ORIGINAL: The Universe
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| About the image: Artist’s conception of the newly born alien planet LkCa 15 b and its parent star, which are found about 450 light-years from Earth in the constellation Taurus. CREDIT: Karen L. Teramura, UH IfA |
As if the immense distances weren't enough of a challenge to overcome when exoplanet hunting, there are other factors we have to work around. First, only certain objects emit light in the visible wavelengths. Most of them (particularly the cooler celestial objects) emit light in infrared, ultraviolent waves, x-rays or gamma rays. Another "bugaboo" happens to be observing stars that are caressed in thick pockets of interstellar gas and dust clouds that compose the bulk of the interstellar medium. Although these regions are beautiful to behold in the form of cosmic nebulae or supernovae remnants, they propose a significant amount of problems to astronomers when they're trying to hone in on specific stars that are encircled by disks of gas and dust debris; the raw materials that form rocky terrestrial planets such as Earth.
So, it should come of no surprise that these areas are of particular interest to astronomers in the search for "exo-Earth" planets. First, it's important to note that protoplanetary debris disks are thought to only exist for a short amount of time. Most of the dust will eventually be scattered from their respective solar system into interstellar space. Due in part to gravitational disturbances between any existing gas-giant planets as they tug on their parent star. After a relatively short time (cosmically speaking), most of the debris disks should be cleared and only replenished if, or when collisions between inner rocky planets and large asteroids or comets occurs.
In order for debris disks to exist around an older star, the system it's located in would have to be very stable. The environment would have to be relatively calm without the larger planets migrating inwards or causing any gravitational perturbations that would disrupt the rest of the planets. The same is needed for the formation of terrestrial planets to commence. Therefore, it's an obvious conclusion to draw, that these two outcomes would be correlated to one another in such a fashion. According to astronomer Sean Raymond from the Observatory of Bordeaux in France, "The most important implication of the findings is that, if I am right, then debris disks can act as signposts for systems with a high probability of having terrestrial planets and, in some cases, Earth-like planets."
To further explore his hypothesis, Raymond and his colleagues created planet formation models to simulate how inner debris disks surrounding a young star would coalesce into rocky planets like the four innermost planets in our solar system. What their models discovered; was that terrestrial planets form somewhat quickly; within 10 to 100 million years after the formation process begins. Most of which, form as a result of impacts between large planetesimal objects similar in size to Pluto, Ceres and Eris; to larger terrestrial protoplanets that range in size from the moon -- to a Mars-sized body. The gas-giants are thought to form even more quickly in an early time-frame of a solar system's evolutionary history. (within the first few million years of the star's protoplanetary disk forming) The inner dust disks don't have the proper amount of mass for the core accretion process to occur for gas-giants like Jupiter or Saturn. (Both are thought to harbor molten cores consisting of large quantities of iron, nickel or a combination of various metals, but the jury is still out on that one) Therefore, It's hypothesized that these gaseous planets must form much farther out in the solar system's outer disks than their rocky counterparts do, somewhere beyond the "snow-line," where the temperatures are cold enough for the hydrogen compounds to condense into icy grains.
If this in indeed correct, gas-giants could play a crucial role in influencing the rocky planets that are in the process of forming in the inner debris disk. To further investigate, Raymond and his team studied hundreds of simulations of a solar system that has an inner disk consisting of 500 planetesimal objects, three gas-giants similar in mass and composition to Jupiter and Saturn, an outer disk of planetesmial objects that are between 50 to 100 Earth masses; along with 50 planetary "embryos" that are nine Earth masses, but located between 1/2 to four AU (Earth - Sun distances) from the disks. The gas giants were each located between five and ten AU from their parent star, while the frozen planetesmial objects linger about 10 to 20 AU from there star, in what is almost certainly an area that's very similar to the Kuiper Belt for more reasons than just one.
After the system "evolved" for 100 million to 200 million years, hundreds of various simulations were ran that spanned a few weeks to six months for completion; he and his team discovered that the gravitational instabilities that reoccurred with the massive planets, can eliminate planetesimal objects and sometimes, they can even rearrange the positions and orbits of the planets in their solar system. This is especially interesting where hot-Jupiter exoplanets are concerned. Their formation and subsequent migration inwards (to dangerously close orbits, I might add) has been of great interest to astronomers. That's not all either. It was discovered that the width, composition and mass of the outer disks play a large role in whether a system is likely to host rocky planets along with the gas-giants. The wider and more massive the disk is, the better the chances are that planetesimal objects could potentially stabilize the gravitational eccentricities in the orbits of the gas giants, which in turn, greatly reduces the odds that a hot-Jupiter migrated inwards, causing the rocky planets to be slung to the outer trenches of the solar system where water would permanently remain in a frozen state. Even worse, sometimes this process causes the planets to be slung out of their solar system entirely, doomed to wonder about the galaxy with no parent star or sister planet in sight.
"Raymond and collaborators have identified a path that can guide us toward terrestrial exoplanets at Earth-like distances from their star," said planetary scientist Rory Barnes at the University of Washington, who did not take part in this research. "These planets are extremely challenging to detect, so any clues that narrow down the search is of great value."
"The good thing about this idea is that it's testable simply by searching for low-mass planets around stars with debris disks. That work is in progress," Raymond noted.
- Jaime
Source Material:
You can read more about Raymond's models in the original published paper:
"Earth’s twin could be hiding in a thick dust cloud:"
"Signposts of Planet Formation & Destruction:"
"Earth-like Alien Planets May Hide in Dust Around Distant Stars:"
"Jupiter Could Have Earth-like Rocky Core:"
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