As the Kepler observatory’s March 5th launch approaches,
there is building interest on the Internet and in the news media on its planned
historic census of Earthlike planets in our Milky Way galaxy.
What has been largely in the shadow of this grand quest is
the fact that Kepler will carve out new "discovery space" in characterizing
the sunlike stars that the planets orbit.
"Kepler is the stealth stellar physics mission,"
quipped stellar astronomer Dave Soderblom (Space Telescope Science Institute). Kepler will scrutinize 1,000 stars measuring small precise fluctuation in their brightness that are previously unheard of for observing any star
other than the sun. "This is a domain we have never entered before,"
adds Ron Gilliland (STScI), a science investigator on Kepler.
A lot of our stellar physics is derived from monitoring the sun with dedicated ground and space based facilities. Solar observatories do helioseismology, where they measure light
fluctuations on the sun's surface caused by acoustic waves rippling across the sun's visible
surface, like a drumstick hitting a bass drum. These penetrating waves
can be used to probe the sun's interior much like seismic waves are used to
probe Earth's interior.
But imagine if you grew up alone on a deserted island and had no idea
how other people looked and behaved.
The sun is just one very nearby example of a star. And, we simply don't
know much about other stars, and their behavior down to a time scale of
minutes.
Kepler will scrutinize 1,000 stellar candidates, out of a
170,000 stars in its galactic field-of-view, to take their pulse at one-minute
intervals. This will yield an unprecedented new census of stellar ages, sizes,
rotation rates, and even chemical composition. This will be an important prerequisite
before a definitively Earth-sized planet is identified. You need to know the star's
size for relative comparison with a planet's true diameter.
And, all this can be accomplished simply by looking at tiny fluctuations
of 1/10 of a percent of the star's light -- which is what Kepler was precisely
designed to do.
As with our sun, what's happening on the star's surface is a
deep probe of its interior. Star are essentially big acoustic chambers where
energy from the top of upwelling convection cells of hot gas sends
reverberations through the planet like hitting a bell with a hammer. Essentially, heat energy is turned into mechanical energy, which we've all seen demonstrated in those novelty drinking toy birds.
The interior reverberations bounce off the dense stellar
core of accumulating helium ash - the byproduct of nuclear fusion in healthy solar-type stars.
So by measuring these seismic waves astronomers can get the core's "shoe
size." As a star ages, the core grows in size -- like a filling trashcan with
waste. The bigger the core, the older the star.
Knowing the interior structure yields the star's density,
which in turn can be used to calculate the star's physical size.
Kepler's photometry will even identify the waxing and waning
of dark starspots. Sunspots are a powerful diagnostic of activity on our sun
because they are linked to the behavior of the sun's magnetic field. As the sun
rotates the field tangles up like spaghetti until field lines snap like rubber
bands, unleashing deadly solar flares. The magnetic fields are driven by sun's differential
rotation - where the equator spins faster that the poles. With Kepler we'll be
able to gauge differential rotation rates of other stars by watching starspots,
and see how their behavior matches a star's age.
Because exoplanets are expected to take three to six hours
to transit stars, Kepler's normal data acquisition mode will look at the
stellar heartbeat once every 30 minutes. But for stellar seismology, Kepler
will switch into high gear by taking measurements at a frantic rate of one-minute
intervals.
This will be mandatory for stars where transiting planets
are found. Kepler's stellar seismology will make a precise measurement of the
stars' diameters.
Only when that data are collected and analyzed will astronomers
be able to nail the true size of the transiting planet. This is turn will improve our understading of the nature of the exoplanet candidates. What of the physics?
How many exoplanets will be big and puffy, or small and rocky?
For its modest cost and size, the Kepler mission promises a quantum leap in our understanding of our galactic neighbors - both
stars and planets.
Photo Credit: Carter Roberts
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