"Long before it's in the papers"
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To understand far-off worlds, astronomer looks closer to home

April 26, 2013
Courtesy of the University of Washington
and World Science staff

An as­tron­o­mer has de­vised a way to bet­ter un­der­stand some stars too far away to be di­rectly mea­sured—and the plan­ets they may host—by com­par­ing the stars to bet­ter-known ones near­by.

Sar­ah Bal­lard, a post-doctoral re­search­er at the Uni­vers­ity of Wash­ing­ton, uses that meth­od to judge the prop­er­ties of cer­tain very com­mon stars that are small and rel­a­tively cool. Bal­lard is the lead au­thor of a study ac­cept­ed for pub­lica­t­ion in The As­t­ro­phys­i­cal Jour­nal, and posted here, using the tech­nique along with da­ta from the Kep­ler Space Tel­e­scope to de­scribe the dis­tant star Kep­ler-61.

This artist's con­cep­tion shows the Kep­ler tele­scope's search zone, a re­gion shaped like a cone about 3,000 light-years long with our Solar sys­tem at its point. (Cred­it: NA­SA/Jon Lom­berg)


Our un­der­standing of the size and tem­per­a­ture of plan­ets de­pends on the size and tem­per­a­ture of the stars they or­bit. As­tro­no­mers al­ready have a good way to dis­cern the prop­er­ties of Sun-like stars by meas­ur­ing the light they emit at dif­fer­ent wave­lengths, or roughly speak­ing, col­ors.

But “small stars are in­credibly dif­fi­cult” to un­der­stand, Bal­lard ex­plained: such meth­ods don’t work well for so-called M-dwarf stars, which are light­er and about half the size of the sun or smaller. It’s too bad, be­cause such stars make up about three-quarters of the uni­verse, she added.

Bal­lard built on work by as­tron­o­mer Ta­be­tha Boy­a­jian, now at Yale Uni­vers­ity. Boy­a­jian uses a near-infra­red in­ter­fer­om­eter—an ar­ray of tele­scopes work­ing in un­ison stu­dying light slightly less en­er­get­ic than the vis­i­ble kind—to re­solve the size of near­by stars. Bal­lard said her meth­od takes “full ad­van­tage that there now ex­ists this pre­cious sam­ple” of rel­a­tively near­by stars that have been di­rectly meas­ured.

For their re­port, Bal­lard and col­leagues used com­parison to this samp­le, a me­thod they call Char­ac­ter­iz­a­tion by Proxy, to as­sess Kep­ler-61b, a plan­et cir­cling near the in­ner edge of the “hab­it­able zone” of the dis­tant, low-weight star Kep­ler-61. A star’s hab­it­a­ble zone is that swath of space suit­a­ble for an or­biting plan­et’s sur­face wa­ter to be liq­uid, giv­ing life a chance. The star lies in the di­rection of the Cyg­nus con­stella­t­ion and about 900 light-years away, a light-year be­ing the dis­tance light trav­els in a year.

The sci­en­tists com­pared the star to tem­per­a­ture size av­er­ages from four stars si­m­i­lar in their light spec­trums, and which lie be­tween 12 and 25 light-years away in the Ur­sa Ma­jor and Cyg­nus con­stella­t­ions. Kep­ler-61 turned out to be big­ger and hot­ter than ex­pected, Bal­lard and col­leagues said, which in turn re­cal­i­brated plan­et Kep­ler-61b’s rel­a­tive size up­ward as well. That meant it, too, would be hot­ter than pre­vi­ously thought and no long­er a res­i­dent of the star’s hab­it­a­ble zone.


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An astronomer has devised a way to better understand some stars too far away to be directly measured—and the planets they may host—by comparing the stars to better-known ones nearby. Sarah Ballard, a post-doctoral researcher at the University of Washington, uses that method to judge the properties of certain very common stars that are small and relatively cool. Ballard is the lead author of a study accepted for publication in The Astrophysical Journal that used the technique, along with data from the Kepler Space Telescope, to describe the distant star Kepler-61. Our understanding of the size and temperature of planets depends on the size and temperature of the stars they orbit. Astronomers already have a good way to discern the properties of Sun-like stars by measuring the light they emit at different wavelengths, or roughly speaking, colors. But “small stars are incredibly difficult” to understand, Ballard explained: such methods don’t work well for so-called M-dwarf stars, which are lighter and about half the size of the sun or smaller. It’s too bad, because such stars make up about three-quarters of the universe, she added. Ballard is building on work by astronomer Tabetha Boyajian, now at Yale University, who uses a near-infrared interferometer—an array of telescopes working in unison studying light slightly less energetic than the visible kind—to resolve the size of nearby stars. Ballard said her method takes “full advantage that there now exists this precious sample” of relatively nearby stars that have been directly measured. For their report, Ballard and colleagues used this reasoning, which they call Characterization by Proxy, to assess Kepler-61b, a planet circling near the inner edge of the “habitable zone” of the distant, low-weight star Kepler-61. A star’s habitable zone is that swath of space suitable for an orbiting planet’s surface water to be liquid, giving life a chance. The star lies in the direction of the Cygnus Constellation and about 900 light-years away, a light-year being the distance light travels in a year. The scientists compared the star to temperature size averages from four stars similar in their light spectrums, and which lie between 12 and 25 light-years away in the Ursa Major and Cygnus constellations. Kepler-61 turned out to be bigger and hotter than expected, Ballard and colleagues said, which in turn recalibrated planet Kepler-61b’s relative size upward as well. That meant it, too, would be hotter than previously thought and no longer a resident of the star’s habitable zone. All this caused Ballard to informally subtitle the paper, “How Nearby Stars Bumped a Planet out of the Habitable Zone.” closer to home