Copyright 2009 Pearson Education, Inc. Life in the Universe

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Julianna Hancock
6 years ago
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1 Life in the Universe

2 Life in the Universe The only place we know life exists is here on Earth One of humanity s Big Questions is whether it exists elsewhere We can get some clues by considering life s history here on Earth When we do, we can get an idea how likely life as we know it is We should keep in mind that life as we know it may not be the only kind possible But it is the kind that we will be best able to recognize, if it does exist

4 When did life arise on Earth? What do these events tell us about the possibility that life exists elsewhere in the universe?

5 When did life arise on Earth? The first evidence of life appears ~150 million years after it became possible It is not fossil evidence, but trace chemical evidence The evidence is in the ratio of carbon-12 to carbon-13

6 Carbon isotope evidence for life In >3.8-billion-year-old rocks like these in Greenland there is a higher than normal ratio of 12 C: 13 C Living things incorporate 12 C more easily than 13 C So the higher ratio is taken as indirect evidence for life

7 When did life arise on Earth? The oldest fossils of living things date to ~3.5 billion years ago.

8 Earliest Fossils The oldest fossils of living things date to ~3.5 billion years ago. Fossil stromatolite in 3.5-billionyear-old rock

9 Earliest Fossils The oldest fossils of living things date to ~3.5 billion years ago. Fossil stromatolite in 3.5-billionyear-old rock This is a living stromatolite Stromatolites are layered structures formed by colonies of bacteria They still exist today, typically in extreme environments like hypersalty lakes and lagoons

10 Earliest Fossils The oldest fossils of living things date to ~3.5 billion years ago. Fossil stromatolite in 3.5-billionyear-old rock This is a living stromatolite Stromatolites are layered structures formed by colonies of bacteria They still exist today, typically in extreme environments like hypersalty lakes and lagoons Here are some in a lagoon in Australia

11 When did life arise on Earth? So fossil evidence shows that life certainly existed on Earth by 500 million years after conditions would permit it to survive And chemical evidence suggests it probably existed much earlier But how did it come to be? We don t know

12 How did life arise on Earth? We do know that all life as we know it has an inside and an outside These are separated by a lipid membrane (along with a cell wall in plants) Vesicles made of lipids are easy to make in the laboratory

13 How did life arise on Earth? Life as we know it also has a nucleic acid genome containing instructions for building the organism And we know that all of that was accomplished here on Earth surprisingly quickly So we believe that given similar conditions elsewhere, life will also arise

14 Necessities for Life As We Know It Nutrient source Energy (starlight, chemicals, heat) Liquid water (hardest to come by) And can only exist if planet is in habitable zone

15 Are habitable planets likely?

16 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. There still needs to be sufficient time for life to evolve And certain other requirements as well

17 Constraints on star systems: 1. Old enough to allow time for origin and evolution (rules out high-mass stars 1%) 2. Need to have stable orbits (might rule out binary/multiple star systems 50%) 3. 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.

18 Finding them will be hard Looking for an Earth-like 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. But new technologies should soon show the way.

19 The Kepler Space Telescope is monitoring 156,000 stars for transit events for 4 years.

25 Planned orbiting interferometers will obtain spectra and crude images of Earth-size planets.

26 Spectral Signatures of Life Venus Earth oxygen/ozone Mars

27 Are Earth-like planets rare or common?

28 Elements and Habitability Some scientists argue that proportions of heavy elements need to be just right for the formation of habitable planets. If so, then Earth-like planets are restricted to a galactic habitable zone.

29 Impacts and Habitability Some scientists argue that Jupiter-like planets are necessary to reduce the rate of impacts. If so, then Earth-like planets are restricted to star systems with Jupiterlike planets.

30 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.

Jovian Moons An intriguing possibility is that life as we know it could exist on jovian moons If, that is, those moons are in the HZ of their star Of the 542 confirmed exoplanets, 124 of

31 Jovian Moons An intriguing possibility is that life as we know it could exist on jovian moons If, that is, those moons are in the HZ of their star Of the 542 confirmed exoplanets, 124 of them are in the HZ Only 2 of those are near Earthsize, and they are close with highly elliptical orbits A number of the jovian planets are in more Earth-like orbits Good for jovian moon life

32 The Bottom Line We don t yet know how important or negligible these concerns are. The general feeling among most scientists is that microbial life is likely to be common But how common intelligent, technological life like us humans is, is unknown

33 How many civilizations are out there?

34 The Drake Equation Number of civilizations with whom we could potentially communicate = N HP f life f civ f now N HP = total number of habitable planets in galaxy f life = fraction of habitable planets with life f civ = fraction of life-bearing planets with civilization at some time f now = fraction of civilizations around now

35 We do not know the following values for the Drake equation: N HP : probably billions f life :??? Hard to say (near 0 or near 1) f civ :??? It took 4 billion years on Earth f now :??? Can civilizations survive long-term?

36 Are we off-the-chart smart? Humans have comparatively large brains. Does that mean our level of intelligence is improbably high?

37 How does SETI work?

38 SETI experiments look for deliberate signals from E.T.

39 We ve even sent a few signals ourselves Earth to globular cluster M13: Hoping we ll hear back in about 42,000 years!

40 Your computer can help! Home: a screensaver with a purpose

41 Where are the aliens?

42 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 haven t we detected them?

43 Possible solutions to the paradox 1. We are alone: life/civilizations much rarer than we might have guessed Our own planet/civilization looks all the more precious

44 Possible solutions to the paradox 2. Civilizations are common, but interstellar travel is not, perhaps because: interstellar travel is more difficult than we think. the desire to explore is rare. civilizations destroy themselves before achieving interstellar travel. These are all possibilities, but they are not very appealing.

45 Possible solutions to the paradox 3. There IS a galactic civilization and someday we ll meet them.

46 Exocivilizations If there are other civilizations as advanced as ours. What would they be like? What would the people look like? How would they think? Presumably they would have similar science to ours What would their art be like? What would their philosophy be like? What would their religion be like?

Copyright 2009 Pearson Education, Inc. Life in the Universe

Copyright 2009 Pearson Education, Inc. Life in the Universe Life in the Universe Life in the Universe The only place we know life exists is here on Earth One of humanity s Big Questions is whether it exists elsewhere We can get some clues by considering life s