To know more about the planets, we have to look to the stars
In February, NASA announced the first known system of seven Earth-sized planets around a single star. Located about 40 light-years, or 235 trillion miles, from Earth, the planets are outside of our solar system, and so are scientifically known as exoplanets. This exoplanet system is called TRAPPIST-1, named for The Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile.
We caught up with Steve Kawaler, an Iowa State astrophysicist, to get a better understanding of how planets are found.
"To know the planet you need to know the star," Kawaler, professor in the Department of Physics and Astronomy, said. "We don't observe the planets, we only observe the shadows of the planets as they cross the star. So anything we know about the star translates into more information about the planet."
Kawaler is part of a large collaboration of astrophysicists studying stars using seismology, much like observing the seismic waves propagated through the earth during and after an earthquake.
"Stars don't really undergo ‘starquakes’ per se, like the earth," Kawaler said. "But they're continually undergoing oscillations. Those oscillations are very low level, which is why the sun doesn't seem to be vibrating, but with sufficient precision you can see those vibrations of the shape and size of the star as changes of brightness."
By plotting the brightness over time, Kawaler can see these vibrations or oscillation frequencies of a star. That data leads to measuring the radius of the star, the luminosity of the star, the rotation rate, and the age of the star.
This sort of data also can shed light on planets that orbit the star. Knowing the star’s radius is vital to knowing the radius of the planet. The age of the star also gives the age of the planetary system, as well as how stable the system is — an older system is generally considered more stable. The internal structure of a star reveals how turbulent it is, which plays a role in the star’s interaction with the planet, affecting its orbit and magnetic interactions.
A picture is worth a thousand… pieces of data
Though the recent discovery was found from the ground, much of the data Kawaler studies comes from The Kepler Mission, a spacecraft that was launched in 2009 to survey a portion of our region of the Milky Way galaxy to discover dozens of Earth-size planets in or near the habitable zone and determine how many of the billions of stars in our galaxy have such planets.
Kepler orbits the sun at a slightly slower pace than the earth – 370 days – causing it to slowly increase distance from Earth. About the size of a minivan, the spacecraft holds a telescope about a meter in diameter in a steady location.
In the original mission, Kepler looked consistently at one area of the sky 100 square degrees in size, or about the size of a fist if you held it up to the sky. Kepler takes pictures of that swath of sky once every minute or thirty minutes with a 96-megapixel camera (the newest iPhone camera boasts eight megapixels).
"That's all we need," Kawaler said. "We just need those precise measures of how bright each star is in that field."
This data allows Kawaler to study the seismological data of a star, and it also gives other scientists looking for planets the data they need to see a planet traverse a star.
However, identifying a planet is a long process. To verify it's a planet, it must be observed traversing the star three times, with the same change in brightness and the same time gaps between each time. For a planet in an orbit like that of Earth, which traverses the sun once per year for a period of about 12 or 13 hours, that takes three years of data – the majority of which shows no sign of a planet at all.
"The data is overwhelmingly just of the star," Kawaler said. "For those who study the planets, that's not interesting, they want to know what transits are happening. For us, we want to study the stars — so we are happy to use the 99.99% of the data from Kepler that is not interesting to the scientists looking for planets. It's sort of the needle in the haystack: they're looking for the needle; we're eating the hay."
Keeping Kepler relevant
Because of the three-year requirement to find planets, Kepler was designed to last for four years. It took data of the same area of sky for exactly four years and one day before it ran into trouble. Then one of the three stabilizers that held the telescope steady broke. Through brilliant engineering, NASA was able to use the energy of the sun to keep the telescope relatively stable if it was pointed in the plane of the earth. This allowed the telescope to take pictures of the same area for about three months at a time before needing to be readjusted to stay in the same plane.
The new mission, dubbed K2, began to take pictures around the orbital plane of Earth. In a three-month window, planets couldn't be found in orbits like the Earth, so study shifted to low-mass stars, like the star in the TRAPPIST-1 system.
Low-mass stars are not as hot, making their habitable zone — the area with temperatures such that water would not boil away — much closer to the star. With a closer distance to the star, the planet may orbit the star in only a month or less, thus it could be found in a three-month window. Low-mass stars are also more plentiful than stars like our sun, so this opened the possibility of finding many more planets. To date, Kepler has found 2,330 planets, and photographed even more stars.
"It's been a lot of fun transitioning from doing one or two stars a year before Kepler to doing hundreds of stars at a pop," Kawaler said.
With all the new star data at his fingertips, Kawaler might be compared to a kid in a candy store.
"I grew up in the 60's which was the height of the UFO craze and the space race," he said. "That's what was imprinted in my brain as something that was worth doing. I still haven't outgrown that."
Editor’s note: If this story piqued your interest in Kawaler’s work, tune in to IPR’s River to River program (weekdays at noon), where he is featured as an occasional guest. Join the conversation as he shares his expertise in space and astrophysics and takes questions from listeners.