Stellar Surface Gravity Tells Where to Find Habitable Exoplanets

Summary: The autocorrelation function timescale technique identifies habitable exoplanets from stellar bright variations, surface gravity, turbulence, and vibration.

Depiction of autocorrelation function timescale technique for identifying habitable exoplanets ~ Brightness variations suggest stellar surface turbulence and vibration; if stellar surfaces were solid, humans would register different weights on different stars; an adult weighing 75 kilograms on Earth would weigh 20 times more on the sun and 50 times lighter on a red giant star (the sun’s far-future fate); credit Jaymie Matthews and Thomas Kallinger: No usage restrictions, via EurekAlert!

Astrophysicists in Australia, Austria, Denmark, France and the United States are measuring stellar surface gravity in order to identify habitable exoplanets, according to a study published in Science Advances Jan. 1, 2016.

Rafael García, Saskia Hekker, Daniel Huber, Thomas Kallinger and Jaymie Matthews bring to scientific attention the new autocorrelation function timescale technique method regarding stellar brightness variations. The new timescale technique calculates for a star whose distance defies conventional methods surface gravity from the surface turbulence and vibration suggested by stellar brightness variations. It draws upon data from the Kepler space observatory launched March 7, 2009, in order to turn its photometer on more than 145,000 main sequence stars.

The research team examines about 1,300 stars.

The technique follows a precept articulated by Jaymie Matthews, Professor in the Department of Physics and Astronomy at the University of British Columbia in Vancouver, Canada.

Professor Matthews, one of the study’s six co-researchers and co-writers, gives the habitable exoplanets rule: “If you don’t know the star, you don’t know the planet.”

The sizes of habitable exoplanets exterior to the solar system have to be measured relative to the energy outputs and sizes of host or parent stars. The technique identifies from luminosity, mass and radius whether “a planet around it is the right size and temperature to have water oceans, and maybe life.”

Gravity-dependent graininess and oscillation join gravity-independent magnetism and rotation to affect stellar brightness.

Astronomers know that separate measurements of acoustic oscillation indicative of sun-like outer layer turbulence and of graininess indicative of sun-like outer layer heat yield surface gravity. But stellar pulsations that help understand stellar internal structures let surface gravity be inferred for only relatively bright stars and not for distant and fainter stars.

Eight-hour flicker amplitude methods measure both, but will be compromised by magnetism, photon noise and rotation and useless for M dwarfs, red giants and young dwarfs.

The study’s six co-researchers and co-writers note the surer, wider applicability of the timescale technique, which draws upon time-series data, not amplitude, regarding stellar brightness variations. Time-scaling overcomes instrumental and photon noises to identify sun-like granulated, pulsating stellar signals.

The timescale technique provides surface gravity to average precisions of about 4 percent because of self-adapting filters against instrumental perturbations, magnetic activity, rotation and uncorrelated noises.

Guesstimating main sequence and subgiant surface gravities within ten-minute intervals quantifies flicker’s advantage.

Timescale techniques nevertheless reveal unique capabilities to measure large samples of stars with high accuracy and undetectable oscillations modes or noisy light curves with reasonable accuracy. They also succeed regardless of activity for luminous red giants, main sequence stars and red giants with masses 0.8 to 3 times that of the sun.

Accuracy and versatility trigger Professor Matthew’s observation regarding time-scaling that “I expect someone will announce the discovery of life on an exoplanet within about 20 years.”

graph of relation between asteroseismically determined surface gravity and ACF (autocorrelation function) time scale for a sample of Kepler light curves: Science Advances @ScienceAdvances, via Twitter Jan. 6, 2016

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:

Depiction of autocorrelation function timescale technique for identifying habitable exoplanets ~ Brightness variations suggest stellar surface turbulence and vibration; if stellar surfaces were solid, humans would register different weights on different stars; an adult weighing 75 kilograms on Earth would weigh 20 times more on the sun and 50 times lighter on a red giant star (the sun’s far-future fate); credit Jaymie Matthews and Thomas Kallinger: No usage restrictions, via EurekAlert! @ https://www.eurekalert.org/multimedia/531855; (EurekAlert! news release @ https://www.eurekalert.org/news-releases/461068; (former URL @ http://www.eurekalert.org/multimedia/pub/106005.php)

graph of relation between asteroseismically determined surface gravity and ACF (autocorrelation function) time scale for a sample of Kepler light curves: Science Advances @ScienceAdvances, via Twitter Jan. 6, 2016, @ https://twitter.com/ScienceAdvances/status/684767164881068033

For further information:

Altergott, Briana. 5 January 2016. “Astrophysicists Discover New Technique to Find Habitable Planets.”MyAJC > News Science.
Available @ http://www.myajc.com/news/news/science/astrophysicists-discover-new-technique-find-habita/npxg4/

Kallinger, Thomas; Saskia Hekker; Rafael A. García; Daniel Huber; and Jaymie M. Matthews. 1 January 2016. “Precise Stellar Surface Gravities from the Time Scales of Convectively Driven Brightness Variations.” Science Advances 2(1), e1500654, pages 1–6. DOI: 10.1126/sciadv.1500654
Available @ advances.sciencemag.org/content/2/1/e1500654.full

Newsy Science. 4 January 2016. "Astrophysicists Discover New Technique To Find Habitable Planets -- Newsy." YouTube.
Available @ https://www.youtube.com/watch?v=FsAubojxW7A

Science Advances @ScienceAdvances. 6 January 2016. "New #ScienceAdvances: Pinpointing A Star's Surface Gravity." Twitter.
Available @ https://twitter.com/ScienceAdvances/status/684767164881068033

Shekhtman, Lonnie. 3 January 2016. “How Calculating the Gravity on Distant Stars Could Tell of Habitable Planets.” Christian Science Monitor > Science. Available @ http://www.csmonitor.com/Science/2016/0103/How-calculating-the-gravity-on-distant-stars-could-tell-of-habitable-planets

University of British Columbia. 1 January 2016. “Astronomers Find New Way to Measure the Pull of Gravity at the Surface of Distant Stars.” EurekAlert! > Public Releases.
Available via EurekAlert! @ http://www.eurekalert.org/pub_releases/2016-01/uobc-afn122915.php