Life on Other Worlds? Please pick up your transmitter and swipe your ID

The chemistry of all known life forms on Earth is based on which essential element? :10 0 of 5 1.Hydrogen 2.Helium 3.Carbon 4.Oxygen 5.Iron

The Physical Basis of Life All life forms on Earth, from viruses to complex mammals (including humans) are based on carbon (C) chemistry. Carbon-based DNA and RNA molecule strands are the basic carriers of genetic information in all life forms on Earth. The Tobacco Mosaic Virus contains a single strand of RNA, about 0.1 mm long This complex mammal contains about 30 AU of DNA.

Do we find evidence for carbon-based compounds anywhere else than on Earth? :10 0 of No 2.Yes, but only in the atmospheres of other terrestrial planets 3.Yes: in the atmospheres of most planets (and moons) which have an atmosphere. 4.Yes: On the surfaces of other planets and moons throughout the solar system. 5.3 and 4.

The basic building blocks of life Simple carbon-based (organic) compounds are found not only in the solar system (on planets and moons), but also in interstellar space: Methane Ethane Formaldehyde …

The Miller Experiment Miller Experiment in 1952: Simulating conditions on Earth when life began ~ 4 billion years ago: Water (oceans), primitive atmosphere gases (hydrogen, ammonia, methane), and energy from electric discharges (lightning). Experiment produced some of the fundamental building blocks of life: amino acids, fatty acids,...

Requirements of Life Liquid water (for chemical reactions and as transport medium). Atmosphere (to avoid rapid vaporization of water; gases needed for organic compounds) Moderate temperatures (keep water liquid; avoid disintegration of organic compounds; activate complex chemical reactions) Time for life to evolve from simple organic compounds into higher life forms: several billion years.

Geologic Time In geologic terms, higher life forms, in particular mammals and humans, have evolved only very recently. Humans have existed for only ~ 3 million years.

Do we have evidence for planets around other stars? :10 0 of No. 2.Yes, but only very few might have planets. 3.Yes, many (single) stars might have planets.

Requirements for Life in Other Planetary Systems Planetary systems are probably common.

Do you expect that virtually all stars will be single, rather isolated stars like our sun? :10 0 of Yes. Most stars should be single stars. 2.No. Stars form in clusters, so many stars could still be bound in small groups (double-, triple-star systems). 3.No. Stars form in clusters, so virtually all stars should still be bound in large groups of several hundreds to thousands of stars.

Requirements for Life in Other Planetary Systems Planetary systems are probably common. Stable orbit around the star → consider only single stars. About 50 % of all stars in our Milky Way are bound in double- or triple-star systems.

If we have a planet with a reasonable atmosphere, water, and carbon-based compounds, could life always evolve on them? :10 0 of No, it needs to have seasons. 2.No, it needs to be at an appropriate distance from the star to provide reasonable temperatures. 3.No, it needs to be around a star that lives at least several billion years. 4.All of the above 5.2. and 3. 6.Yes.

Requirements for Life in Other Planetary Systems Planetary systems are probably common. Stable orbit around the star → consider only single stars. Time for evolution → consider only stars that live for several billion years. Moderate temperatures → Life zone around the star Orbit of Mars Orbit of Earth Orbit of Venus Sun Too cold Life zone Too hot

Are we alone? The Drake Equation Factors to consider : Let’s try to estimate the number of civilizations (N c ) in our Milky Way that are currently in a state of their development that they are intelligent, advanced enough, and willing to communicate with us. 1)Number of stars in the Milky Way: N * ≈ 2* )Fraction of stars with planets: f p ≈ 0.01 – 0.5

Factors to consider (contd.) 3)Number of planets per star that lie in the life zone for longer than 4 billion years: n LZ ~ Orbit of Mars Orbit of Earth Orbit of Venus Sun Too cold Life zone Too hot

Factors to consider (contd.) 4)Fraction of suitable planets on whith life actually begins: f L ~ )Fraction of eco-systems in which a life form evolves to inteligence f I ~ )Fraction a star’s life during which there is a communicative civilization F S ~ – 10 -4

The Drake Equation The number of technologically advanced civilizations per galaxy that are currently able and willing to communicating with others: N c = N * · f p · n LZ · f L · f l · F S Most of the factors are highly uncertain. N c ~ 2*10 -5 – 2*10 7

What does it mean if N c ~ 2*10 -5 ? :10 0 of There are currently 2*10 -5 communicating civilizations in the Milky Way. 2.There are currently 50,000 (= 1/[2*10 -5 ]) communicating civilizations in the Milky Way. 3.If you take 2*10 -5 galaxies, you can expect to find one communicating civilization in them. 4.If you take 50,000 galaxies, you can expect to find one communicating civilization in them.

The Drake Equation Possible results range from 1 communicative civilization within a few dozen light years to us being the only communicative civilization in the Milky Way. N c ~ 2*10 -5 – 2*10 7

So, what’s the final word on the question: “Are we alone in the Milky Way?” :10 0 of There is most likely no other communicative civilization in the Milky Way at this time. 2.There is a reasonable chance that there are a few other communicative civilizations in the Milky Way at this time. 3.There are probably thousands or even millions of other communicative civilizations in the Milky Way at this time. 4.We have no clue!