Solar Variability after Dark: Photometric Evidence from Stars and Planets
Wes Lockwood [gwl@lowell.edu], Lowell Observatory, Flagstaff, Arizona
Two long photometric projects seeking evidence of solar variability in the light reflected
from planets ended in frustration as one target after another proved intrinsically unstable. U. S.
Weather Bureau Chief of Scientific Services Harry Wexler sponsored the first effort at Lowell
Observatory from 1950 to 1966. Delays in proposed solar irradiance measurements from Earth
orbit prompted NOAA climatologist J. Murray Mitchell, Jr. to urge resumption of planetary
measurements in 1971. Ambiguous results from Uranus, Neptune, and Titan kept luring us
into proposing various solar related hypotheses, but we ultimately came to realize that outer
solar system planetary atmospheres exhibit mainly seasonal variations.
All was not in vain, however, as the planetary measurements spawned long-term studies of
Sun-like stars, historically thought to be paragons of stability. Observations of sixteen stars
from 1953 to 1966 yielded upper limits of variability below 1% and the important conclusion
that “…if the Sun acts in similar fashion, its variability over a 15-year period probably does
not exceed one half of one percent.” New measurements beginning in 1984 at Lowell and
continuing at Fairborn Observatory after 2000 pushed the detection threshold down to the 0.1%
level and solidified the demographics of variability among Sun-like stars including several
“solar twins.” Parallel Ca II observations from Lowell’s Solar Stellar Spectrograph add to an
increasingly comprehensive assessment of the patterns of chromospheric and photometric
variation of the stars most like the Sun.