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

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Can we find Earth-mass habitable planets orbiting our nearest star, α Centauri? John Hearnshaw, Dept. of Physics and Astronomy University of Canterbury collaborators: Stuart Barnes (AAO, Australia) and Mike Endl (Austin, Texas) ree ways of finding Earth-like planets he Doppler method: periodic radial-velocity ransits of planets across disk of a star Gravitational microlensing All three are poised for success in the next few years. All use latest cutting-edge technology. 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 m V : +0.01 1.33 13.1var spectral type: G2V K1V M5Ve absolute mag M V : 4.37 5.71 15.53 luminosity L : 1.6 0.45 5.1 × 10 –5 mass M : 1.10 0.91 0.12 left: α Centauri true and projected orbits 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) 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. above: α Centauri sizes 1 M E 10 M E 1 M E 10 M E a (A U) K (m /s) K (m /s) P (d) K (m /s) K (m /s) P (d) 0.05 0.39 3.92 3.88 0.43 4.26 4.23 0.1 0.28 2.77 10.99 0.30 3.01 11.95 0.4 0.14 1.38 87.9 0.15 1.51 95.6 0.6 0.11 1.13 161.5 0.12 1.23 175.6 1.0 0.09 0.88 347.5 0.10 0.95 377.9 2.0 0.06 0.62 982.8 0.07 0.67 1069. 3.0 0.05 0.51 1805. 0.05 0.55 1964. 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 M E and 10 M E planets in orbits of different size, a. α Cen A α Cen B 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 at Mt John 2009 Jan 24 A fibre scrambler for Hercules stabilizes the fibre illumination a the exit inside Hercules. This shou allow ~2 m/s velocity precision, even without I 2 cell. 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 progra over several years. • We can observe α Centauri all yea even in November and December when α Cen passes through lower culmination (altitude ~ 15°). . 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! The data used were 963 actual Hercules data recorded April 2009, ntauri (left) and β Centauri (right) Iodine cell velocity precision ~2.5 m/s Above: early trials with I 2 , May-Jun 2007. Left: 963 spectra of α Cen A, in 2009 Apr show 2.68 m/s precision Above: α Cen at lower culminatio 1-m Mt John telescope and Hercule fibre-fed échelle spectrograph Right: McLellan 1-m telescope MJUO Hercules optical layout Left: Hercules 4k × 4k CCD camera; above Hercules vacuum tank 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). Stable planetary orbits must be within 2 or 3 A.U. of each star and coplanar with the binary star orbit, i = 79°. 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. More data for the α Cen triple system The answe r is yes!

Transcript of Can we find Earth-mass habitable planets orbiting our nearest star, α Centauri? John Hearnshaw,...

Page 1: Can we find Earth-mass habitable planets orbiting our nearest star, α Centauri? John Hearnshaw, Dept. of Physics and Astronomy University of Canterbury.

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

α Centauri?John Hearnshaw,

Dept. of Physics and AstronomyUniversity of Canterbury

collaborators: Stuart Barnes (AAO, Australia) and Mike Endl (Austin, Texas)

Three ways of finding Earth-like planets

1. The Doppler method: periodic radial-velocity

2. Transits of planets across disk of a star

3. Gravitational microlensing

All three are poised for success in thenext few years. All use latest cutting-edgetechnology.

α Centauri data

R.A.: A: 14h 39m 36.4951s B: … 35.0803sDec.: A: -60° 50′ 02.308 B: … 13.761″parallax p = 0.75distance d = 1.34 parsecs = 4.37 light-yrα 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 AUAngular separation of stars varies 2 to 222008: 8.32009: 7.52016: 4.0apastron: 1995, 2075periastron: 1955, 2035

A B Proxima

apparent mag mV: +0.01 1.33 13.1varspectral type: G2V K1V M5Ve absolute mag MV: 4.37 5.71 15.53luminosity L: 1.6 0.45 5.1 × 10–5 mass M: 1.10 0.91 0.12radius R: 1.227 0.865 0.12temperature Teff (K): 5790 5260 3240colour index (B-V): 0.69 0.90 1.81age (Gyr): 6.52±0.3 6.52±0.3

left: α Centauri true and projected orbits

Habitable zone planets

The habitable zone of a star is the zone where a planet can have liquid water.

right: Artist’s impressionof habitable zone Earth-likeplanet 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)

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.

above: α Centauri sizes

1 ME 10 ME 1 ME 10 ME a (AU) K (m/s) K (m/s) P (d) K (m/s) K (m/s) P (d) 0.05 0.39 3.92 3.88 0.43 4.26 4.23 0.1 0.28 2.77 10.99 0.30 3.01 11.95 0.4 0.14 1.38 87.9 0.15 1.51 95.6 0.6 0.11 1.13 161.5 0.12 1.23 175.6 1.0 0.09 0.88 347.5 0.10 0.95 377.9 2.0 0.06 0.62 982.8 0.07 0.67 1069. 3.0 0.05 0.51 1805. 0.05 0.55 1964.

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 Bcaused by 1 ME and 10 ME planets in orbits of different size, a.

α Cen A α Cen B

α Cen A + iodine cell spectrum: 2009 Jan 22

Sample spectra of α Cen B through I2 cell showing thousands of fine I2 lines superimposed on stellar spectrumRecorded at Mt John 2009 Jan 24

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.

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°). .

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!The data used were 963 actual Herculesdata recorded April 2009, which were used to generate 46000 simulated observations over 4 years.

α Centauri (left) and β Centauri (right)

Iodine cell velocity precision ~2.5 m/s

Above: early trials with I2, May-Jun 2007.

Left: 963spectra of α Cen A, in2009 Aprshow 2.68 m/s precision

Above: α Cen at lower culmination

1-m Mt John telescope and Herculesfibre-fed échelle spectrograph

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Hercules optical layout

Left: Hercules 4k × 4kCCD camera; aboveHercules vacuum tank

An Earth-mass planetin the habitable zone ofα Centauri A, with starB at a distance of about 20 AU. (Artist’simpression).

Stable planetary orbits must be within 2 or 3 A.U. of each star and coplanar with the binary star orbit, i = 79°.

α Cen A is 23% larger than Sun and alittle hotter. B is cooler than Sun and less than half Sun’s luminosity. Notethat B is 20% less massive than A.

More data for the α Cen triple system

The a

nswe

r is ye

s!