Prior to the discovery of exoplanets, astronomers assumed that our solar system's configuration was typical. But now, some 1,715 exoplanets later, we know that we're far from ordinary. So what passes for "normal" in the annals of solar systems? Here's what we know now.
Top Image: Artistic impression of Kepler 186f by Ron Miller (used with permission).
Astronomers used to believe that our solar system was representative of most — if not all — planetary systems.
"I think it's fair to say that most astronomers assumed that our solar system was unlikely to be an anomaly," says Eric B. Ford. He's with the Center for Exoplanets and Habitable Worlds, and a professor of astronomy and astrophysics at the Pennsylvania State University in State College, PA.
For some cases the assumption was explicit, but in other cases, we simply didn't think about planetary systems that were very different from our own," he told io9.
This made perfect sense at the time given the lack of empirical evidence to suggest otherwise. It was a valid application of the Copernican Principle (i.e. we shouldn't assume that we're special in the large scheme of things). Moreover, our solar system has a kind of logical and consistent flavor to it, one in which small, rocky planets are parked on the inside, and large gas giants hang out on the outside — and all in tidy, nearly circular orbits.
But as Ford points out in a paper that now appears in Proceedings of the National Academy of Sciences, we now know that our solar system is atypical in multiple ways. The discovery of exoplanets, he says, is providing many opportunities for improving our understanding of the formation and evolution of planetary systems.
No Two Are The Same
Planet-hunting projects, like NASA's Kepler mission and the California Planet Survey, are providing a slew of data for scientists to pour over. Back in February of this year, for example, NASA confirmed the existence of 715 new exoplanets — an announcement that increased the figure of known exoplanets by a factor of 70%. And it was less than two weeks ago that astronomers found the first Earth-sized, habitable zone planet. These days, the challenge for planetary astronomers is to make sense of all these findings.
Needless to say, and as Ford told me, it's still premature to put a single figure on of the total number of solar system architectures.
"We're still trying to make sense of all the planetary systems were finding, so there's not a single number," he noted. "That said, we are already starting to recognize that some architectures are showing up many times in our planet searches."
I asked Ford why there are so many different types of solar systems.
"When planets start forming somewhere, they don't know how big they will become," he explained. "So often multiple planets start growing up so close to each other that some sort of violent outcome is inevitable as they grow in mass."
Planet formation starts when gas and dust swirls around what will become a star. Some of the dust sticks together to form into pebbles, some pebbles grow into planetessimals, and planetessimals grow into planets.
"Of course, growing from a collection of gaseous atoms to a giant or even rocky planet, there are humongous changes in the sizes, masses and relevant physics," Ford told io9. "If it were simple, we would have already figured it out."
That said, Ford says there are several planet-types and solar system architectures that appear to be fairly common throughout the galaxy.
One of the first planetary models to emerge was one featuring a giant planet orbiting very closely to its host star — a so-called hot Jupiter. These systems typically feature a gap between the gas giant and any additional planets further away from the host star. Hot Jupiters feature orbital periods of up to several days and masses comparable to that of Jupiter or Saturn. As more observations have been made, however, astronomers have extended the median orbital period of these planets to about a year. They also know that they're rarer than initially presumed (a product of observational selection effects — and a problem that plagues every observation).
In terms of how hot Jupiters form, the going theory is that they begin as a rocky core far away from their host star, followed by the accretion of a gaseous envelope, and then the migration to an uncomfortably close orbital location (sometimes as close as ~2-5 day orbital periods).
Scientists aren't entirely sure why the migration occurs, but it could happen as the result of a gradual migration through the protoplanetary disk, or the result of dramatic gravitational effects (i.e. the excitation of a large eccentricity followed by tidal circularization). This process essentially cleans out the inner solar system by scattering any rocky planets in the inner planetary system into the star or the outer regions of the planetary system.
Giant Planets Near Snow Lines
Image: ESO/M. Kornmesser/Nick Risinger.
A second architecture involves giant planets with greater orbital periods, typically around 300 days to about four years. These systems contain several planets significantly larger than the Earth, but smaller than Neptune. They're packed tightly together, all orbiting closer to their star than the Earth is to our sun, and typically on nearly circular orbits.
Some of these planets may have formed farther out in the disk and migrated to their current location. And it may not be a coincidence that many of these planets reside near the water-snow line (the location in the protoplanetary disk where the solid surface density increases due to condensation of water ice).
"Because the snow line affects the formation of planetesimals, migration of giant planets
toward the snow line could be accommodated by a variety of migration models, including migration through a gaseous disk, migration via planetesimal scattering, or even via scattering of multiple planets or planetary cores," writes Ford in his paper.
Long-Period Giant Planets
Astronomers are also discovering giant planets with orbital distances ranging from a few AU to several AU. These systems contain about two to four giant planets orbiting their star with a distance comparable to that of Venus to our Sun, often with significantly elongated orbits.
These wide, eccentric orbits suggest that some of these planets have ejected others from the host planetary system. And in fact, studies have shown that this "planet scattering" effect can naturally produce a broad range of eccentric orbits observed by astronomers.
Another intriguing possibility is that planets in wide orbits formed around a star from a different solar system, eventually making their way to a new host star.
Super-Earths and Mini Neptunes
Some systems feature Neptune- and super-Earth mass objects at short orbital periods. These planets appear to be much more common than giant planets. Interestingly, most solar-type stars host a sub-Neptune size planet.
Short-Period Tightly Packed Inner Planetary Systems (STIPS)
But while mini-Neptunes may be extremely common, we're also learning that our galaxy is home to an abundance of systems with multiple planets. A typical system contains planets with relatively short orbits, ranging from about one to 100 days. These systems tend to be tightly packed, suggesting correlated orbital periods. The mass of most of these planets is dominated by rock, ice, or water, but not gas. Ford says these planets didn't accrete a rocky core before clearing the protoplanetary disk, and it's conceivable that their atmospheres are the result of outgassing (rather than accretion of gas from the disk).
Ford also told me about two other types: A giant planet on a very slow and wide orbit much further from its host star than any of the planets in our solar system are from the Sun, and a giant planet orbiting around a binary star system.
Life On Fairly Odd Planets
Ford's work also carries implications to the search for alien life.
"The history of how planets formed can have a significant impact on their habitability," he says. "For example, most of Earth's original water may have been lost to space. In that case, much of Earth's present day water may have been acquired by collisions with asteroids and comets. The rate of those collisions depends on the locations and masses of other planets, in our solar system Jupiter's orbit is particularly important. Therefore, we don't just want to find individual planets. Instead, we want to characterize all the planets orbiting a star and combine our knowledge about each of them to piece together their story."
Image: ESO/L. Calçada.
His paper thus proposes some interesting questions, such as, is it worth searching for signs of life on planets larger than the Earth and likely enshrouded in a dense atmosphere? And if so, what would we look for? Also, if we don't find any evidence of life, would that be a meaningful finding? Or would our search be so primitive that it's not really an interesting result?
Thankfully, we may be able to answer some of the questions in the coming years.
"You can expect that astronomers will searching for and find rocky planets orbiting bright and nearby stars," he says. "Those will be easier to study in more details. With ground-based direct imaging searches and the James Webb Space Telescope, we'll likely learn lots more about the atmospheres of Jupiter-size and Neptune-size planets."
Read the entire study at PNAS: "Architectures of planetary systems and implications for their formation ."
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