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

43
n we find Earth-mass planets orbiting our nearest star, α Centauri? hn Hearnshaw, iversity of Canterbury, ristchurch, NZ Humboldt Symposium University of Otago Dunedin 29 January 2010

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

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

α Centauri?

John Hearnshaw,University of Canterbury, Christchurch, NZ

Humboldt SymposiumUniversity of OtagoDunedin29 January 2010

“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 thatbiochemical 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’ atVanderbilt University, spring 1958.

Harlow Shapley (1885-1972)Harvard College Observatory

The masses of planets

Jupiter mass planets: 1 MJ = 10-3 MSun

Earth-mass planets: 1 ME = MJ = MSun 300

1

000,300

1

• No Earth-mass planets have yet been detected, buta few planets slightly bigger than the Earth have beenfound.

• Mainly they are much closer than the Earth-Sundistance (= 1 AU) so very hot.

1 A.U. = 1 astronomical unit = Earth-Sun distance = 150 million km.

Three ways of finding Earth-like planets1. 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 581e (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)

HD114762: original data ofLatham et al. (small dots),and more precise data fromMcDonald (Cochran et al.)(large dots). 51 Pegasi: Marcy and Butler

mp sin i (mJ) = 11.0 0.47a (AU) = 0.3 0.05P (days) = 84.03 4.22e = 0.334 0.00

Radial-velocity (Doppler) method

Three ways of finding Earth-like planets

2. Transits of planets across disk of a star• 62 planets detected by precise photometry• First was HD209458 (Charbonneau et al. 2000)• Typical change in light ~ 10-4 to 10-5

• Smallest is CoRoT-7b: size = 1.7 REarth; mass = 4.8 MEarth a = 0.017 AU; P = 0.85 d (20.5 h) (Leger et al. Aug 2009)

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.

Finding planets by the transit method

KEPLER (NASA)

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

Kepler search areaas function of stellar mass and orbitalsemi-major axis.

KEPLER: planet sensitivity regionand the habitable zone

Three ways of finding Earth-like planets

3. Microlensing• Nine planets found, first in 2004• Lowest mass objects: (i) OGLE-05-BLG-390Lb, mass = 5.4 MEarth

at 2.1 AU (projected on sky plane) (ii) MOA-07-BLG-192Lb, mass = 3.2 MEarth

at 0.62 AU

Left: light curve ofOGLE-05-BLG-390Below: light curve ofMOA-07-BLG-192

Microlensing light curves for two lens stars withlow mass planets

α Centauri: position in the sky

Right ascension: A: 14h 39m 36.4951s B: … 35.0803sDeclination: A: -60° 50′ 02.308 B: … 13.761″Hence: culminates at midnight in early MayIn NZ (lat 44°S), lower culmination at altitude 15° (Nov).

α Centauri: name and brightness

Names: α Centauri, Rigil Kent, 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 3rd brightest star in the sky.

α CentauriA (V = +0.01) is 4th brightest after Sirius, Canopus and Arcturus.

α CentauriB (V = 1.33) is 21st 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 starAngular separation of stars varies from 2 to 22 arc sec2008: 8.32009: 7.52016: 4.0

max separation: 1995, 2075closest approach: 1955, 2035

α Centauri: the stellar components

A B apparent mag mV +0.01 1.33 luminosity L 1.6 0.45 mass M 1.10 0.91 radius R 1.227 0.865 temperature Teff (K) 5790 5260 (of surface) age (billions of yr) 6.52±0.3 6.52±0.3

relative sizes of α Cencomponents 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)

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 zones

Habitable zone planetsfor α Centauri A and B

Can planets form in the α Cen system?An example of planet formation in a circumstellar disk around α CenB. 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 aboutthe 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 ApJ 679, 1582 (2008)

The detection of planets by the Doppler method

masses)Earth in ( (m/s) sin

0.094

masses)Jupiter in ( (m/s) sin

8.29

1

1.

)(

sin2

p

p

232

31

maM

im

maM

im

emM

im

P

GK

p

p

p

p

K = velocity amplitude of star’s ‘wobble’ caused by planetmp = mass of planet in Jupiter or Earth massesM* = stellar mass in solar mass unitsa = size of planetary orbit in AUi = 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 periodicRV variations in α CenA and B, and found no planets.Typical velocity precision ~ 10 m/s.

For α CenA and α CenB : No Jupiter-mass planets weredetected. Conclusion: There are no Jupiters in α Centauri!

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

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.

α Cen A α Cen BEarth super-Earth Earth super-Earth

α Cen A + iodine cell spectrum: 2009 Jan 22

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

Hercules

High Efficiency and Resolution CanterburyUniversity Large Echelle Spectrograph

Hercules in the spectrograph room at MJDecember 2006

Tw

o sp

ectr

a of

ζ T

rA (

F9

V)

show

ing

a 15

km

/s s

hift

inth

e st

ella

r li

nes

ζ TrA: a test spectroscopic binary T

wo

spec

tra

of ζ

TrA

(F

9 V

)sh

owin

g a

15 k

m/s

shi

ft in

the

stel

lar

line

s

Spectral Instruments4k × 4k CCD cameraon 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. I2 absorption lines superimposed onstellar spectrum act as a precise wavelength calibration.

Cell length 15 cmIodine vapour temperature 50.0 ± 0.1 °C.

2009 April data for α Cen A showing a precision of 2.68 m/s from 963 observationsusing 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 overabout 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 spectrain 2010.

• 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.

How precisely do the positions of Doppler line shifts need to be measured?

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 Centaurito confirm the existence of a planet?

Answer: Yes, may be!

• If we can travel at 0.1c (30,000 km/s), the journey would take about 50 years.• To reach that speed, we need to accelerate at 0.04 m/s2 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 km2

• 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