Putting Everything Together Ruslan Belikov1, Maxwell Moe2, Stephen Bryson1, Lily Zhao3, Jack Lissauer1 1NASA Ames Research Center, Moffett Field, CA 2University of Arizona, Tucson, AZ 3Flatiron Institute, New York, NY Folding in Other Effects and Priors Summary • Alpha Centauri AB is a promising system in the search for potentially habitable planets, representing the closest (by a large margin) Sun-like stars to the Solar System. • Planets can and do form around binary stars like Alpha Centauri, and the habitable zones of both stars are stable. However, planet formation around binaries may not be as efficient as around single stars • This work estimates the number of potentially habitable planets around Alpha Centauri AB system as 0.28−0.12 +0.25, but if at least one planet is confirmed, this number can rise to 1.2 −0.3 +0.7. A lot of uncertainty still remains Expected Occurrence Rates and Types of Planets Possible in the Alpha Centauri AB System Occurrence Rates from Kepler (around single and binary stars) Selected References Total expected number of planets [0.5-13 Re, 0-2.5AU] Expected number of Potentially habitable planets [0.5-1.5 Re, Kopparapu et al. 2013 conservative HZ] Combined single+binary occurrence from Kepler (per star) ~ 3 0.40−0.2 +0.5 after correction for binarity (per system, i.e. counting both stars) ~ 2 0.30−0.13 +0.27 after accounting for RV non- detections (both stars) ~ 2 0.28−0.12 +0.25 if NEAR candidate is real, and assuming it does not disrupt HZ (both stars) ~ 8 ~ 1.2 −0.3 +0.7 Ruled out by dynamical stability Ruled out by RV non-detections (note: this region boundary is very uncertain due to mass-radius relationship uncertainty) 1.92aCenBaCenA Caveat: due to numerous uncertainties, and evolving understanding of various effects, the above numbers are continuing to evolve a Cen AB Kraus et al. 2016: Moe and Kratter, 2020: Howell et al. 2021 a Cen AB Suppression factor ~0.3 Suppression factor ~0.25−0.10 +0.15 Due to observation bias against binaries, suppression factor estimates may be too aggressive hearth(aCenA) ~ hearth(aCenB) ~ 0.40−0.2 +0.5 x 1.44−0.12 +0.12 x 0.25−0.1 +0.15 = 𝟎. 𝟏𝟓−𝟎.𝟎𝟗 +𝟎.𝟏𝟗 Conservative HZ single+binary hearth Conversion to single- star hearth Binary suppression hearth(aCenAB) ~ hearth(aCenA) + hearth(aCenA) = 𝟎. 𝟑𝟎−𝟎.𝟏𝟑 +𝟎.𝟐𝟕 Alpha Centauri contains the closest (by a large margin) Sun-like stars to the Solar system Interest in observing aCen planets is mounting, e.g.: • VLT / NEAR (thermal IR direct imaging, Wagner et al. 2020) • JWST / MIRI (F1550C, 15.5mm, Beichman et al. 2020) • ALMA (astrometry, Akeson et al. 2020) • Toliman (astrometry w/ small space telescope, Bendek et al., Tuthill et al., ) • Window is opening for new RV observations due to increasing separation • Work is on-going to enable aCenAB direct imaging on Roman, IR/O/UV mission (Belikov et al., Bendek et al.) • 2021 Breakthrough Discuss conference was dedicated to Alpha Centauri: https://breakthroughinitiatives.org/initiative/5/discuss2021) MSWC Simulation SNWC Lab Demo : 1.4RE in HZ of aCenB (Gibbous phase) : 7RE @1.1AU, aCenA (quadrature) SAG13 (2016) Dulz et al. 2020 Bryson et al. 2021 • Meta-analysis of Kepler occurrence rates as of 2016 • Extended SAG13 to larger periods • Used for LUVOIR / HabEx yields • Latest / final analysis by Kepler team • Accounts for updated stellar radii, reliability, etc. • Treats occurrence rates in terms of instellation flux instead of period SAG13 / Dulz et al Bryson, low extrapolation Bryson, high extrapolation Conservative Kopparapu HZ, 0.5-1.5 Re 0.40−0.14 +0.48 𝟎. 𝟑𝟕−𝟎.𝟐𝟏 +𝟎.𝟒𝟖 0.60−0.36 +0.90 Optimistic Kopparapu HZ, 0.5-1.5 Re 0.58−0.2 +0.7 0.58−0.33 +0.73 0.88−0.51 +1.28 Wagner et al. 2020 R ~ 3.3 – 7 Re Maxwell Moe • If C1 is a real planet, then the suppression factor may not apply (or not as strongly) – aCenAB occurrence rates may become similar to single stars. • However, because C1 is in the HZ, it will “carve out” some of the HZ, leaving less space for rocky HZ