Astronomer studies far-off worlds through 'characterization by proxy'

Apr 26, 2013 by Peter Kelley

(Phys.org) —A University of Washington astronomer is using Earth's interstellar neighbors to learn the nature of certain stars too far away to be directly measured or observed, and the planets they may host.

"Characterization by proxy" is the technique used by Sarah Ballard, a post-doctoral researcher at the UW, to infer the properties of small, relatively cool stars too distant for measurement, by comparing them to closer stars that now can be directly observed.

Ballard is lead author of a study accepted for publication in The Astrophysical Journal that used this method and observations from the Kepler to learn the nature of the Kepler-61.

Our understanding of the size and temperature of planets depends crucially on the size and temperature of the stars they orbit. Astronomers already have a robust way to discern the physical properties of solar-type stars—those like the sun—by measuring the light they emit at different wavelengths and matching that to synthetically created spectra.

"The challenge is that small stars are incredibly difficult to characterize," Ballard said. Those theoretical methods don't work well for what are called M-dwarf stars, lower-mass stars about half the size of the sun and smaller—which is too bad, because such stars make up about three-quarters of the universe.

Ballard is using the characterization by proxy method to try to fill this . She is building on what she calls "truly remarkable" work by astronomer Tabetha Boyajian, now at Yale University, who uses a near-infrared interferometer—an array of telescopes working in unison studying light wavelengths a bit longer than visible light—to resolve the physical size of relatively .

Ballard said her characterization by proxy method takes "full advantage that there now exists this precious sample" of relatively nearby stars that have been directly measured. You could say the method borrows a bit from Greek mathematician Euclid, whose first "common notion" held that things that equal the same thing must necessarily also equal each other.

In the new paper, Ballard and co-authors used this reasoning to learn about Kepler-61b, a planet orbiting near the inner edge of the of the distant, low-mass star Kepler-61, about 900 light-years away in the Cygnus Constellation. A star's habitable zone is that swath of space just right for an orbiting planet's surface water to be in liquid form, thus giving life a chance.

She did this by comparing it to temperature size averages from four spectroscopically similar stars between 12 and 25 light-years away in the Ursa Major and Cygnus constellations. A light-year is about 6 trillion miles.

A funny thing also happened along the way: Kepler-61 turned out to be bigger and hotter than expected, which in turn recalibrated planet -61b's relative size upward as well—meaning it, too, would be hotter than previously thought and no longer a resident of the star's habitable zone.

All of this caused Ballard to informally subtitle the paper, "How Nearby Bumped a Planet out of the Habitable Zone."

Explore further: Astrophysicists offer new research, tool for identifying habitable zones

More information: iopscience.iop.org/0004-637X

Related Stories

Kepler Set to Launch Tonight on Planet Finding Mission

Mar 06, 2009

(PhysOrg.com) -- The Kepler spacecraft and its Delta II rocket are "go" for a launch tonight that is expected to light up the sky along Florida's Space Coast at 10:49 p.m. EST as the rocket lifts off from ...

Scientists discover planetary system orbiting Kepler-47

Sep 13, 2012

News flash: The Milky Way galaxy just got a little weirder. Back in 2011 astronomers were amazed when NASA's Kepler spacecraft discovered a planet orbiting a double star system.  Such a world, they realized, ...

Astronomers identify three extrasolar planets

Apr 26, 2012

(Phys.org) -- It's not little green men, but it could be a step in that direction: Cornell astronomers, using data from the NASA Kepler Mission, have identified three Earthlike planets orbiting their own suns, all of which ...

Recommended for you

Image: Multicoloured view of supernova remnant

14 hours ago

Most celestial events unfold over thousands of years or more, making it impossible to follow their evolution on human timescales. Supernovas are notable exceptions, the powerful stellar explosions that make ...

Ultra-luminous X-ray sources in starburst galaxies

14 hours ago

Ultra-luminous X-ray sources (ULXs) are point sources in the sky that are so bright in X-rays that each emits more radiation than a million suns emit at all wavelengths. ULXs are rare. Most galaxies (including ...

When a bright light fades

15 hours ago

Astronomer Charles Telesco is primarily interested in the creation of planets and stars. So, when the University of Florida's giant telescope was pointed at a star undergoing a magnificent and explosive death, ...

Image: Horsehead nebula viewed in infrared

15 hours ago

Sometimes a horse of a different color hardly seems to be a horse at all, as, for example, in this newly released image from NASA's Spitzer Space Telescope. The famous Horsehead nebula makes a ghostly appearance ...

The Milky Way's new neighbour

15 hours ago

The Milky Way, the galaxy we live in, is part of a cluster of more than 50 galaxies that make up the 'Local Group', a collection that includes the famous Andromeda galaxy and many other far smaller objects. ...

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.