Can we find Earth-mass habitable planets orbiting our nearest star, α Centauri?

1. Can we find Earth-mass habitable planets orbiting our nearest star, α Centauri? The answer is yes! John Hearnshaw, Dept. of Physics and Astronomy University of Canterbury collaborators: Stuart Barnes (AAO, Australia) and Mike Endl (Austin, Texas) An Earth-mass planet in the habitable zone of α Centauri A, with star B at a distance of about 20 AU. (Artist’s impression). α Centauri (left) and β Centauri (right) Habitable zone planets The habitable zone of a star is the zone where a planet can have liquid water. right: Artist’s impression of habitable zone Earth-like planet in the α Cen system For α Centauri A: habitable zone 1.1 – 1.3 AU (1 from A) For α Centauri B: habitable zone 0.5 – 0.9 AU (0.6 from B) 1-m Mt John telescope and Hercules fibre-fed échelle spectrograph • Three ways of finding Earth-like planets • The Doppler method: periodic radial-velocity • Transits of planets across disk of a star • 3. Gravitational microlensing The challenge of detecting Earth-mass planets Earth-mass planets require velocity precision of ~ 1 m/s. The table gives velocity amplitudes of α Cen A and B caused by 1 ME and 10 ME planets in orbits of different size, a. Right: McLellan 1-m telescope MJUO α Cen A α Cen B Hercules optical layout α Cen A + iodine cell spectrum: 2009 Jan 22 Left: Hercules 4k × 4k CCD camera; above Hercules vacuum tank All three are poised for success in the next few years. All use latest cutting-edge technology. A fibre scrambler for Hercules stabilizes the fibre illumination at the exit inside Hercules. This should allow ~2 m/s velocity precision, even without I2 cell. Stable planetary orbits must be within 2 or 3 A.U. of each star and coplanar with the binary star orbit, i = 79°. α Centauri data R.A.: A: 14h 39m 36.4951s B: … 35.0803s Dec.: A: -60° 50′ 02.308 B: … 13.761″ parallax p = 0.75 distance d = 1.34 parsecs = 4.37 light-yr α Centauri is a double star (G2V + K1V) Orbital elements: Period P = 79.91 yr Eccentricity e = 0.52 Semi-major axis a = 23.4 A.U. Inclination i = 79° Separation of stars 11.2 to 35.6 AU Angular separation of stars varies 2 to 22 2008: 8.3 2009: 7.5 2016: 4.0 apastron: 1995, 2075 periastron: 1955, 2035 A B Proxima apparent mag mV: +0.01 1.33 13.1var spectral type: G2V K1V M5Ve absolute mag MV: 4.37 5.71 15.53 luminosity L: 1.6 0.45 5.1 × 10–5 mass M: 1.10 0.91 0.12 radius R: 1.227 0.865 0.12 temperature Teff (K): 5790 5260 3240 colour index (B-V): 0.69 0.90 1.81 age (Gyr): 6.52±0.3 6.52±0.3 Can planets form in the α Cen system? Results from planet formation simulations by Guedes et al.for α CenB. All simula- tions yield 1 to 4 Earth-mass planets of which 42% lie inside the star’s habitable zone (dashed lines). The planetary con- figuration of the solar system is shown for reference. Starting conditions: N lunar-mass bodies in a disk with 1/a surface density. • Why observe α Centauri from Mt • John Observatory New Zealand? • We have a high resolution spectro- • graph able to deliver ~1 m/s • precision. • We have a 1-m telescope available • for an intensive observing program • over several years. • We can observe α Centauri all year, • even in November and December • when α Cen passes through lower • culmination (altitude ~ 15°). . Sample spectra of α Cen B through I2 cell showing thousands of fine I2 lines superimposed on stellar spectrum Recorded at Mt John 2009 Jan 24 Iodine cell velocity precision ~2.5 m/s Above: early trials with I2, May-Jun 2007. left: α Centauri true and projected orbits Left: 963 spectra of α Cen A, in 2009 Apr show 2.68 m/s precision Above: α Cen at lower culmination RV simulation on α Cen A to find a one Earth-mass planet at 1 A.U. More data for the α Cen triple system The simulation assumed 11,500 spectra per year each with σ = 3 m/s. The planet induces a signal with K = 8 cm/s, P = 370 d. The power spectrum shows this planet is easily detectable, even after 2 years! The data used were 963 actual Hercules data recorded April 2009, which were used to generate 46000 simulated observations over 4 years. above: α Centauri sizes α Cen A is 23% larger than Sun and a little hotter. B is cooler than Sun and less than half Sun’s luminosity. Note that B is 20% less massive than A.