A Tight Little Planet System, Dancing to the Tune of Gravity

Careful measurements using information from the Kepler planet-hunting spacecraft has allowed astronomers to figure out one of the tightest and most synchronized planet systems ever found.

Kepler 80 is a dim star in the constellation of Cygnus the swan, 1100 light years away, that has five planets orbiting it. That by itself is not unusual -- astronomers have now discovered a number of stars with 7, 6, or 5 planets around them and more are likely to be discovered as time goes by. What IS unusual is how tightly these five planets all hug their star.

The five planets take one, three, four, seven and nine earth days to orbit the star. (Think about that, their year is 1, 3, 4, 7, or 9 DAYS! If you lived on the inner planet, you would be 365 of its years old after one Earth year had gone by.)

Even more interesting, the outer 4 planets have orbits that are synchronized -- they are in tune, you might say. They return to the same arrangement of their positions around their star every 27 days. This is called a gravitational resonance and it's something gravity prefers to do when bodies can exchange energy.

Think of gravity like your uncle who is careful with his money -- they both prefer arrangements that are cheap! For example, gravity, when it can, tries to make bodies round, because then the pull on every point on the surface is the same -- which is the arrangement that takes the least energy.

When objects in space move around or with each other in a synchronized way, where one orbit is related to the other with numbers like 1 to 2 or 2 to 3, this requires less energy too. Pluto, for example, shows an extreme example of such resonances: Pluto takes 6.4 Earth days to spin and Pluto's big moon Charon takes 6.4 days to orbit the dwarf planet. So Pluto's day and month are the same length. Charon also takes 6.4 days to spin, so we have a 1 to 1 to 1 resonance of these motions.

Calculations show that the planets dancing in resonance around Kepler 80 are rocky world like our own Earth, although at least some of its planets are 4 to 6 times Earth's size. Imagine these giant Earths whipping around that little star in Cygnus in 1 to 9 days, year in and year out, lining up every 27 days in a kind of cosmic dance. Nature seems to permit so many odd and wonderful line-ups of planets out there -- science fiction is having trouble catching up to science fact.

(Just for comparison, the innermost planet in our own planetary system, Mercury, takes 88 days to orbit the Sun, and it is much smaller than Earth. So we have no large planets tightly hugging our star -- but a number of other stars besides Kepler 80 show such tight arrangements.)

Billions of Earth-like Planets

This week, the University of California, Berkeley and NASA's Kepler Telescope project jointly announced that the ongoing discovery of planets around other stars had yielded some exciting statistics: It now appears that one out of five Sun-like stars has an Earth-like planet!

One out of five! This means there are likely to be BILLIONS of earth-sized planets orbiting at comfortable distances from BILLIONS of stars in our Milky Way Galaxy. All of us involved with SETI -- the Search for Extra-Terrestrial Intelligence -- are, of course, thrilled to hear this news. Our hope is that among all those planets, there are some where intelligent creatures with an interest in astronomy have developed and perhaps enjoy their own blogs with astronomy news like we do.

The latest stats about known planets around other stars are also record-breaking.  As of early November 2013, we have found 1039 planets around 787 stars beyond our solar system! (There are 173 stars so far where we have discovered more than one planet in the same system.)  In addition, the Kepler mission (which is searching for such planets from space) already has over 3000 candidate planets which are still being checked out!  And they still have a whole year's worth of data to go through.

I was interviewed on KQED, the San Francisco Bay Area public radio station, by Michael Krasny, the host of the Forum program, about all this -- and we were joined by the Berkeley graduate student who had done the basic work of making the estimates. If you want to hear the interview, it is available at:
http://www.kqed.org/a/forum/R201311060930 

Earth-like Planet Discovered in a Ridiculous Orbit

 Astronomers from two continents made a startling announcement last week. They had found a world similar in size and composition to the Earth that orbited its star in only eight and a half hours.... Just think about that for a minute. Our planet takes 365 and a quarter days to complete its orbit. The innermost planet in our system, Mercury, takes 88 days to circle the Sun. The new planet, designated Kepler 78b, takes only about a third of one of our days to orbit its star. In other words, a year on Kepler 78b is only 8.5 hours long -- a two-Earth-year-old toddler on this alien world would already be 2,063 years old in local time!

The planet is a bit larger in size than our Earth, but made of dense rock like our inner planets (and not gas and liquid, like our outer planets.) Since it circles so close to its star, it must be torridly hot, so we imagine its surface is molten rock and not solid like our own crust.  Some are calling it a lava planet.

How can astronomers know so much about a distant world like Kepler 78b? As its name implies, the planet was discovered around a faint star in the constellation of Orion by the Kepler telescope in space. Kepler's camera measures the size of a planet when the planet is seen going across the face of its star and diminishing the star's light briefly. But that can only tell us how big the planet is across, and not what kind of material it's made of.

But once Kepler found the planet, astronomers in the U.S. and Europe used giant telescopes on the ground to find the tiny wiggle the pull of the planet causes in the motion of its star. This "wiggle method" tells us how much pull (gravity or mass) the planet has.

When astronomer combine the size of the planet from Kepler and the mass of the planet from the wiggle method, they can calculate the planet's "density" (mass per unit volume). In this case, all the measurements made it clear this was a dense world, made of rock, just like our Earth.

The mystery is: how did an Earth get SO outrageously close to its star. If it was falling in, what made it stop? We know it couldn't have been born so close to the star, because the star was larger when it was young, and the planet would have been inside the star, where no planet can exist. Kepler 78b is part of a group of strange planets Kepler has been discovering -- all of them too close to their stars for their own good and for our peace of mind.