AST 309part 2:

Extraterrestrial Life

A new science:

Astrobiology

• Sometimes called “exobiology” and “bioastronomy”

• Literally means the study of life in the universe

• Trying to answer the age-old questions “Are we alone?”

• The discovery of life elsewhere can be regarded the single most profound discovery in human history

A new science:

Astrobiology

• Interdisciplinary (physics, chemistry, geology, etc…)

• Scientists need to learn to talk to each other!

• The astronomical part is concerned with the conditions for life in the cosmos

• the biological part is concerned with questions like what is life in the first place and how did it emerge?

A new science:

Astrobiology• The major problem: astrobiology has no gravity!

– We observe and measure gravity everywhere in the cosmos: it is truly a universal law!

– The processes we observe for life on Earth we cannot extrapolate to the rest of the universe

– The history of human exploration of the universe (aka science) has removed us more and more from the center of the universe, except for one remaining issue: life itself

A new science:

AstrobiologyJust over the past 10-20 years:

• The “astro” part discovered a large sample of exoplanets and is making excellent progress toward finding potential habitats for alien life.

• The “biology” part discovered “alien” life-forms here on Earth: the extremophiles can survive and thrive in conditions previously thought impossible (this opens up a completely new parameter space for life in the universe)

Overview

• What is life?• What do we know about life on Earth?• What about life in the Solar System (Mars)?• The Drake Equation• Life on extrasolar planets?• Extraterrestrial Intelligence? (SETI)

What is life?

• Definition is very difficult as life is a process!• “We know it when we see it” (really true?)• Biology uses the combination of certain

characteristics as signs of life, like metabolism, growth, reaction to stimulus, reproduction, etc. (every single one is not enough e.g. fire grows & breaths oxygen, crystals reproduce…)

• Darwinian Evolution is another main feature of life (even valid for viruses)

• Physics uses entropy (level of discorder) as parameter, life is “negative entropy” (Schroedinger) i.e. it is reducing its own entropy at the expense of external sources

What is life?

“I’d look for an entropy reduction, since this must be a general characteristic of life.”

James Lovelock 1964

NASA asked “How can we identify martian life?”

Life on Earth

In order to be able to find life outside our Earth, we have tounderstand life in our own planet. The chemistry of life and thedifferent processes during the formation and evolution of theEarth have played a crucial role.

Is life on Earth a very special thing ?Can life spawn spontaneously elsewhere ?

There are roughly 10^11 (100 billion) stars inthe Milky Way and similar number of galaxies in the observable Universe.

Are we that special ?

Planets, if they form at all, must form as part of the star formation process: Planets are literally what formed

out of the “debris” that didn’t find its way into a star

These images are the Orion and Omega Nebulae: star-forming regions as observed in the visual part of the spectrum. These regions are typical, containing hundreds to thousands of newly-formed (and forming) stars from ~ 0.1 to 100 solar masses. The gas is glowing because of the radiation from the massive young stars. The “dark lanes” are dense regions in front of the glowing gas; they are dark because of the dust they contain. Planets will have to form amidst this energetic activity due to the massive stars (winds, jets, explosions), so it is not obvious whether the formation of planets is likely.

One promising result is that many molecules, some complex, are observed, mostly through their spectral lines due to rotational transitions in the radio part of the spectrum. Some examples of molecular rotational spectra, from simple to more complex, are shown below.

MethanolGlycine

Spectral lines in interstellar clouds: Evidence that organic molecules form easily, even in extremely harsh environments

Elements of life: H, C, N, O common only here, or in our neighborhood, but not elsewhere? Are these produced in

special,rare, events? No, H, C, N, O are the most common elements in just about every object in the universe. Only the total amount of ‘metals” (heavier than He) varies, but their proportions are amazingly constant.

Consider composition of Sun: 75%H by mass, ~ 1% C, N, O, everything else either helium (useless for life--inert), or much less abundant. Just about same for all known stars!Same is found in the gas of the interstellar medium, and in the stars and gas of the most distant galaxies.

Does it seem odd to you that the four most abundant elements are just those elements on which life is based, if those elements indeed have special properties? The “special properties” could have been some rare element, but no….

A supernova remnant

Simulation of supernova explosion (20 milliseconds)

Why are abundances of elements so universal?

Hydrogen has been around from the beginning (universe was originally only fundamental particles, including protons = hydrogen (rest was electrons, photons, …, no other elements)

Carbon, oxygen produced in red giant stars, which later explode as supernovae. All stars become red giants, but only massive stars produce supernovae; massive stars are rare (~ 1% of stars). So why is there carbon and oxygen everywhere?

Nitrogen the Earth’s atmosphere is mostly nitrogen.Important? (Yes: Nitrogen doesn’t react well with oceans, rocks, so our atmosphere is stable. But weirdpart to this: If not for the nitrogen cycle, involving bacteria, the nitrogen would have disappeared long ago.

Abundances of elements vs. atomic number

All planetary systems formed as part of the star formation process

The standard model of the formation of the sun is that it collapses under gravity from a proto-cloud

Because of rotation it collapses into a disk.

Jets and other mechanisms provide a means to remove angular momentum

So we had our Earth forming by so-called planetesimal accretion at 1 AU from our young Sun

Life on Earth

Life is a self-sustained set of chemicalreactions based on

with carbon (C) as the key chemical element

The atomic structure of carbon allows forthe formation of long chains of C-Cchemical bonds on which other elementscan be attached, giving a wide range ofchemical properties.

Life on Earth

Life is a self-sustained set of chemicalreactions based on

+ carbon as the key chemical element

and

+ water as the key solvent

Water comprises ~70% of the masscontent of living organisms and constitutes the medium on which biochemicalreactions take place.

Life on Earth

All known life on Earth isDNA-based => we all sharea common ancestor!

Life on Earth

How and where did life form in the beginning?

Life on Earth

How and where did life form in the beginning?

Nobody knows!

What about Mars?

Viking lander

What about Europa?

What about Titan?

What about Enceladus?

What about Earth-like planets around other stars?

The Drake Equation

Where should we search for extraterrestrial life? How should we search? What is required to have life? Complex life? Life we could communicate with?

The “Drake Equation” simply organizes these supposed requirements into separate factors, a sort of list of possibilities for our consideration.

We want to estimate the likelihood that there are stars with planets with life that developed into complex “intelligent” technological forms that might be sending or receiving signals.

What we really want is the total number of them, because that tells us how far we might have to search.The Drake equation assigns a symbol for each one of these key factors, representing its probability of occurrence, and multiplies all of them together. It is not something that is actually solved, or that you will have to work with except to see a few basic things.

At the end you should see this “equation” as a map of our class topics:

The Drake Equation:

N = N* fpl nhab fL fC fT L/T

Stars ? Planets? Habitable Origin Complex Intelligence, Lifetime planets? of life? life? technology? of civilization

The Drake equation is just a symbolic way of asking what the probabilities are that a sequence of events like those below

(and more) might occur in other planetary systems.

Our place in the Galaxy The disk is ~100,000 l.y. across, Sun is about 30,000 l.y. from center.

Think about times for communication at the speed of light!Clearly we can only search for life among the nearest stars, and for that

to be successful, it must be the case that a significant fraction of all stars must have life, “N” must be very very large

If we leave out fi and fc (i.e. assume they are unity—all life forms develop our kind of intelligence and technology and try to communicate), we are calculating the number of life-bearing planets in our Galaxy at any given time (like now). We know there has been life on our planet for 3 billion years, so take L = 3 billion. Let’s be optimistic about fP (0.1), nP (1), and fL= (0.1). Then

Nlife ~ 1011 x 0.1 x 1 x 0.1 x (3 billion/10 billion) = 300 million

300 million planets with life in our Galaxy! That’s roughly1 out of 1000 stars. This means that the nearest life-bearing planet might only be 10-100 light years away, close enough that in the future we may be able to detect such planets and obtain their spectra (that is the primary goal of astrobiology space missions for the next decade).

This result is a major reason for exerting most of our effort toward detecting signatures of biochemistry in the spectra of planets orbiting nearby stars. You will be reading and hearing a lot about “biosignatures” in this class soon!

Now estimate number of planets with life in our Galaxy(not number with intelligent, communicating life)

The reason for searching for signs of life from only nearby stars is not only a matter of communication times.

We are also interested in detecting signs of life in the spectrum of light emitted from a planet’s atmosphere.

In order to find such “biosignatures,” you need an extremely large telescope, and for the planet + star to be as nearby as possible, both inorder to maximize the light received.

Illustration is “zoom-in” on the region of our Galaxy from which we can plausibly detect life, or signals from communicating civilizations.

A timeline for the very early history of the EarthAnother way of looking at the sequence of the required events that are (symbolically) represented by the Drake

equation:

Habitable planets Origin of life on planets Development of complex life

Drake EquationFactors that determine the likelihood of life, intelligence, and

technological civilizations in our Galactic neighborhood.

StarsThe only thing everyone agrees on is that, to get life, you need a planet, and a planet orbiting a star (nearly all probably do). So first we need the number of stars in our Galaxy, which is about 1011.We denote this “N subscript star” or N*, so

N* = 1011 starsThe Sun is an average star. We’ll see which stars might be best for life later. See picture above—young stars are very active: Dangerous radiation environment.

Average separation of stars is a few light years.(Compare with size of Galaxy: about 100,000 light years; nearest other galaxies ~ millions of light years distant.

This means that if there are only 106 (a million) communicating civilizations in the Galaxy, the average one will be too distant for

two-way communication. (Think about this.)

The planet factor

Molecules can only react and survive at temperatures like those of planets. This is ~ room temperature ~ 300 degrees Kelvin (300K).

Temperatures of stellar surfaces are ~ 3,000-10,000 K: too hot for molecules, water, anything we think you need for life. It all vaporizes to a simple gas of atoms. Why are planets so much cooler than stars? (Important for rest of course)The Drake equation question: What is the probability that a star has a planet?Or, what fraction fpl of stars have planets? In equation form, we could write

N(planets) = N* times fpl .This just says: number of stars with planets in the Galaxy is the number of stars times the fraction of them that have planets.Evidence: Observations of extrasolar planets show that giant planets (like Jupiter, or even Neptune).But how about Earth-like (much smaller, rocky) planets? With oceans? One may be found this semester! Kepler mission)

Life almost certainly requires complex molecules. Complex

molecules require planets. Why?

Planets: Can’t have life without them

Artist’s conception of an extrasolar planet orbiting A faint red parent star

Planets of our Solar System

Planets: Can’t have life without them

Artist’s conception of an extrasolar planet orbiting A faint red parent star

Planets of our Solar System

OOPPSS….

The temperature of a planet’s surface is mostly controlled by it’s distance to its parent star, and its parent star’s luminosity, because

that determines how much energy it receives. The illustration below shows the Sun as it would appear from Pluto:

Way too cold for liquid water (but plenty of water ice) => Pluto is far outside the “habitable zone.”

What controls a planet’s surface temperature?

The “Habitable Zone” (HZ)around different stars

• Conditions just right to allow liquid surface water on a rocky planet.

The galactic habitable zone• There may also be a

preferred time and locationwithin the galaxy forhabitable planets to exist

• Stars that are too close tothe center of the galaxy aresubject to frequent near-collisions and moresupernovae and gamma raybursts

• Stars that are too far out inthe galaxy (or that evolvedtoo early in its history) maybe too metal-poor

• Fortunately, though, theGHZ is probably largecompared to the local solarneighborhood

Ref.: Lineweaver et al., Science (2004)

What about Earth-like planets around other stars?

Searching for Earth-like planets and so-called “bio-signatures” intheir atmospheres

Terrestrial Planet Finder:Kepler:

ELT (42m)

What about Earth-like planets around other stars?

Search for ExtraTerrestrial Intelligence (SETI):

•How do we search for ETI?

•What has been done?

•What have we found?

Main issues of Astrobiology:

• Understanding the origin of life on Earth

• How likely is it for life to appear under conditions similar to Earth’s?

• How common are planets like Earth’s

• How likely is it for intelligent life to evolve?

• How likely is it for a civilization to survive over stellar lifetimes?