Are Main-Sequence K-type Stars the "Goldilocks" Stars for Hosting Long-term Habitable Planets

Details Are Main-Sequence K-type Stars the "Goldilocks" Stars for Hosting Long-term Habitable Planets Are Main-Sequence K-type Stars the "Goldilocks" Stars for Hosting Long-term Habitable Planets

Jan 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010aas...21542310w&link_type=abstract

American Astronomical Society, AAS Meeting #215, #423.10; Bulletin of the American Astronomical Society, Vol. 42, p.327

Astronomy and Astrophysics

Astronomy

Scientific paper

Main-sequence K-type (dwarf K = dK) stars have masses and luminosities ranging from ˜0.6-0.85 M&sun; and ˜0.1-0.45 L&sun;. In addition, dK stars have significantly longer main-sequence lifetimes than our Sun -- lasting ˜20-50 billion yrs. Moreover, these cool, low luminosity orange dwarfs are much more numerous (˜6-10×) than solar-type stars and also have been found to host an increasing number of planets. Their liquid-water habitable zones (HZs) extend from ˜0.4-1.2 AU. Because dK-stars evolve more slowly than G-stars, their HZs are essentially fixed for billions of years.
As an extension of the Villanova "Sun in Time" program, we have been studying the suitability of dK stars as hosts to habitable planets. To this end we have measured the coronal X-ray and chromospheric emissions of dK0-8 stars with wide ranges of age, rotation, and magnetic-dynamo generated coronal and chromospheric X-UV activity. We have established well defined age-rotation-activity relations for this sample. We have used archival X-ray (mostly ROSAT) and UV data (from FUSE and IUE). The rotation periods were determined using photometry from starspot modulations. Although their optical luminosities remain essentially fixed up to ˜10+ Gyrs, the magnetic dynamo X-UV radiances decay rapidly with age. Young dK stars rotate rapidly and have correspondingly strong magnetic dynamos and strong coronal X-ray and chromospheric UV emissions (as well as frequent flaring). Here we discuss the suitability of dK stars as hosts for life-supporting planets where long-term life is sustainable and compare them with properties of planets hosted by G and M-type stars. From this study we conclude that these orange dwarf stars may be the best choices for hosting planets with evolved, complex life.
This research is supported by grants from NSF/RUI (AST5-07542) and NASA/FUSE (NNG04G0386)
which we gratefully acknowledge.

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