Alien Planetary Systems Are Both Familiar and Surprising
Over the past 15 years much planet-hunting has focused on simply cataloging individual worlds whirling about other stars. This search was turbo-boosted last week. Two teams of astronomers reported that they are at the point where they can study the architecture of entire planetary systems.
By being able to characterize entire planetary systems — from Jovian gas giants to Earth-sized planets — we will gain important insights into the construction of alien solar systems in our galactic neighborhood.
In what seemed like a battle of press releases, last Tuesday the European Southern Observatory announced that five confirmed planets are circling the sun-like star HD10180, which is just 27 light-years away in the southern constellation Hydrus.
Two days later, NASA's Kepler space observatory reported possibly three planets (two of which have been confirmed) around a star called Kepler-9, which is a whopping 2,000 light-years away in the summer constellation Lyrae. Two of the confirmed Kepler planets are the size of Saturn. This adds to the fifteen planetary systems known to have at least three planets.
I find the HD 10180 system more intriguing than Kepler-9 because it has five Neptune-mass planets huddled close to the star — within the radius of Mars’ orbit. The outermost of these, HD 10180g, is within this star’s habitable zone where liquid water would remain stable. Though the planet is a 24 Earth-mass gas giant, it probably has a family of moons. Some may be big enough to be habitable by possessing an atmosphere and seas. This might be a great place to go looking for the blue-skinned "Avatar" aliens.
We see stark similarities and differences when comparing both systems to our solar system. The layout of the worlds offers insights into the assembly and evolution of planetary systems:
The planets in the Kepler-9 system follow orbits that are coplanar. Kepler detects them passing in front of their star. This means their orbits are nearly edge-on to our line-of-sight. This solidifies the centuries-old notion that our solar system — also coplanar — formed by the agglomeration of dust in a flattened disk. The HD 10180 system was studied through
Both systems have one "candidate" super-Earth. But these are no places to call home. They each orbit so close to their star that they might have 3,000 degree Fahrenheit oceans of molten magma. The Kepler-6 super-Earth is 1.5 times the radius of our planet. The HD 10180 super-Earth is 1.4 times the mass of Earth. This is the smallest exoplanet found to date along the road to pinpointing an Earth clone out there.
Each system has gas giant planets crowding in close to their stars. This means that the worlds migrated inward after amassing ices and gasses at a farther and colder distance from the star. This could have been caused by drag within a thick gas disk, or due to a gravitational billiards game with a clutter of smaller bodies. By contrast, dynamical simulations show that Jupiter barely migrated at all after it formed. But these simulations imply that our solar system is a fluke — only 10 percent of star systems may have non-migrating planets.
What's intriguing is that the HD 10180 planets are roughly spaced in the same proportions as our solar system. This suggests there is some common type of resonance in a planet-forming disk that builds worlds according to an empirical relationship called the
The mapping of these two systems will also allow us to eventually put our solar system into a broader context and determine how typical it is among what seems to be incredible diversity among planetary systems.
HD 10180 would be an intriguing system for a far-future interstellar probe to reconnoiter. The artificially intelligent machine would be like the Saturn Cassini mission on steroids. The probe would tour all the moons and planets in HD 10180. It would obediently send back streams of spellbinding pictures to the descendants of its earthbound builders — who would have to patiently wait 27 years for the first snapshot following the probe’s arrival. The probe might dispatch nanobot landers to do biology experiments on the surfaces of earth-like moons.
No doubt other bizarre and wondrous stellar systems await planet-hunters. It's a booming industry.