2000 April 20, SPS 1020 (Introduction to Space Sciences) - Reading: today was TNSS 27 & 28 and PBD 5 & 20. - If you haven't done so already, read TNSS 27 and PBD 5 & 20 by Tuesday, April 25. - Papers due Thursday, April 27. No late papers will be accepted. --------------- Other Solar Systems [and, this year, Life in the Universe] ------------------- -------------------- Since 1995, it has become possible to detect planets around other star systems! 1995 November: Michel mayor & Didier Queloz (Geneva Observatory) anounce discovery of a companion to the star 51 Pegasi => 51 Peg B. 51 Peg is a normal G2.5V star, quite similar to the Sun. Shortly afterwards and ever since: Geoff Marcy (SFSU) and Paul Butler (now at Anglo-Australian Obs) confirm this, and discover many more planets, more than are in our own Solar System. Total number of extrasolar planets now exceeds 30. 1991: Alex Wolszczan discovered the first extrasolar planets, around pulsars. He isn't in general given credit for discovery of extrasolar planets, though; he jokes that pulsars "aren't politically correct", since they're burned-out cinders of massive stars, not normal stars I think he was robbed... Interestingly, though, he _did_ find that there are _at least three planets_ around the pulsar, from the effects their gravity has on each other's motion, which modulates the pulses from the pulsar. (To make a very long story short, pulsars emit pulses of light, like lighthouses.) Pulsars are made in supernova explosions (massive stars explode; among the most powerful explosions in the Universe) => Pulsar planets are probably made _after_ the supernova => It's very easy to make planets, anywhere! Also, pulsar planets are _terrestrial planets_: they have similar mass to Earth's. All others so far, found around normal star, are massive planets, probably gas giants like Jupiter. Types of Extrasolar Planets Found So Far: - Hot Jupiters (or Jovians) - massive, but very close to parent star! - Eccentric Jupiters - massive, in eccentric orbits - Classical Jupiters - massive, in long-period orbit, like Jupiter - Pulsar Planets - have masses of terrestrial planets Terrestrial planets have so far _not_ been found around normal stars. Methods so far used cannot find them: but just wait! Binary Stars ------------ Before we discuss how to dectect extrasolar planets, note that > 50% of nearly any sample of stars one might choose are _binary_ stars: stars with companions. (bi-= two, e.g. bicycle) The general types of binary stars are: - Spectroscopic binaries: found by their shifting velocities (Annoying demo of the Doppler effect) - Eclipsing binaries: found by their eclipses - Astrometric binaries: found by their shifting positions (We see one star, but it shifts back and forth, as it moves through space, pulled around by an unseem companion) - Visual binaries: We _see_ two stars orbit each other, directly! (special case of astrometric binaries) Extrasolar Planet Detection Methods ----------------------------------- 1) Radial Velocity Variations: (as with spectroscopic binaries) 2) Eclipses (as with eclipsing binaries) 3) Astrometric detection (as with astrometric binaries) 4) Direct imaging! With Hubble Space Telescope, particularly its (now defunct) near-infrared camera, NICMOS. Other proposed methods: ---------------------- 5) Gravitational Microlensing Einstein's General Theory of Relativity predicts that gravity deflects starlight. This is a well-observed effect, first observed during the solar eclipse of 1919 by A. S. Eddington & collaborators, and it made Einstein a household name. The first gravitational lenses, in which the gravitational deflection of starlight focuses the images of distant objects, making them brighter to us than the original objects (as a regular lens does), were discovered in 1979. They were distant galaxies, focused by quasars. _Microlensing_ is when individual stars do this. These are rare events, but one can take advantage of _multiplexing_: observe fields with many stars, to check many at the same time. (Digital imaging, fast computers, and fancy software work wonders.) => This is the only method that can detect extrasolar terrestrial planets around normal stars with technology IN HAND NOW. => The MACHO survey: MAssive Compact Halo Objects Carried out since 1992 at Siding Spring, Australia A dedicated 1.3-m telescope takes deep, very wide-field images of dense star fields, i.e. the Magellanic Clouds (a companion galaxy to the Milky Way that they can resolve into 8 million stellar images) and the bulge of the Milky Way (> 10 million stellar images). The events have distinctive light curves, which look like: These are independent of color, which is why images are obtained through both blue and red filters. => Has found 4 events in Magellanic Clouds, and 45 in Galactic Bulge! The whole project was supposed to detect unseen cosmic matter (usually called "dark matter"---duh!), the presence of which had been inferred by its gravitational pull on stars. It hasn't really found nearly as much as should be there, though (i.e., the "dark matter problem" still is unsolved: the stuff could be black holes, white dwarfs, or really anything.) => The big deal here is this suggests a potential way to detect extrasolar terrestrial planets, since: _the shapes of the light curves tells us their masses_! Several projects are now getting underway... 6) Null Interferometry Uses the phenomenon of: _destructive interference_ between waves of light. => Synchronize waves, so that they cancel each other out. This solves a major problem for extrasolar planet detection: the parent star can be > 10^9 times brighter than its planets Tests carried out last year in Arizona; multi-mirror telescopes will be used for this. 7) Interferometric imaging Destructive interference is the opposite of _constructive interference_, in which light waves, properly synchronized, can amplify each other. Several ground-based systems coming online (Mt. Wilson, Hawaii, Arizona) SIM: Space Interferometry mission, planned for 2005? TPF: Terrestrial Planet Finder, 2nd generation, 2011? Goldin's goal: imaging continents on extrasolar terrestrial planets Will take 3rd or 4th-gen system: 100-m space-based (membrane) mirror... (Roger Angel & Neville Woolf showed off a _very_ preliminary design at January AAS meeting) Remember also: to detect life, we don't need this much capability. We could make do with spectra of the planets' atmospheres, to look for gases wildly out of chemical equilibrium, e.g. O_2. What Planets Have Been Found? ---------------------------- - Pulsar planets - Hot Jupiters - Eccentric Jupiters - Classical Jupiters Already there are surprises! First 3 types are unknown in Solar System. Also, note that the methods used are very much biased to finding what we have found, especially hot Jupiters. 51 Peg B has an orbital period of 4 days (Mercury's is 88 days). => Definintely hot! Jupiter itself has an orbital period of 12 years. Surveys haven't been going on that long, must wait until we can expect to find similar planets. Eccentric Jupiters: Have orbits as eccentric as those of comets in Solar System, known from the shapes of the radial velocity curves: => In all probability, the amazing diversity of worlds we see in the Solar System is only the very tip of the iceberg. Minimum mass distribution is strongly skewed to < 5 M_J (Jupiter masses) => These are probably mostly planets, not brown dwarfs. (Which themselves are turning up aplenty in the new deep infrared survey, 2-MASS.) Note also: we get only minumum masses from spectroscopic binaries, since we don't know the inclinations of the orbits. _Could_ improve on this, if we could _also_ resolve them with direct imaging, making these also (what kind of binaries)? Formation and stability of stellar planetary systems ---------------------------------------------------- Why do hot Jupiters exist? Should form only beyond the "snow line", far out in a stellar system. => Do they move inward? Preliminary ideas: perhaps they all did move inward at some time (Jupiter included), because sprial waves in the disks dissipate angular momentum. => They stop moving in, when the disk interior to them clears out. Many questions emerging, as happens with good science: - If there were significantly more giant planets in our Solar System (than the 4 we have now: Jupiter, Saturn, Uranus, Neptune), would gravitational resonances between them eventually kick some out of the Solar System? (Giving rogue planets, perhaps seen in the MACHO survey?) - If Jupiter, etc. weren't in the Solar System, would Earth be pelted by the comets they flung out into the Oort cloud? - Would a hot Jupiter fling terrestrial planets out of its stellar system? About 6% of Solar-type stars have giant planets within 2 AU. Simulations: not necessarily, once formed and stable. - Might the _moons_ of a stabilized hot Jupiter be habitable? (Example: in the 3rd Star Wars movie...) Planetary habitability: ---------------------- Planetary habitable zones: where liquid H_2 O can exist. (Note Earth is right near inner edge of ours...) Have found extrasolar planets on both sides of theirs; also, planets of 16 Cyg and other stars are a eccentric Jupiters that move through it. Stabilizing effect of the Moon on Earth's obliquity---and climate? ============================================================================ Life in the Universe -------------------- "Is it reasonable to suppose that in a large field, that only one shaft of wheat should grow, and in an infinite Universe, to have only one living world?" -- Metrodorus, c. 500 A.D. "If we could conceive, in some warm little pond, with all sorts of ammonium and phospheric salts---light, heat, electricity etc. present, that a proteine compound was chemically formed ready to undergo still more complex changes..." -- Charles Darwin, 1871 What is life? No universally agreed-on definition. 1) Life _evolves_, by _mutation_ (random changes) and by _natural selection_ (survival of the fittest). 2) Even more basic: life _reproduces_. Even the most primitive life forms, if only collections of molecules, must reproduce, in order to be considered alive. Viruses are *not* considered alive: they don't reproduce by themselves. (Typically, a virus is a bit of DNA that attaches itself to a living cell, and causes the cell to help it replicate itself.) => The Darwinian definition of life: life is a self-sustained chemical system capable of Darwinian evolution by natural selection. More loosely, "Life is the preservation and evolution of DNA." DNA - deoxyribonucleic acid, the giant molecule with the double-helix shape, which carries hereditary _information_. We must stick with discussing _life-as-we-know-it._ It's awfully hard to say anything sensible about _life-as-we-don't-know-it_: we don't know it! Life-as-we-know-it requires: 1) Water 2) Energy 3) Access to organic (carbon-based) compounds. Water is thought to be special: - Large temperature range (100 C) between boiling and freezing - Has freezing-point depression (floats when it freezes, so oceans don't freeze solid) Carbon may be special, too: no other atom bonds to itself nearly as readily, making long, complex molecules capable of carrying _information_. Life does *not* require: - Oxygen: anaerobic bacteria (formerly "blue-green algae") were most common form of life on Earth, before 2 billion years ago. In fact, O_2 is highly poisonous to them! - Comfortable temperatures: "Extremophile" bacteria (love extremes), thrive near boiling (in Yellowstone geysers!) and near freezing (in the dry valleys of Antarctica!). But be careful what you generalize. Life is _very_ good at adapting to conditions; but then, many extremophiles are very primitive bacteria. - Sunlight: in the 1990s, deep drilling has been finding life at increasing depth in the Earth. Also near "black smokers", volcanic vents on the ocean floor. Recall the Miller-Urey experiment: Collect the most primitive and plentiful molecular gases in the Universe (H_2 O, NH_3, CH_4), add energy => Amino acids, which make up proteins, the stuff of life Harold Urey always assumed the "cold origin" of the planets. It didn't happen that way, though. Impacts were plentiful! Charles Darwin, successors, assumed life originated in a "warm, little pond", but there's really little evidence for it. Lynn Margulis has recently reclassified life, on its most basic scale: Old: Plant Kingdom Animal Kingdom (There are plant bacteria, and animal bacteria) New: Plants Animals Fungi Bacteria Archaeobacteria (archaeo = ancient) Some of the most primitive archaeobacteria known are found near "black smokers", at ocean floors, forever out of touch with sunlight. => Might life have originated _there_? Would have been more stable environment than early Earth's surface. => Implications loom large for Europa, which has: 1) Water 2) Energy (from tidal heat) 3) Organic material---if from nowhere else, C-type asteroids! Possibly also Ganymede, Callisto? Titan: probably too cold for life, but you never know: CH_4, N_2 out of equilibrium, must be actively replenished, unknown how (You heard it here first!) Signature of life on Earth: what it does to atmospheric chemistry. Many gases (e.g. CH_4, ) are amazingly out of chemical equilibrium in such a highly oxidizing atmosphere. => Replenished daily by life! Indeed, so much O_2 in atmosphere is a sign of life: nearly all in Earth's atmosphere was made by blue-green algae, gradually, around 2 billion years ago. The Search for Life on Mars --------------------------- It's really amazing how tenacious life is on Mars. No matter how many times we kill it, it keeps coming back! - Percival Lowell's "Canals" on Mars => pure optical illusion - Before the 1965 Mariner 4 flyby of Mars: seasonal changes in color were thought to be from vegetation => were in fact from global sandstorms - Microbial life on Mars: searched for by the Viking 1 and 2 landers (landed 1976): the first-ever instruments purpose-built to look for life on another world. 1) Pyrolytic release: Are CO_2 and CO assimilated? Used radioactive carbon (C14), as tracer. 2) Labeled release: put C14 into nutrient-rich broth; was any eaten? 3) Gas exchange: nutrient-rich broth added to samples: were metabolic gases given off? ALL 3 EXPERIMENTS GAVE POSITIVE REACTIONS!!!!!!!!!!!!!!!!!!!!!!!!!! BUT: ---- 4) Mass spectrometer (analyzes samples atom by atom): There is NO organic material on Martian surface! ------------------------------------------------ This is VERY surprising: Mars is near asteroid belt, and should be pelted by carbonaceous meteorites. => UV light from Sun at Mars surface breaks down organic compounds; also, oxidizing properties of surface material. A generation went by, and life on Mars seemed a dead issue. - 1996 claim by David McKay (NASA Johnson Space Center, Houston): SNC meteorite (from Mars): ALH 84001 found in Allan Hills in Antarctica, 1st in 1984. 4.5 billion years old, part of early Mars crust. Blown off Mars by impact; flew through space for 16 million years. Lay for 13,000 years in Antarctica. Signs of life claimed by McKay et al.: 1) Carbon compounds (PAHs, or polycyclic aromatic hydrocarbons) suggestive of decayed organic matter; 2) Magnetite crystals, similar to those made only by Earth bacteria; 3) Apparently incompatible minerals close together, suggesting organic action if the rock were from Earth; 4) Magnified images show elongated "things", resembling bacteria. No one of these is the "smoking gun", sufficient evidence. McKay emphasized that the case is much stronger because all of them appear together. A vigorous debate ensued: 1) PAHs are also found in cigarette smoke, and on a burnt steak. => Could be from anything. 4) The "bacteria" are much too small: only 0.1 microns long, 0.1 the length and 1/1000 the volume of terrestrial bacteria. => Can't contain the essentials of a living cell. Might be residue of sample preparation process! On the other hand, "nanobacteria" native to Earth have been claimed, too. Somehow, living bacteria had to have their precursors, and this rock _is_ very old... The debate rages on (see 1999 April Sky & Telescope), with the only consensus emerging that we need to find some unambiguous indicator of life ("biomarkers"). When _must_ something *be the product of life? (Sagan during Viking landings: If the cameras see something bigger on top than on the bottom, it may be alive, e.g., a tree or an animal.) Where _Did_ Life Come From? --------------------------- In 1969 November, Apollo 12 astronauts Pete Conrad and Alan Bean landed near the Surveyor 3 robot soft-lander spacecraft, which had landed 31 months earlier. The astronauts stripped some parts off, and brought them back to Earth. Apparently someone had coughed on it; a freeze-dried colony of bacteria was found. Although freeze-dried, they were easily brought out of hibernation! => If we do find life on Mars, and it's just like Earth life, do we really know it's Martian? Might _Earth_ life have originated there? Idea is called _panspermia_, championed by Fred Hoyle. Origin-of-life researchers hate it, because it doesn't solve the problem of how life originated: it just puts it on another planet. OK, so how did it originate there?