Astronomy 162: Professor Barbara Ryden

Friday, March 14

LIFE IN THE UNIVERSE

``It would be strange if a single ear of corn grew in a large plain, or were there only one world in the infinite.'' - Metrodoros of Chios

Key Concepts

Scientists have long speculated about the possibility extraterrestrial life.
Planned searches for life involve examining the spectra of Earth-like planets.
Searches for intelligent life involve scanning the sky for artificial radio signals.

(1) Scientists have long speculated about the possibility of extraterrestrial life.

Early in the history of astronomy, there were two views on life in the universe:

(1) The universe is finite, and the Earth is unique - the only place in the universe suitable for life. (This was the view of Ptolemy, for instance, with his finite, Earth-centered universe.)
(2) The universe is infinite, and contains innumerable inhabited worlds. (This was the view of the ``atomist'' philosophers of ancient Greece, such as Democritus, Epicurus, and Metrodoros of Chios.)

It is highly likely, in my humble opinion, that life exists elsewhere in the universe. Just consider the numbers:

about 100 billion galaxies in the universe
about 100 billion stars per galaxy
THEREFORE, about 10 billion trillion stars in the visible universe

If only one star in a trillion has inhabited planets, that's still 10 billion worlds with life. (However, they would be spread very widely apart.)

Life on Earth is based on complex carbon compounds (proteins, fats, starches, DNA, you name it) suspended in liquid water. Although science fiction writers have speculated about silicon-based life and even more exotic lifeforms, I'll concentrate on the search for life as we know it - carbon-based life dependent on the existence of liquid water. A key question (and one that remains unanswered) is whether life forms very readily on planets with liquid water, or whether it's an extremely rare accident. If life existed on other planets in our Solar System, it would support the hypothesis that life forms readily.

Although liquid water once flowed on Mars, the only evidence that life ever existed on Mars are some structures in a Martian meteorite that have been identified as tiny fossils. (For more information about life on Mars, you can read the lecture notes I prepared on this topic for my Astronomy 161 class.) A dark horse in the ``life in the Solar Systems'' race is Europa, one of the Galilean moons of Jupiter. Europa seems to have an ocean of liquid water beneath its icy surface. It would be interesting to drill through the ice and see what lies beneath...

(2) Planned searches for life involve measuring the spectra of Earth-like planets.

Where should we look if we wanted to find life (as we know it) outside the Solar System? First of all, we can forget the short-lived O and B stars on the main sequence. Once the Earth cooled enough for liquid water to exist on its surface, it took about 400 million years longer for life to arise. Any star with a lifetime of less than a few hundred million years probably won't have life on its surrounding planets. Second, we can forget the dim, cool M stars on the main sequence. In order to be warm enough for liquid water to exist, any planets around an M star would have to be in a very small region very close to the star.

In looking for Earth-like planets (terrestrial planets with oceans and life), it's best to look around Sun-like stars. So far, planets around other stars have only been found indirectly, by their gravitational effect on the parent star. (For more information about planet hunting, you can read the lecture notes I prepared on this topic for my Astronomy 161 class.) This method only finds massive, Jupiter-like planets; low-mass, Earth-like planets have a much tinier gravitational effect.

Potentially, it is possible to take direct images of Earth-like planets. To do this, it helps to look at infrared light. Sun-like stars have a surface temperature of 6000 Kelvin; thus, their spectrum peaks at visible wavelengths. Earth-like planets, however, have a surface temperature of 300 Kelvin (about 80 Fahrenheit); thus, their spectrum peaks at infrared wavelengths. It also helps, when you are searching for Earth-like planets, to have high resolution. From 10 parsecs away, the Earth and Sun would be separated by a maximum angle of only 0.1 arcseconds.

A proposed space-based telescope, designed to look for Earth-like planets, is the Terrestrial Planet Finder. The TPF will not only take snapshots of planets around other stars, it will measure their spectra, looking for absorption lines of ozone (O₃), carbon dioxide (CO₂), and water (H₂O) at infrared wavelengths. The Earth has lots of ozone and little carbon dioxide compared to sterile terrestrial planets like Mars and Venus. (Don't hold your breath; the Terrestrial Planet Finder is currently scheduled for launch in the year 2012.)

(3) Searches for intelligent life involve scanning the sky for artificial radio signals.

For purposes of discussion, I will define ``intelligent life'' as consisting of beings that have sufficiently advanced technology to send a spacecraft or a message between stars. (By this definition, human beings just barely qualify as intelligent.) Even if life is common in the universe, intelligent life may be rare. It took 400 million years for life to evolve on Earth, but 4 billion years for (just barely) intelligent life.

If intelligent extraterrestrial life exists, why hasn't it visited Earth recently? Actually, when posed this way, the question sounds pretty egotistical. You might equally well ask the question, ``What's so great about our planet that would cause aliens to flock to it?'' Nevertheless, an insatiably curious and extremely thorough alien civilization might decide to visit all the stars of the galaxy in turn, eventually visiting the Sun in the course of their survey.

Interstellar travel requires either (A) lots of time, or (B) lots of energy.

(A) The slow route between stars.

The spacecraft Voyager 1 and Voyager 2 are leaving the Solar System, after completing their surveys of the Jovian planets. Currently, they're traveling at about 25 km/second (pretty fast by Earthbound standards: about 50,000 mph). However, if either of the Voyagers were pointed at Proxima Centauri, it would take them 50,000 years to get there.

(B) The fast (but expensive) route between stars.

For spaceships traveling near the speed of light, the length of the journey (as measured on the home planet) is little longer than the light travel time. In addition, the relativistic effect of Time Dilation makes the trip even shorter for observers aboard the spacecraft. The drawback: travel at speeds close to the speed of light requires vast quantities of energy. In doing a bit of research for this lecture, I unearthed an amusing factoid: At current energy costs, accelerating one human being to 20% of light speed costs $2 billion dollars.

Travel at speeds close to that of light is not economically and technically feasible at humanity's current technological level. An extremely advanced civilization (one which is able to harness the entire energy output of a star, for instance) might be able to do it? What would bring such highly advanced beings to Earth? We can only speculate.

Travel is expensive; talk is cheap. That is, sending material objects between stars requires the investment of large amounts of energy; sending messages using electromagnetic radiation requires a much smaller amount of energy. If we wanted to send a message to a civilization orbiting a distant star, what wavelength would we use? Microwaves seem to be the best choice. The sun is not a tremendously powerful source at microwave wavelengths, so there's little danger of our message from Earth getting lost in the Sun's glare. Wavelengths longer than 30 centimeters are impractical because there's too much background `noise' from emission by interstellar gas. Wavelengths shorter than 3 centimeters are impractical because they are absorbed by water vapor and oxygen in the Earth's atmosphere.

In fact, we have been (inadvertently) beaming radio messages into space for three-quarters of a century, since large-scale commercial radio broadcasts first began in the 1920's. An expanding sphere of FM and TV broadcasts surrounds the Earth. (They become increasingly faint with distance, however, so it's not likely that other civilizations are intently watching the original broadcasts of ``I Love Lucy''.) If we ever succeed in detecting radio waves from an alien civilization, it will probably be the result of the civilization deliberately broadcasting a very strong signal into outer space saying ``WE ARE HERE! WE ARE HERE!''

Search for Extraterrestial Intelligence (SETI) programs are currently ongoing (although Congress cancelled a NASA-sponsored SETI program as a cost-saving measure). Privately funded SETI searches are using telescopes that range from small amateur radio telescopes to the huge Arecibo radio telescope. Nothing so far...

Prof. Barbara Ryden (ryden@astronomy.ohio-state.edu)

Updated: 2003 Mar 13