires a large volume of solid material (in the form of ices) which then become the cores of the gas giants. While there may be just as much rocky material like iron, nickel and silicon outside the frost line, these materials are not abundant enough to play a significant role in forming giant planets and any planetesimals they may form are either gobbled up by the giants or flung out of orbit.
However, within the frost line, rocky materials are the dominant basis for planet forming – since most light gas is blown out of the region by force of the stellar wind and other light compounds (such as H2O and CO2) are only sustained by accretion within forming planetesimals of heavier materials (such as iron, nickel and silicates). Appreciably-sized rocky planets would probably form in these regions within 10-100 million years after the star’s birth.
So, perhaps a little parochially, it is assumed that you start with a system of three regions – an inner terrestrial planet forming region, a gas giant forming region and an outer region of unbound planetesimals, where the star’s gravity is not sufficient to draw material in to engage in further accretion.
From this base, Raymond et al ran a set of 152 variations, from which a number of broad rules emerged. Firstly, it seems that the likelihood of sustaining terrestrial inner planets is very dependent on the stability of the gas giants’ orbits. Frequently, gravitational perturbations amongst the gas giants results in them adopting more eccentric elliptical orbits which then clears out all the terrestrial planets – or sends them crashing into the star. Only 40% of systems retained more than one terrestrial planet, 20% had just one and 40% had lost them all.
The Moon has retained a comprehensive record of the Late Heavy Bombardment from 4.1 to 3.8 billion years ago - resulting from a reconfiguration of the gas giants. As well as clearing out much of debris disk of the early Solar System, this reconfiguration flung material into the inner solar system to bombard the rocky planets.
Debris disks of hot and cold dust were found to be common phenomena in matured systems which did retain terrestrial planets. In all systems, primal dust is largely cleared out within the first few hundred million years – by radiation or by planets. But, where terrestrial planets are retained, there is a replenishment of this dust – presumably via collisional grinding of rocky planetesimals.
This finding is reflected in the paper’s title Debris disks as signposts of terrestrial planet formation. If this modeling work is an accurate reflection of reality, then debris disks are common in systems with stable gas giants – and hence persisting terrestrial planets – but are absent from systems with highly eccentric gas giant orbits, where the terrestrial planets have been cleared out.
Nonetheless, the Solar System appears as unusual in this schema. It is proposed that perturbations within our gas giants’ orbits, leading to the Late Heavy Bombardment, were indeed late with respect to how other systems usually behave. This has left us with an unusually high number of terrestrial planets which had formed before the gas giant reconfiguration began. And the lateness of the event, after all the collisions which built the terrestrial planets were finished, cleared out most of the debris disk that might have been there – apart from that faint hint of Zodiacal light that you might notice in a dark sky after sunset or before dawn.
