Life in the Universe Are We Alone? Illustration by Dana Berry

Standards Examine investigations of current interest in science. Understand the scale and contents of the universe Identify important questions that science cannot answer (e.g., is there life outside our solar system?)

Is There Life in the Universe? This is a question with profound implications for humans. It is, however, one for which we have no data. Earth is the only place in the universe where we know for certain that life exists.

Cosmic Evolution There are 7 major phases in the history of the universe: 1.Particulate 2.Galactic 3.Stellar 4.Planetary 5.Chemical 6.Biological, and 7.Cultural evolution

Arrow of Time

Cosmic Evolution These evolutionary stages make up the grand sweep of cosmic evolution – the continuous transformation of matter & energy that has led to the appearance of life & civilization on Earth. The universe has evolved from simplicity to complexity.

Cosmic Evolution We are the result of an incredibly complex chain of events that spanned billions of years. Were those events random, making us unique, or are they in a sense natural, so that technological civilization is inevitable? Are we alone in the universe, or just one of many intelligent life-forms?

Definition of Life Defining life is not an easy task. There are no clear-cut definitions.

Definition of Life The following are, in general, the characteristics of living organisms: 1.They can react to their environment. 2.They can grow by taking in nourishment and processing it into energy. 3.They can reproduce, passing along their characteristics to their offspring. 4.They have the capacity for genetic change & can therefore evolve from generation to generation & adapt to a changing environment.

Definition of Life These rules are not strict. Stars, for example, react to the gravity of their neighbors, grow by accretion, generate energy & “reproduce” by triggering formation of new stars, but no one would suggest that they are alive.

Case in Favor of Extraterrestrial Life Assumptions of mediocrity: 1.Because life on Earth depends on just a few basic molecules, and 2.Because the elements that make up those molecules are (to a greater or lesser extent) common to all stars, and 3.If the laws of science that we know apply to the entire universe, then – given sufficient time – life must have originated elsewhere in the cosmos.

The Opposing View Maintains that intelligent life on Earth is the product of a series of extremely fortunate accidents – astronomical, geological, chemical & biological events unlikely to have occurred anywhere else in the universe.

Chemical Evolution Conditions on early Earth were right for amino acids and nucleotide bases to form. These are the building blocks of life as we know it. Amino acids build proteins and nucleotide bases form genes.

Chemical Evolution Many experiments have been done in which energy has been applied to a mixture of compounds found in atmosphere of early Earth. These experiments have created amino acids and nucleotide bases, showing that the building blocks of life can be formed from the raw materials found on early Earth.

Diversity and Culture Simple one-celled organisms such as blue-green algae appeared on Earth more than 3.5 Ga. These were followed by more complex one-celled organisms, like amoeba, about 2 Ga. Multicellular organisms, such as sponges did not appear until about 1 Ga. The evolution of the rich variety of life on our planet occurred as chance mutations.

Diversity and Culture What about the development of intelligence? Many anthropologists think intelligence is favored by natural selection and closely linked to language. Cultural evolution on Earth began around 10,000 years ago.

Life in the Solar System Life as we know it means carbon- based life that originated in a liquid water environment. Might such life exist elsewhere in our solar system?

The Moon & Mercury Lack liquid water, protective atmospheres & magnetic fields. They are bombarded by UV radiation, solar wind, meteoroids & cosmic rays. Simple molecules could not survive in such harsh environments.

Venus By contrast, has far too much atmosphere. Its dense, dry, scorchingly hot atmospheric blanket rules it out as a home for life, at least like us.

The Jovian Planets Have no solid surfaces (though some researchers have suggested life might have evolved in their atmospheres).

Pluto and the Jovian Moons Are too cold for life. However, the possibility of liquid water below Europa’s icy surface has caused speculation about the possibility of life there. This moon of Jupiter is a prime candidate for future exploration & is high on both NASA’s & the ESA’s priority list for missions.

Mars Is the planet most likely to have life, or to have had it in the past. It is harsh by Earth standards: liquid water is scarce, the atmosphere is thin, UV radiation & high-energy particles reach the surface.

Mars The Martian atmosphere was thicker & the surface probably warmer & much wetter in the past. There is strong photographic evidence from Viking Orbiter & Mars Global Surveyor for flowing & standing water on surface in past. The European Mars Express confirmed water ice at the poles in Opportunity reported strong geological evidence that the area of its landing site was once drenched in water for an extended period.

Mars No life has yet been detected on Mars, but it has not been ruled out.

Extremophiles There is lots of research into life in extreme locations on Earth – called extremophiles. The thought is that if there is life elsewhere in our solar system, it will be in the form of extremophiles.

Examples of Extreme Locations Include: Undersea hydrothermal vents (or black smokers)

Examples of Extreme Locations Include: Caves such as Lechuguilla, in NM

Examples of Extreme Locations Include: Cold environments such as Antarctica and ice caves

Alternative Biochemistries There may be life very different from that on Earth. Some scientists suggest that maybe life could be silicon based (instead of carbon) and have formed in an ammonia environment (instead of water). But, we know nothing about non-carbon, nonwater biochemistries for the very good reason that there are no examples on Earth to study.

Alternative Biochemistries Note: an organism with an alternative biochemistry was recently (Dec 2010) found to exist in Mono Lake, CA: substitutes arsenic for phosphorus in its cells This was then disproven in a study released in 2012

Intelligent Life in the Galaxy At galactic distances, we have little hope of actually detecting life w/current equipment. Instead, we must ask “how likely is it that life in any form exists?

The Drake Equation Is a statistical approach to whether there is life in the Galaxy. Several of the factors in the equation are a matter of opinion. We do not have nearly enough information to determine every factor in the equation. Its value is that it provides a framework within which the problem can be addressed & divides the responsibility among different scientific disciplines.

The Drake Equation # of technological, intelligent civilizations now present in Galaxy = (rate of star formation, averaged over lifetime of Galaxy) x (fraction of stars having planetary systems) x (average # of habitable planets within those planetary systems) x (fraction of those habitable planets on which life arises) x (fraction of those life-bearing planets on which intelligence evolves) x (fraction of those intelligent-life planets that develop technological society) x (average lifetime of a technologically competent civilization).

Rate of Star Formation Can be estimated by dividing current # of stars in Galaxy by the 10 Ga lifetime of our Galaxy. We obtain a formation rate of 10 stars per year.

Fractions of Stars Having Planetary Systems No planets like our own have yet been discovered (would be too faint against parent star). Note: The Kepler mission has discovered 961+ exoplanets, a few of which are Earth-like. Accepting the condensation theory of star formation, it is believed that nearly all stars form with planetary systems.

# of Habitable Planets per Planetary System Temperature is the biggest deciding factor. There is a zone of “comfortable” temperature – a stellar habitable zone- around every star. The hotter the star, the larger the zone.

Galactic Habitable Zone Galactic center too violent for life Galactic halo has too few heavy elements to form terrestrial planets or populate them with technological civilizations

# of Habitable Planets per Planetary System Taking uncertainties into account as best as possible, we get a value of 1/10. In other words, there is one potentially habitable planet for every 10 planetary systems in our Galaxy.

Fraction of Habitable Planets on Which Life Actually Arises Experiments suggest that the combination of elements into the molecules necessary for life are likely to occur. Therefore, given time, life is likely

Fraction of Life-Bearing Planets on Which Intelligence Arises Appearance of well-developed brain unlikely if only by chance. Some say if evolution plays a part, intelligence is inevitable, given enough time. Others argue that there is only one known case of intelligent life.

Fraction of Planets on Which Intelligent Life Develops & Uses Technology This is unknown. Some scientists say that species on other planets will probably always rise to fill the technological niche.

Average Lifetime of a Technological Civilization This is totally unknown. Humans on Earth are our only example. Humans have been technological for about 100 years.

The Drake Equation If we go with the optimistic value for all factors, then we end up with: # of technologically intelligent civilizations now present in our Galaxy = average lifetime of a technologically competent civilization in years

The Drake Equation Thus, if civilizations typically survive for 1000 years, there should be 1000 of them currently in existence in our Galaxy. If they live for a million years, we would expect a million advanced civilizations, and so on. If the life expectancy of a civilization is only a few thousand years, it is unlikely to have time to communicate with its nearest neighbor (communications can’t go faster than the speed of light).

Meeting Our Neighbors Let’s assume there are 1000 technological civilizations in our Galaxy. Given the size & shape of our Galaxy, they would be ~30 pc (or 100 l. y. ) apart. Therefore, any 2-way communication – using signals at the speed of light - would take at least 200 years.

Meeting Our Neighbors One obvious way to search for ET life would be to travel far outside our solar system. This may never be a practical possibility. At a speed of 50 km/s (fastest probes today) the round trip to a distance of 30 pc would take 600,000 years (just to get to the nearest star would take 50,000 years). Going the speed of light is currently beyond our technology.

Meeting Our Neighbors We have already launched IS probes. Pioneer 10 launched in the 1970’s and is now well beyond the orbit of Pluto, on its way out of the solar system.

Meeting Our Neighbors Replica of plaque mounted on board Pioneer 10. The important features of the plaque include a scale drawing of the spacecraft, a man, and a woman; a diagram of the hydrogen atom undergoing a change in energy (top left); a starburst pattern representing various pulsars and the frequencies of their radio waves that can be used to estimate when the craft was launched (middle left); and a depiction of the solar system, showing that the spacecraft departed the third planet from the Sun and passed the fifth planet on its way into outer space (bottom). All the drawings have computer-coded (binary) markings from which actual sizes, distances, and times can be derived. (C. Sagan)

Meeting Our Neighbors The Voyager probes were launched in 1978 with similar information. Although incapable of reporting back to Earth, scientists hope any encountered civilization could unravel the contents using the universal language of mathematics.

Meeting Our Neighbors Aside from practical problems, communication may not be a good idea. Any encountered civilization would be more advanced (since we are so technologically young). The most advanced civilization may try to dominate the others (remember example of European explorers and their domination of “primitive” cultures).

Radio Communication Is a cheaper & more practical alternative to direct contact. Radio can travel through dusty IS space. Radio telescopes on Earth listen passively for radio signals emitted by other civilizations.

The Water Hole But, at what frequency, among the many that make up the radio part of the spectrum, should we listen? Basic arguments suggest a wavelength of 20 cm. Hydrogen (H) atoms radiate at 21 cm & hydroxl (OH) radiates at 18 cm. Together, these form water (H 2 O), the substance out of which life as we know it was formed.

The Water Hole Therefore, researchers have proposed this interval between 18 & 21 cm (called the water hole) as the most likely place to search.

Radio Searches A few are now in progress in and around the water hole. One of the most sensitive & comprehensive searches for ET intelligence was Project Phoenix (known to many by its acronym SETI), carried out during the late 1990’s.