Can we find Earthmass planets orbiting our nearest

Can we find Earth-mass planets orbiting our nearest star, α Centauri? John Hearnshaw, University of Canterbury, Christchurch, NZ Humboldt Symposium University of Otago Dunedin 29 January 2010

Harlow Shapley (1885 -1972) Harvard College Observatory “Millions of planetary systems must exist. Whatever the method of origin, planets may be the common heritage of all stars … “Our kind of chemistry, the chemistry of our Sun, our Earth, is the common chemistry of the universe… “On some of these planets is there actually life? Or is that biochemical operation strangely limited to our planet? … “Is life thus restricted? Of course not. We are not alone. ” Harlow Shapley in a lecture on ‘Religion in an age of science’ at Vanderbilt University, spring 1958.

The masses of planets Jupiter mass planets: 1 MJ = 10 -3 MSun Earth-mass planets: 1 ME = MJ = MSun 1 A. U. = 1 astronomical unit = Earth-Sun distance = 150 million km. • No Earth-mass planets have yet been detected, but a few planets slightly bigger than the Earth have been found. • Mainly they are much closer than the Earth-Sun distance (= 1 AU) so very hot.

Three ways of finding Earth-like planets 1. The Doppler method: periodic radial-velocity variation of a star, detected spectroscopically • 346 planets orbiting 294 stars discovered since 1995 • Most are Jupiter-mass objects (~300 MEarth ) • Lowest mass is Gliese 581 e (mass ≥ 1. 94 MEarth) at 0. 03 AU • Note also Gliese 581 d (mass ≥ 7. 1 MEarth) at 0. 22 AU (Mayor et al. June 2009)

Radial-velocity (Doppler) method HD 114762: original data of Latham et al. (small dots), and more precise data from Mc. Donald (Cochran et al. ) (large dots). mp sin i (m. J) a (AU) P (days) e = 11. 0 = 0. 3 = 84. 03 = 0. 334 51 Pegasi: Marcy and Butler 0. 47 0. 05 4. 22 0. 00

Three ways of finding Earth-like planets 2. Transits of planets across disk of a star • 62 planets detected by precise photometry • First was HD 209458 (Charbonneau et al. 2000) • Typical change in light ~ 10 -4 to 10 -5 • Smallest is Co. Ro. T-7 b: size = 1. 7 REarth; mass = 4. 8 MEarth a = 0. 017 AU; P = 0. 85 d (20. 5 h) (Leger et al. Aug 2009)

Finding planets by the transit method The principle of planet detection by the transit technique. Jupiter would cause a fall in brightness of the Sun by ~1% if it was in a transit event for a distant observer.

KEPLER (NASA) Launch 5 March 2009 • Search for planetary transits • Monitor >100, 000 stars for 4 yr continuously and simultaneously • Stars are brighter than m. V = 14 • Sensitivity to Earth-size planets as well as giants • Kepler satellite

KEPLER: planet sensitivity region and the habitable zone Kepler search area as function of stellar mass and orbital semi-major axis.

Three ways of finding Earth-like planets 3. Microlensing • Nine planets found, first in 2004 • Lowest mass objects: (i) OGLE-05 -BLG-390 Lb, mass = 5. 4 MEarth at 2. 1 AU (projected on sky plane) (ii) MOA-07 -BLG-192 Lb, mass = 3. 2 MEarth at 0. 62 AU

Microlensing light curves for two lens stars with low mass planets Left: light curve of OGLE-05 -BLG-390 Below: light curve of MOA-07 -BLG-192

α Centauri: position in the sky Right ascension: A: 14 h 39 m 36. 4951 s B: … 35. 0803 s Declination: A: -60° 50′ 02. 308 B: … 13. 761″ Hence: culminates at midnight in early May In NZ (lat 44°S), lower culmination at altitude 15° (Nov).

α Centauri: name and brightness Names: α Centauri, Rigil Kentaurus, Toliman Distance: d = 1. 34 parsecs = 4. 37 light-yr ~ 40 × 1012 km Brightness: When viewed by naked eye as a single star, V = – 0. 27, α Centauri is the 3 rd brightest star in the sky. α Centauri. A (V = +0. 01) is 4 th brightest after Sirius, Canopus and Arcturus. α Centauri. B (V = 1. 33) is 21 st brightest in visual apparent magnitude.

α Centauri: a double star • α Centauri is a double star with star A similar to the Sun and star B a little cooler than the Sun • Orbital elements: Period P = 79. 91 yr Eccentricity e = 0. 52 Semi-major axis a = 23. 4 A. U. (separation varies between Inclination i = 79° 11. 2 and 35. 6 AU)

α Centauri: a double star Angular separation of stars varies from 2 to 22 arc sec 2008: 8. 3 2009: 7. 5 2016: 4. 0 max separation: 1995, 2075 closest approach: 1955, 2035

α Centauri: the stellar components apparent mag m. V luminosity L 1. 6 mass M radius R temperature Teff (K) (of surface) age (billions of yr) A +0. 01 B 1. 33 0. 45 1. 10 1. 227 5790 0. 91 0. 865 5260 6. 52± 0. 3 relative sizes of α Cen components and the Sun

Stability of planetary orbits in α Cen AB Wiegert & Holman found stable orbits inside 2. 34 AU, but unstable 3 to 70 AU from each star, provided i = 0° (coplanar with binary orbit). Planets in inclined orbital planes are much less stable. Beyond 75 AU from the barycentre, stable orbits are again possible.

Habitable zone planets The habitable zone of a star is the zone where a planet can have liquid water. More precisely, the continuously habitable zone about a star is the zone in which an Earth-like planet will undergo neither a runaway greenhouse effect in the early stages of its history nor runaway glaciation after it develops an oxidizing atmosphere. 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)

Habitable zones The relative sizes of habitable zones around four of the nearest stars. Sirius A, alpha Centauri A and B, and Proxima Centauri. At this scale, the habitable zone around the red dwarf Proxima is so small that it is only about the size of the full stop at the end of this sentence.

Habitable zone planets for α Centauri A and B

Can planets form in the α Cen system? An example of planet formation in a circumstellar disk around α Cen. B. The disk is initially populated by 600 lunar-mass planetary embryos in nearly circular orbits. The radius of each circle is proportional to the size of the object. After 200 Myr four planets have formed. One planet has about the mass of Mercury and is at a = 0. 2 AU, two 0. 6 ME planets form at a = 0. 7 and a = 1. 8 AU, and a 1. 8 ME planet forms at a = 1. 09 AU. Javiera Guedes et al Ap. J 679, 1582 (2008)

The detection of planets by the Doppler method K = velocity amplitude of star’s ‘wobble’ caused by planet mp = mass of planet in Jupiter or Earth masses M* = stellar mass in solar mass units a = size of planetary orbit in AU i = inclination of orbit to line of sight (i = 90º is edge on)

α Centauri: Existing upper mass limits for planets The study of M. Endl et al. (2001) looked for periodic RV variations in α Cen. A and B, and found no planets. Typical velocity precision ~ 10 m/s. For α Cen. A and α Cen. B : No Jupiter-mass planets were detected. Conclusion: There are no Jupiters in α Centauri!

The challenge of detecting Earth-mass planets require velocity precision of ~ 1 m/s. The table gives velocity amplitudes of α Cen A and B for 1 ME and 10 ME planets in orbits of different size, a. α Cen A Earth super-Earth α Cen B Earth super-Earth

α Cen A + iodine cell spectrum: 2009 Jan 22

Sample spectra of α Cen B through I 2 cell showing thousands of fine I 2 lines superimposed on stellar spectrum Recorded by JBH at Mt John 2009 Jan 24

Hercules High Efficiency and Resolution Canterbury University Large Echelle Spectrograph

Hercules in the spectrograph room at MJ December 2006

Two spectra of ζ Tr. A (F 9 V) showing a 15 km/s shift in the stellar lines

Two spectra of ζ Tr. A (F 9 V) showing a 15 km/s shift in the stellar lines ζ Tr. A: a test spectroscopic binary

Spectral Instruments 4 k × 4 k CCD camera on Hercules Dec 2006

Latest developments in the Hercules instrument • Tests of an iodine vapour cell (S. Barnes, M. Endl) The cell was placed at the Cassegrain focus just before the fibre entrance. I 2 absorption lines superimposed on stellar spectrum act as a precise wavelength calibration. Cell length 15 cm Iodine vapour temperature 50. 0 ± 0. 1 °C.

2009 April data for α Cen A showing a precision of 2. 68 m/s from 963 observations using iodine cell

Why observe α Centauri from Mt John Observatory New Zealand? • We have a high resolution spectrograph able to deliver 1 m/s precision on late-type star velocities. • We have a 1 -m telescope with enough time available for an intensive observing program over several years. • We are the only observatory in the world able to observe α Centauri all year, even in November and December when α Cen passes through lower culmination (altitude ~ 15°). In Chile, Australia, S Africa the lower culmination is at altitude 0° and the observing season is 9 to 10 months through large air mass.

What do we need to do to detect Earth-mass planets? For K ~ 1 m/s, about 300 spectra at S/N 300: 1 over about 3 years, with σ ~ 2. 5 m/s to detect super-Earths. For K ~ 0. 1 m/s, about 30, 000 spectra (or more) at S/N 300: 1 over about 3 years, with σ ~ 2. 5 m/s to detect Earths-mass planets. Typical exposure times for this S/N, using R = 70, 000: α Cen A: ~40 s α Cen B: ~ 2 min in typical 2 arc s seeing.

α Centauri program: progress in last year From 2008 August to 2009 October we have acquired Hercules spectra with an iodine cell as follows: • α Centauri A: 6536 spectra • α Centauri B: 3359 spectra Observers: Kilmartin, Hearnshaw and Barnes. S/N ratio ~ 300: 1 Our aim is to increase the rate of acquisition of spectra in 2010.

How precisely do the positions of Doppler line shifts need to be measured? • Hercules pixel size = 15 μm ≡ 1. 2 km/s • A precision of 1 m/s requires measuring line position to 1 in 300 million. • A 1 m/s velocity shift ≡ a line displacement of ~ 12 nm (~ 10– 3 pixels!) on the CCD detector. • A 10 cm/s shift ≡ 1. 2 nm ~ 10 -4 pixels ~ 5 × diameter of a Si atom in the CCD chip.

Semi-empirical RV data for α Cen A. Based on actual spectra taken April 2009, but (a) reproduced over a 4 -yr period; (b) with an 8 cm/s 370 -d period signal added; (c) binned into 17 equally spaced bins (each of about 3 weeks)

RV simulation on α Cen A to find a one Earth-mass planet at 1 A. U. 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!

Can we send a space probe to α Centauri to confirm the existence of a planet? Answer: Yes, may be! • If we can travel at 0. 1 c (30, 000 km/s), the journey would take about 50 years. • To reach that speed, we need to accelerate at 0. 04 m/s 2 for 25 years, and then decelerate for another 25 yr. • To do that we need a light sail driven by radiation pressure (sunlight or lasers) • Sail area needed ~25 km 2 • Technology may be available in 50 yr from now. Arrival at α Cen ~ 100 yr from now. Return of first images 4. 3 yr after that.

α and β Centauri The End