24.2 Life in the Solar System Our goals for learning • How do we find/identify Life? • Could there be life in the Solar System and where would it be?
Finding life in the Universe • Solar System -close enough to visit, active exploration possible. • Look for signs of life in likely places • Other star systems -too far away to visit, passive exploration dependant on naturally received data only. • Finding stars with planets (preferably Earth-like), identifying right materials for habitability, looking for signals from intelligence.
What Does Life look Like? • Only one known example- Earth! • We use Earth’s fossil record and environmental niches to gain an idea of where, what and how life exists. (Terrestrial analogues)
Brief History of Life on Earth • 4.4 billion years - early oceans form • 3.5 billion years - cyanobacteria start releasing oxygen. • 2.0 billion years - oxygen begins building up in atmosphere • 540-500 million years - Cambrian Explosion • 225-65 million years - dinosaurs and small mammals (dinosaurs ruled) • Few million years - earliest hominids
Hardest to find on other planets Necessities for Life • Nutrient source • Energy (sunlight, chemical reactions, internal heat) • Liquid water (or possibly some other liquid) • Stable environment for life to form (e.g. not devastated by volcanoes or impacts every 5 minutes)
Looking For Life in the Solar System • Look for water and the other necessities • Examine environments that life can flourish (extremophiles) • Examining fossils to recognize past life (identifying fossil bacteria)
Life in Unexpected Places(Extremophiles) Poison tolerant Salt tolerant Acid tolerant Alkali tolerant Radiation tolerant Lives in minerals No sunlight No oxygen Extreme cold Extreme heat Extreme pressure No sunlight Low water content
1. Searching for Life on Mars • Mars had liquid water in the distant past, still has subsurface ice; possibly subsurface water • Energy sources: solar, possible old volcanic vents • Carbon compounds from meteorites
In 2004, NASA Spirit and Opportunity Rovers sent home new mineral evidence of past liquid water on Mars. Also looking for evidence of micro-fossils
2. Jovian Ice Worlds Evidence for global subsurface ocean on Europa
Ganymede, Callisto also show some evidence for subsurface oceans. • Relatively little energy available for life, but still… • Intriguing prospect of THREE potential homes for life around Jupiter alone… Ganymede Callisto
3. Titan? • Surface too cold for liquid water (but deep underground?) • Liquid ethane/methane on surface • Lots of hydrocarbons • Relatively stable, energy sources could be a problem.
What have we learned? • Identifying Life • The most common and longest life forms are single-celled. They arose at least 3.85 billion years ago on Earth and occur in a variety of environments that complex life cannot endure. • What are the necessities of life? • Nutrients, energy, and liquid water • Could there be life In the Solar System? • Evidence for present or past liquid water occur on Mars, Europa, possibly Ganymede and Callisto. Mars is the easiest to explore and exploration is ongoing. • Any life on Mars would be simple and small though
24.3 Looking for Life Around Other Stars Our goals for learning • Are habitable planets likely? • Hunting for Planets • Are Earth-like planets rare or common?
Habitable Planets Definition: A habitable world contains the basic necessities for life as we know it, including liquid water. • It does not necessarily have life.
Constraints on star systems: • Old enough to allow time for evolution (rules out high-mass stars - 1%) • Need to have stable orbits (might rule out binary/multiple star systems - 50%) • Size of “habitable zone”: region in which a planet of the right size could have liquid water on its surface. Even so… billions of stars in the Milky Way seem at least to offer the possibility of habitable worlds.
The more massive the star, the larger the habitable zone — higher probability of a planet in this zone.
Finding them will be hard Recall our scale model solar system: • Looking for an Earthlike planet around a nearby star is like standing on the East Coast of the United States and looking for a pinhead on the West Coast — with a VERY bright grapefruit nearby.
Hunting For Planets • Have to use light coming to us- no interstellar exploration :( • Direct: Pictures or spectra of the planets themselves • Indirect: Measuring the effects of planets on the properties of their parent stars. (Stellar wobble, Doppler shift effects, brightness changes during transits/eclipses)
Stellar Wobble • Sun and Jupiter orbit around their common center of mass • Sun therefore wobbles around that center of mass with same period as Jupiter
Astrometrics (Stellar Wandering) • Sun’s motion around solar system’s center of mass depends on tugs from all the planets • Astronomers around other stars that measured this motion could determine masses and orbits of all the planets
Astrometrics (Stellar Wandering) • We can detect planets by measuring the change in a star’s position on sky • However, these tiny motions are very difficult to measure (~0.001 arcsecond)
Doppler Technique • Measuring a star’s Doppler shift can tell us its motion toward and away from us • Current techniques can measure motions as small as 1 m/s (walking speed!)
Transits and Eclipses • A transitis when a planet crosses in front of a star • The resulting eclipse reduces the star’s apparent brightness and tells us planet’s radius
Finding Planets • Current technologies cannot detect a planet as small as Earth, most exo-planets found so far are Jupiter-sized. • One System Gliese 581 has a planet that is calculated to be about the mass of Neptune and 2 planets called super-Earths (about 5-8 Earth masses).
Spectral Signatures of Life Venus oxygen/ozone Earth Mars
Elements and Habitability • Some scientists argue that proportions of heavy elements need to be just right for formation of habitable planets • If so, then Earth-like planets are restricted to a galactic habitable zone
Impacts and Habitability • Some scientists argue that Jupiter-like planets are necessary to reduce rate of impacts • If so, then Earth-like planets are restricted to star systems with Jupiter-like planets
Climate and Habitability • Some scientists argue that plate tectonics and/or a large Moon are necessary to keep the climate of an Earth-like planet stable enough for life
The Bottom Line We don’t yet know how important or negligible these concerns are.
What have we learned? • Are habitable planets likely? • Billions stars have sizable habitable zones, but we don’t yet know how many have terrestrial planets in those zones • Finding Exo-planets • Over 200 planets have been found by direct or indirect means. The smallest is Neptune-sized, no earth-sized worlds found yet. • Are Earth-like planets rare or common? • We don’t yet know because we are still trying to understand all the factors that make Earth suitable for life
24.4 The Search for Extraterrestrial Intelligence Our goals for learning • How many civilizations are out there? • How does SETI work?
The Drake Equation Number of civilizations with whom we could potentially communicate= NHP flifefcivfnow NHP = total # of habitable planets in galaxy flife = fraction of habitable planets with life fciv = fraction of life-bearing planets w/ civilization at some time fnow = fraction of civilizations around now.
We do not know the values for the Drake Equation NHP : probably billions. flife : ??? Hard to say (near 0 or near 1) fciv : ??? It took 4 billion years on Earth fnow : ??? Can civilizations survive long-term?
What have we learned? • How many civilizations are out there? • We don’t know, but the Drake equation gives us a framework for thinking about the question • How does SETI work? • Some telescopes are looking for deliberate communications from other worlds
24.5 Interstellar Travel and Its Implications to Civilization Our goals for learning • How difficult is interstellar travel? • Where are the aliens?
Current Spacecraft • Current spacecraft travel at <1/10,000 c; 100,000 years to the nearest stars. Pioneer plaque Voyager record
Difficulties of Interstellar Travel • Far more efficient engines are needed • Energy requirements are enormous • Ordinary interstellar particles become like cosmic rays • Social complications of time dilation
Fermi’s Paradox • Plausible arguments suggest that civilizations should be common, for example: • Even if only 1 in 1 million stars gets a civilization at some time 100,000 civilizations • So why we haven’t we detected them?
Possible solutions to the paradox • We are alone: life/civilizations much rarer than we might have guessed. • Our own planet/civilization looks all the more precious…
Possible solutions to the paradox • Civilizations are common but interstellar travel is not. Perhaps because: • Interstellar travel more difficult than we think. • Desire to explore is rare. • Civilizations destroy themselves before achieving interstellar travel These are all possibilities, but not very appealing…