Lecture 24 Transits and Upcoming Space Missions

1
Lecture 24 Transits and Upcoming Space Missions

• Meteo 466

2
Transiting planets

• If a planets orbital plane is nearly aligned
  with the observer on Earth, then the planet may
  transit its star, i.e., it passes in front of the
  star (and behind it)
• Transiting planets can be studied in a variety of
  different ways
• The probability of a transit depends on the size
  of the planets orbit relative to the size of the
  star ?

3
Probability of transits
i inclination of planets orbit to the
plane of the sky ?o angle of planets orbit
with respect to the observer ( 90o i) a
planets semi-major axis Rs stellar
radius Then, the probability that a planet will
transit is given by
4
Radial velocity curve for HD 209458 b

• First transiting hot Jupiter
• Planetary characteristics
• M 0.69 MJ
• Orbital period 3.5 d
• Odds of seeing a transit are equal to
• P Rs/a
• where
• Rs radius of star
• 7?105 km for the Sun
• a planet semi-major
• axis
• 0.04 AU (1.5?108 km/AU)
• 6?106 km
• Hence
• P ? 0.1

T. Mazeh et al., Ap. J. (2000) http//obswww.unige
.ch/udry/planet/ hd209458.html
5
Transiting giant planet HD 209458 b
Ground-based (4-inch aperture)
Hubble Space Telescope

• In 1999, about 10 hot Jupiters were known
  hence, the
• chances that one would transit were good
• Jupiters radius is 0.1 times that of the Sun
  hence, the
• light curve should dip by about (0.1)2 1
• Hot Jupiters have expanded atmospheres, so the
  signal is
• bigger

D. Charbonneau et al. Ap. J. (2000)
T. M Brown et al., Ap. J. (2001)
6
Primary transit spectroscopy
Habitable Planets book, Fig. 12-4

• Primary transit is when the planet passes in
  front of the star
• The planet appears larger or smaller at
  different wavelengths
• depending on how strongly the atmosphere
  absorbs
• Hence, the transit appears deeper at wavelengths
  that
• are strongly absorbed, allowing one to form a
  crude spectrum

7
First detection of an extrasolar planet
atmosphere (HD 209458 b)
Sodium D lines

• Sodium was detected in this
• spectrum taken from HST
• H2O was also detected
• (next slide)

Planetary radius vs. wavelength
D. Charbonneau et al., Ap. J. (2002)
8
HST observations of HD209458b
Key Green bars STIS data Red curves
Baseline model with H2O (solid) and without
(dashed) Blue curve No photoionization of Na
and K
T. Barman, Ap.J. Lett., in press (2007)
9
Transit of HD 209458 b observed in Ly ?

• Transit depth in visible 1.6
• Transit depth at Ly ? 14
• Ratio of areas
• ALy?/Avis 14/1.6 ? 9
• Ratio of diameters 3

Vidal-Madjar et al., Nature (2003)
10
Artists conception of transiting giant planet HD
209458 b

• Hydrogen cloud observed in Ly ?, presumably from
  planetary blowoff (Vidal-Madjar et al., Nature,
  2003)
• Note Evidently, this observation is
  controversial (may not be correct)

http//en.wikipedia.org/wiki/HD_209458_b
11
Spitzer Space Telescope

• Transiting extrasolar planets can also be studied
  in the thermal infrared using the Spitzer Space
  Telescope (formerly called SIRFT), currently in
  operation
• 0.85 m mirror, cryogenically cooled,
  Earth-trailing orbit

http//www.spitzer.caltech.edu/about/ index.shtml
12
Secondary transit spectroscopy
http//www.nasa.gov/mission_pages/spitzer/news/070
221/index.html
13
Hot, dry Jupiters (from Spitzer)

• Two separate studies
• HD 209458b (J. Richardson et al.)
• HD 189733b (C. Grillmair et al., Ap.J.Lett., in
  press)
• No sign of H2O!
• Recall that HD 209458b was the planet on which
  H2O was identified in the near-IR using HST

Artists conception http//www.planetary.org/news/
2007/0221_Spitzer_Captures_the_Light _from_Dry.ht
ml
14
http//www.nasa.gov/mission_pages/spitzer/news/070
221/index.html
15
http//www.nasa.gov/mission_pages/spitzer/news/070
221/index.html
16
http//www.nasa.gov/mission_pages/spitzer/news/070
221/index.html
17

• Conclusions from transit data on HD209458b
• Spitzer curves (thermal-IR secondary eclipse
  photometry) show no H2O
• HST curves (visible/near-IR primary eclipse
  photometry) show H2O at approximately solar
  abundance
• Ly ? data (Vidal-Madjar et al., Nature, 2003)
  show evidence for escaping hydrogen (transit is 9
  times as deep in Ly ?)

18
Finding M-star planets using transits

• Presentation to the ExoPTF by Dave Charboneau
  (February, 2007)
• Relative radii
• Sun 1
• Jupiter 0.1
• M star 0.1-0.3
• Earth 0.01
• Thus, the light curve for Earth around a late M
  star is about as deep (1) as for Jupiter
  around a G star
• The HZ around an M star is also close in ?
  transits are reasonably probable

• Transiting giant planet HD 209458b
• (D. Charbonneau et al. Ap. J., 2000)

19
James Webb Space Telescope

• JWST will be a 6.5-m thermal-IR (cooled)
  telescope
• Scheduled deployment 2015
• JWST can be used to measure secondary transit
  spectra (like Spitzer) on planets identified from
  ground-based observations
• Our first spectrum of a habitable world may come
  from a planet orbiting an M star!

http//www.jwst.nasa.gov/about.html
20
Observing transits from space

• Future space-based missions will be able to do
  transit studies at much higher contrast ratios
• RJup/RSun ? 0.1 ? contrast (0.1)2
  0.01
• REarth/RSun ? 0.01 ? contrast (0.01)2 10-4

21
COROT mission (ESA)

• 30-cm aperture
• Launched Dec. 27, 2006
• Must point away from the Sun ? can only look for
  planets with periods lt75 days, i.e., a lt 0.35 AU
  around a G star
• Planetary radius
• R gt 2 REarth
• Could conceivably find hot ocean planets, i.e.,
  water-rich rocky planets orbiting close to their
  parent stars

http//www.esa.int/esaSC/120372_index_0_m.html
22
Kepler Mission

• This space-based telescope
• will point at a patch of the
• Milky Way and monitor the
• brightness of 100,000 stars,
• looking for transits of Earth-
• sized (and other) planets
• 10?5 precision photometry
• 0.95-m aperture ? capable
• of detecting Earths
• Launch February, 2009

http//www.nmm.ac.uk/uploads/jpg/kepler.jpg
23

• Even if it works perfectly, Kepler will only
  find distant Earths
• Wed like to find the ones nearby

24
Number of Earths to be detected

• Monitor 100,000 stars
• Assume orbit at 1 AU around a G star
• Probability of transit
• RSun/1 AU 7105 km/1.5108 km
• 510-3 (i.e., 0.5)
• Expected number of Earths
• N 510-3(105) ? ?Earth
• 500 ? ?Earth
• where ?Earth is the expected frequency of
  Earth-like planets
• Actual numbers are slightly lower than this
  because not all stars being monitored are solar
  type

25

• Earth-sized planets around nearby stars can
  potentially be found by doing accurate
  space-based astrometry ?

26
SIM Space Interferometry Mission

• Narrow-angle astrometry 0.6 ?as precision on
  bright targets
• Could be used to identify Earth-mass (or slightly
  larger) planets around a significant number of
  nearby stars
• Much of the required development work has already
  been done

http//planetquest.jpl.nasa.gov/SIM/sim_index.cfm
Ref Unwin et al., PASP (Jan., 2008)
27
Primary SIM Targets

• 250 A, F, G, K, M dwarfs within 15 pc
• Doppler Recon. _at_ 1 m s-1
• Jupiters Saturns within 5 AU
• SIM 30 obs. during 5 yr (1 mas)
• 3 MEarth _at_ 0.5 - 1.5 AU
• 6 K-giant reference stars _at_ 0.5 - 1 kpc
• Located within 2 deg of each target
• Doppler vetting for binaries _at_ 25 m/s

5 s
Geoff Marcy Roadmap presentation
28
SIM target space Earth analogue survey 129
nearby stars

• Gaia is a small ESA
• astrometric mission
• TPF-C is Terrestrial
• Planet Finder
• Coronagraph

Ref Unwin et al., PASP (Jan., 2008)