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Opportunities in the Study of Extrasolar Planets. Peter R. McCullough, STScI Astrophysics Enabled by the Return to the Moon Nov 30, 2006. Outline (also the summary). Transiting planets are great science.
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Opportunities in the Study of Extrasolar Planets Peter R. McCullough, STScI Astrophysics Enabled by the Return to the Moon Nov 30, 2006
Outline (also the summary) • Transiting planets are great science. • Somewhere in space is a good site for a 0.6-m telescope to monitor known transiting systems. • The Moon is in space. • Polarization may be more practical than spectra for physical characterization of exo-earths. • A 10-m diameter telescope with imaging polarization capability and more modest wavefront quality requirements than TPF-C can detect oceans if they exist on terrestrial exoplanets and nearly map continental boundaries. • Return to the Moon will inspire creative ways to overcome challenges and tap new opportunities.
Captain Cook’s International Expedition • Transit of Venus, 1769 in Tahiti • Hoped to measure the physical scale of the solar system. • How much did it cost? Endeavor
XO’s Site #1 is Haleakala’s Site #1, the SAO BakerNunn Building. 1957 1958 www.ifa.hawaii.edu/users/steiger/post_cook.htm
Exoplanets are relevant to these Satellites: • Hubble • Spitzer • MOST • COROT • Kepler • JWST • TPF * *Flight Opportunity for TPF is TBD.
Ingress Eclipse Egress HD 209458 with HST/STIS Spectrophotometry Brown et al 2001
XO targets Bright Stars that allow … • Absorption Spectra of Planetary Atmosphere • Precise photometry and timing of transits • Oblateness (rotation rate) of planet • Rings, Satellites of planet • Perturbations from TP: d(TOA) ~ many seconds • Limb darkening and star spots • Secondary Eclipse • Temperature, df/f ~ ( Tp/Ts ) ( Rp/Rs )2 ~ 0.4% in IR • Eccentricity better than RV method • Albedo, df/f ~ 0.02% x optical albedo
An Extrasolar Planetary Atmosphere Charbonneau et. al. 2002, ApJ, 568, 377 • Spectrophotometric observations of four planetary transits taken in sodium doublet at 589.3nm. • DI (sodium/cont.): • 2.32 ± (0.57) x 10-4
Habitable worlds • Satellites of warm Jupiters • Could be more prevalent habitable exoworlds than exoplanets (!?) • Detectable by its gravitational influence on its host planet: transits arrive early and late. • At some wavelengths, planet is much darker than its satellite(s). • There are ~10 transiting Jupiters of stars V<12 and periods ~1 year; they will be found before lunar sorties. 70 Vir, available from Extrasolar Visions Inc.
Lunar Deployable Telescope Drake Deming (PI), Peter McCullough, and David Charbonneau
LDT’s terrestrial radius mass diagram Drake Deming (PI), Peter McCullough, and David Charbonneau
LDT’s terrestrial planet transit Drake Deming (PI), Peter McCullough, and David Charbonneau
LDT as concept Drake Deming (PI), Peter McCullough, and David Charbonneau
By 2019 … • Kepler will have completed its mission, detecting Earth-like planets orbiting solar-type stars from 100 to 300 pc distant. • Ground- and space-based transit surveys, augmented by precise radial velocities, will have detected 100 hot Jupiters transiting Sun-like stars and some terrestrial planets transiting nearby K- and M-dwarfs. • Spitzer measurements of thermal emission from exoplanets will have carried over to JWST, which will have measured thermal emission spectra of hot Jupiters and hot Neptunes. JWST will be pushing toward measuring the thermal spectrum of a close-in Earth-like planet around a lower main sequence star, using the secondary eclipse technique. • The most important requirement for transiting planet studies is a site in space, free of the terrestrial atmosphere, and with the capability to observe a given transiting planet system with photon-limited photometric precision for long durations (multiple transits). • LDT will be deployable by two astronauts and operate autonomously. It will … • Observe planets transiting bright, nearby, stars. • Stare at a given star for weeks to months. • Discover additional terrestrial planets of known transiting systems based upon: • Transits of the terrestrial planets: the premise (unproven) is the inclination is favorable. • Timing giant-planet transits. • Measure accurate sizes for terrestrial planets orbiting the nearest stars and measure their visible reflected light (at secondary eclipse). Drake Deming (PI), Peter McCullough, and David Charbonneau
Transiting Planet Spectroscopy (1 of 2) • Proven technique: V=8, Jovian planet HD 209458b orbiting sun-like star, observed with HST STIS, Na etc detected. • Earth-Sun at 10 pc: Absorption-lines due to transiting planet atmosphere feasible with 10-m steerable light bucket in vacuum plus spectrograph. (Gilliland, p.c.) • PSF FWHM = 1 arcmin is ok
Transiting Planet Spectroscopy (2 of 2) • Emission spectrum of planet disappearing behind star. planet atmosphere. • Proven technique: V=8, Jovian planet HD 209458b, etc provides spectral energy distributions of planet in Spitzer bands. • JWST will get true spectra of hot Jupiters by this technique – but probably not terrestrial exoplanets.
Does this help? Size of error bar ~ 1/(A Dn t)1/2
McCullough (2006 astroph 2007 ApJ) A few others independently working on this concept also: Ford, Hough, Williams, Stam, Schmid
Unpolarized Light Satellite images Simulations
Polarized Light S-pol P-pol
10-m or 20-m precision-surface (but not TPF-C) steerable telescope required. Total flux 2x flux difference
What’s great about this? • Speckle pattern (think JWST-like PSF) can be designed to be nearly identical in the two polarizations, so the glare of the star can be suppressed not only by coronagraphy but also by subtracting one polarized image from the other. The unpolarized star cancels out; the polarized planet doesn’t. • Spectra imply dividing light into 100+ bins; linear polarization implies two bins. • Rayleigh scattering is very blue; glint from oceans is achromatic. (so four bins: 2 polarization; 2 wavelengths) • Glint is very localized (~15 degrees of “longitude”). • The glint’s flux difference in the two polarizations is 0.15 photons per second for a 10-m telescope observing Earth-Sun system at 10 pc. Long integrations (days) can pick out the polarized light from an oceanic planet in the glare of the star. One hour integration gives Poisson S/N ~ 20 if star light can be suppressed entirely by a superb technology.
Ocean planet with clear atmosphere;Rayleigh suppressed (long wavelength)
About costs… • Net worth of US households is 50 T$ in 2000. • The aggregate value of corporate equities directly held was 9 T$ in 2000 and was 4 T$ in 2003, so it declined by ~2 T$ per year for three consecutive years. That's 1 B$ per hour of each and every working day for three years. • The median market capitalization of a corporation in the DJIA is 108 B$ (as of Oct 31, 2006). • Pfizer, a pharmaceutical company founded in 1849, in 2005 had annual revenue of 51 B$ and spent 7 B$ on R&D. NASA by comparison was awarded 16 B$ in 2005 by the US Congress.
Various thoughts…sustainability • Is colonization of the Moon a metaphor for solving Earth's geopolitical problems? A quasi-sustainable presence on the Moon wouldn't use fossil fuels; it would use solar or nuclear with a considerable emphasis on conservation. • Making scientific equipment on the moon is desirable strategically even if its possible to bring it there from Earth. • A flywheel with 2-m radius and 2-m height filled with lunar regolith has a mass of 40 metric tons, and if spun to 1000 rpm, has a kinetic energy of 110 kwh, which corresponds to 330 Watts for the 300-hour duration of the lunar night. • On the Moon, a need for UPS may exist for these reasons: • to buffer solar power to the night • to provide for the variable power demand of human habitation • to mitigate risk
Various thoughts…bandwidth • JWST is bandwidth limited from L2. • LSST = 10 Terabyte/night. • That data rate may be impractical to transmit to Earth from sensors on Moon or L2, but can be carried home on a disk if there’s a regular data delivery service: • One kg equals 1 Terabyte in 2006. By 2020, 14 doublings later, that’ll be 8000 Terabytes per kg, so 400 Petabytes will weigh as much as a human. • Alternatively, transmitting from lunar surface to lunar orbit for storage and subsequent transport to Earth may be desirable.
Various thoughts… • Hoyle’s ideas on panspermia could be tested by trying to detect life in lunar regolith, suitably nurtured, much like Viking did on Mars but much more seriously. PCR or equivalent technique(s) might permit detection at very trace levels. • A satellite can be sustained with some propulsion inside L2 so it is in Earth’s shade all the time. Nuclear power required. • Things too dangerous (or not permitted) on or near Earth could perhaps be done on the Moon. One example, Clementine. Another example: perturbing a small asteroid to orbit (or hit) the moon instead of the Earth (in case it misses). • Nuclear explosions could be used for excavations of and transmuting elements of the regolith. Also for pinging the moon for tomography. • The pristine fossil water ice in the lunar polar craters could be burned like fossil fuels are on earth.
Various thoughts… security and economics • Those examples (I hope) make you think that not all things that are possible are desirable or ethical. • The return to the Moon is supposed to enhance our national security. Bored children tend to fight; busy and intrigued children fight less. Perhaps the same could be true for nations and their peoples. • Economic enhancement? • When asked how much the US should spend and on which scientific disciplines, someone (I forget who) replied, “[exactly as much as it takes to be number one in each and every one.]” • Seward’s Folly (buying Alaska) and multi-national Antartica are possible answers to why the US will go to the Moon. • Yankee ingenuity fostered at grad-student level if few-100 kg payloads are delivered regularly to space. (If not, the best brains will do biology, computer science, etc.)
Various thoughts… • Human+robot surgery will develop tools we can use.
Parabola or Sphere? • Rotating Liquid Mirrors produce parabolas naturally. • Two surfaces rubbed together produce a sphere naturally. So does a bubble via surface tension. • Arecibo and Hobby Eberly Telescope are both spheres. • Spheres have advantages over Parabolas: • multiple focal planes can look at very different directions on the sky using a single sphere.
Parabola or Sphere? C F F F
Parabola or Sphere? • Rotating Liquid Mirrors produce parabolas naturally. • Two surfaces rubbed together produce a sphere naturally. So does a bubble via surface tension. • Arecibo and Hobby Eberly Telescope are both spheres. • Spheres can have advantages over Parabolas: • multiple focal planes can look at very different directions on the sky using a single sphere. • segmented optics are identical for a sphere. • replica optics fabricated on the moon, subsequently ion polished (or equivalent) and coated in place courtesy of the lunar vacuum, and actively controlled to maintain figure could be an alternative to rotating liquid mirrors.
Summary • Transiting planets are great science. • Somewhere in space is a good site for a 0.6-m telescope to monitor known transiting systems. • The Moon is in space. • Polarization may be more practical than spectra for physical characterization of exo-earths. • A 10-m diameter telescope with imaging polarization capability and more modest wavefront quality requirements than TPF-C can detect oceans if they exist on terrestrial exoplanets and nearly map continental boundaries. • Return to the Moon will inspire creative ways to overcome challenges and tap new opportunities.
