Day 1140

Thursday.

1608.03074
The Venus Hypothesis
Cartwright

Current models indicate that Venus may have been habitable.  Complex life may have evolved on the highly irradiated Venus, and transferred to Earth on asteroids.  This model fits the pattern of pulses of highly developed life appearing, diversifying and going extinct with astonishing rapidity through the Cambrian and Ordovician periods, and also explains the extraordinary genetic variety which appeared over this period.


1608.00706
Was Venus the first habitable world of our Solar System?
Way, et al

Present-day Venus is an inhospitable place with surface temperatures approaching 750K and an atmosphere over 90 times as thick as present day Earth's.  Billions of years ago the picture may have been very different.  Create a suite of 3D climate simulations using topographic data from the Magellan mission, solar spectral irradiance estimates for 2.9 and 0.715 billion years ago, present day Venus orbital parameters, and ocean volume consistent with current theory and measurements, and an atmospheric composition estimate for early Venus.  Using these parameters, find that such a world could have had moderate temperatures if Venus had a rotation period slower than about 16 Earth days, despite and incident solar flux 46-70% higher than model Earth receives.  At its current rotation period of 243 days, Venus's climate could have remained habitable until at least 715 million years ago if it hosted a shallow primordial ocean.  These results demonstrate the vital role that rotation and topography play in understanding the climatic history of exoplanetary Venus-like worlds being discovered in the present epoch.


Nature
A terrestrial planet candidate in a temperate orbit around Proxima Centauri
Anglada-Escudé, et al

At a distance of 1.295 pc, the red dwarf Proxima Centauri (alpha Centauri , GL 551, HIP 70890 or simply Proxima) is the Sun's coolest stellar neighbor and one of the best-studied low-mass stars.  It has an effective temperature of only around 3050 K, a luminosity of 0.15% of that of the Sun, a measured radius of 14% of the radius of the Sun and a mass of about 12% of the mass of the Sun.  Although Proxima is considered a moderately active star, its rotation period is about 83 days and its quiescent activity levels and X-ray luminosity are comparable to those of the Sun.  Report observations that reveal the presence of a small planet wit ha minimum mass of about 1.3 M_Earth orbiting Proxima with a period of approximately 11.2 days at a semi-major-axis distance of around 0.05 AU.  Its equilibrium temperature is within the range where water could be liquid on its surface.


1608.06672
MOST observations of our Nearest Neighbor: flares on Proxima Centauri
Davenport et al

Present a study of white light flares from the active M5.5 dwarf Proxima Centauri using the Canadian micro satellite MOST.  Using 37.6 days of monitoring data from 2014 and 2015, detected 66 individual flare events, the largest number of white light flares observed to date on Proxima Cen.  Flare energies on the sample range from 1e29-31.5 ergs, with complex, multi-peaked structure found in 22% of these events. The flare rate is lower than that of other classic flare stars of similar spectral type, such as UV Ceti, which may indicate Proxima Cen had a higher flare rate in its youth.  Proxima Cen does have an unusually high flare rate given the slow reported rotation period, however.  Extending the observed power-law occurrence distribution down to 1e28 erg, show that flares with flux amplitudes of 0.5% occur 63 time per day, while super flares with energies of 1e33 erg occur ~8 times per year.  Small flares may therefore pose a great difficulty in searches for transits from the recently announced 1.27 M_earth Proxima b, while frequent large flares could have significant impact on the stellar atmosphere.