When NASA Kepler Space Observatory scientists announced the discovery of over 1,200 planets around other stars last week, my colleague, veteran science writer Dennis Overbye, wrote in the New York Times that astronomers have broken open a Milky Way piñata and more planets fell out than they could imagine!
But a less publicized part of this story I’d call: Astronomers Open a “Six-Pack.” That is, a family of six planets in whirling blender orbits encircle a star called Kepler-11.
Or perhaps a better characterization is “six-packed” because this collection of planets whip around their sun-like star in very tight racetrack orbits, five of which are crammed inside the radius of Venus’s orbit about the sun.
Except for the fact the planets’ orbits are all remarkably coplanar, which makes the Kepler 11 system structurally much like our solar system, there is little else that makes this system conventional. That is, if “conventional” means anything anymore in the fast-paced exoplanet gold rush now underway.
Popular astronomy textbooks of 20 years ago speculated that alien stellar systems would be have planets spaced essentially space like ours, in a sort of harmonic, as described in the purely empirical Bode’s Law.
But this latest finding from Kepler shows that nothing is the “norm” among stellar systems. The variables affecting the architecture of planetary systems must be so numerous, that any kind of orbital pattern may emerge. And we are left to ask once again: is our solar system the exception of the rule?
With orbital periods ranging from 10 to 114 days, the planets strongly gravitationally tug on each other because of their close proximity to each other. If they were any closer the system would be dynamically unstable and fly apart. Compared to our solar system, this looks like an interstellar NASCAR event with bodies whipping around in tight racetrack orbits.
Physically, the planets fill a little understood gray category that falls between Earth and Neptune — from 2 to 13 times Earth’s mass, and 2 to 5 times our planet’s diameter. So are they super-Earths or mini-Neptunes?
By timing how the planet orbital periods slightly change to due the gravitational tug-of-war with their companions, the Kepler team was able to calculate their density and masses.
Kepler scientist Jack Lissauer compared the planets to having the density of a marshmallow. In fact three of them have a lower density than water — so they would float in a big-enough bathtub. Structurally they may be like a marshmallow with a denser hard candy inside he said. Or imagine a giant Nerf ball 12,000 miles across.
Why did so many planets form so close to their star? It’s a mystery says Lissauer, but it should give us more insights into the elusive planet formation process.
Perhaps the circumstellar disk of dust and gas the planets agglomerated out of was much more massive than the one that gave birth to our solar system 4.6 billion years ago. The Kepler-11 disk perhaps had so much hydrogen gas, that the planets were able to gravitationally suck it up within a few million years before the newly ignited star blew the rest away.
The newborn planets must have quickly circularized their orbits deep within the star’s gravitational field. The temperature is too high for ices to have ever condensed, so the smaller planets could be largely made up of water.
This would be an intriguing system to visit. Neighboring planets would be truly brilliant gems in the nighttime, and perhaps even the daytime sky. One planet may occasionally pass so close to another world it would appear as big as the full moon.
Any alien astronomy professors living at Kepler-11 would instruct their students about their equivalent of our ecliptic plane and Zodiacal constellations. But the Kepler-11 planets probably can’t host life as we know it, unless it has the consistency of the Stay Puft Marshmallow Man.
Unfortunately the system is so far away that short of inventing an interdimesional shortcut though space and time, there is no way a probe from Earth would ever get there in any reasonable period.
Kepler 11 will forever remain on the dim periphery of “fascinating” planetary systems — as Mr. Spock might say.
Image credit: NASA