The Search for Extraterrestrial Intelligence (SETI)

Size: px

Start display at page:

Download "The Search for Extraterrestrial Intelligence (SETI)"

Ira Simpson
5 years ago
Views:

1 The Search for Extraterrestrial Intelligence (SETI) Our goals for learning What is the Drake equation? How many habitable planets have life? How many civilizations are out there? How does SETI work? Can SETI be justified rationally?

2 How many civilizations are out there? The Drake Equation Number of civilizations with whom we could potentially communicate = N HP f life f civ f now N HP = total # of habitable planets in galaxy f life = fraction of habitable planets with life f civ = fraction of life-bearing planets with civilizations at some time f now = fraction of civilizations around now.

3 Our activity Number of Sophomore, sociology students in the library on a weekday night with short hair using a laptop Min = 1 Max = 625 Average = 65 Most around 50 Good deal of uncertainty, even on this simple problem

4 How many Habitable Planets in Milky Way? N HP f life f civ f now Best estimates come from Kepler observations:

5 Kepler observes only a small fraction of the sky Kepler Field of View

6 Kepler Mission is becoming more sensitive to smaller planets BLUE = 1 st yr data RED = 2 nd yr data YELLOW = 3 rd yr data Notice smaller planets with longer periods being discovered in 3 rd yr data

7 How many Habitable Planets in Milky Way? N HP f life f civ f now Initial extrapolations from Kepler observations: N stars in the Galaxy with planets ~ 50 billion

$What fraction of our Solar System in Habitable Zone? Solar System Habitable Zone from 0.7 3.$

8 What fraction of our Solar System in Habitable Zone? Solar System Habitable Zone from AU Orbit of Neptune (farthest planet) is ~ 30 AU Area of H.Z. p ( ) Fraction of H.Z. = = ~ 0.01 Area of Sol. Sys. p 30 2

9 How many Habitable Planets in Milky Way? N HP f life f civ f now Initial extrapolations from Kepler observations: N stars in the Galaxy with planets ~ 50 billion But only ~ 1:100 of those stars has a planet in a Habitable Zone N HP ~ 500 million

10 How many Habitable Planets have life? N HP f life f civ f now To date, we only know of 1 example: Most Pessimistic Estimate: f life ~ 1 / 500 million

11 Solid surface with atmosphere conducive to life 40% of Kepler-discovered planets are Super-Earths or smaller

12 However - Moon of Gas Giants Could be Habitable

13 Development of life happened in few 100 million years

14 Earth was heavily bombarded with rocks/comets Possible sterilization of biogensis

15 Age dating craters on Moon are direct evidence of this

16 Indeed the Moon was formed by a large impact

17 And the Moon coalesced from the debris

18 Impacts may have sterilized early Earth shortening time for biogenesis

19 Is Biogensis Common?? If yes, then optimistic estimate: f life ~ 0.5-1

20 Water is common in all Star-Forming Regions

21 Need enough time for SUPERGIANTS Biogensis GIANTS High mass limit of 2 times solar mass for lifetime to be at least 1 billion years for development of complex life. WHITE DWARFS Low mass limit of 30% of solar mass to have a habitable zone spanning region of the terrestrial planets.

22 Red dwarfs are most numerous stars in Milky Way (~90%) But they have the smallest Habitable Zones The less massive the star is, the smaller the habitable zone.

23 Tidal locking can occur with strong interaction (being close) Same side of Moon always faces Earth

24 Tidal locking should occur for planets close to stars Planets in H.Z. around red dwarfs may have same face always facing the star = potentially bad for life

25 Stellar Flares can be detrimental to life Fast rotating large planets can have strong, protective magnetic field Red dwarf are more active than solar-type stars

water Below this line, H 2 can escape planet gravity

26 Gas Speed (km/s) More evidence size matters : retaining an atmosphere Retains hydrogen Retains water Below this line, H 2 can escape planet gravity Below this line, H 2 O can escape planet gravity Temperature (K)

27 Our Solar System may hold the answer Life on Europa? Was there ever life on Mars?

28 How many Habitable Planets have life? N HP f life f civ f now Bottom line is that we don t know and the answer could be anywhere between ~ 0 and 1 Most Pessimistic Estimate: f life ~ 1 / 500 million Is Biogensis Common?? If yes, then Optimistic Estimate: f life ~ 0.5-1

29 How many planets with life develop intelligence? N HP f life f civ f now To date, we only know of 1 example and it took 4.5 billion years:

30 It took almost 4 billion years before Cambrian Explosion

31 How many planets with life develop intelligence? N HP f life f civ f now To date, we only know of 1 example and it took 4.5 billion years:

32 Earth s Moon has stabilized tilt of orbital axis

33 Mars has no moon to stabilize its rotation axis Climatic variability how does this affect life?

34 Earth has plate tectonics which cycles Carbon Plate tectonics favor larger planets?

35 Was Earth lucky? Water from comets? Jupiter protecting us from sterilizing events Large moon, stable rotation Magnetic field Plate tectonics

36 How many planets have intelligent life now? N HP f life f civ f now Major extinction events have occurred in the past

37 f now = L civ /L planet How many planets have intelligent life now? N HP f life f civ f now Ends of civilizations may be self-inflicted

38 We do not know the values for the Drake Equation N HP : ~ 500 million (estimates will improve next few yrs) f life :??? Hard to say (near 0 or near 1) f civ :??? It took 4.5 billion years on Earth f now :??? Can civilizations survive long-term?

39 Optimistic case N HP : ~ 500 million f life : 0.5 f civ : 0.1 f now : L civ = 1 million years, L planet = 10 billion years f now = 10-4 N civ = 2500 Note: this is highly dependent on L civ

40 Pessimistic case N HP : ~ 500 million f life : 1 in a million f civ : 1 in a million f now : L civ = years, L planet = 10 billion years f now = 10-6 N civ = 5*10-10 However, with galaxies life in the Universe is not out of the question

41 A Series of Diminishing Factors

42 So why is this a useful equation? Frank Drake

43 How many planets have communicating civilizations? N HP f life f civ f communicate Humans have had radio communication for only ~ 100 years

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

45 How does SETI work? Looking for deliberate signals from E.T.

46 Best Frequency to search is in Radio lowest noise

47 Your computer can help! Home: a screensaver with a purpose, looking for signals in radio data (mostly just noise).

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