Three Tough Questions

Star field
In a universe brimming with stars, the search for life is in part a numbers game

Q: Approximately how many stars are there in the universe?

A: The number of stars in the visible universe is estimated to be 70 sextillion, or 70,000,000,000,000,000,000,000 [seven followed by twenty-two zeros].

Such a vast population can be compared in a list of the very biggest numbers imaginable, with some terrestrial references borrowed from a combination of science and poetry:

  • ten times more than the number of grains of sand on Earth
  • eleven times the number of cups of water in all the Earth’s oceans
  • ten thousand times the number of wheat kernels that have ever been produced on Earth
  • one hundred million times more than the number of ants in all the world
  • one hundred million times the dollar value of all the market-priced assets in the world
  • ten billion times the number of cells in a human being
  • one hundred billion times the number of letters in the 14 million books in the Library of Congress


The Terestrial Planet Finder will search for Earth-like planets orbiting 250 of the closest stars.
Credit: NASA

In the realm of astrobiology, it may be said that most meaningful terrestrial analogies to the number of stars in the known universe are indeed biological: only a fertile biosphere can yield such large numbers. The earth’s biological census currently lists around 28,000 species with a backbone–a miniscule number of advanced species (vertebrates) relative to the microbial ecosphere.

"On this single planet called Earth, there co-exist (among countless other life forms), algae, beetles, sponges, jellyfish, snakes, condors, and giant sequoias. Imagine these seven living organisms lined up next to each other in size-place. If you didn’t know better, you would be hard-pressed to believe that they all came from the same universe, much less the same planet". –Neil DeGrasse Tyson, director of the Hayden Planetarium
Credit: NGS

So to further the comparison, one may ask: how many living ‘things’ the Earth itself can accomodate in its volume?

If one cubic inch can hold ten billion animal or plant cells, and if one stacked these cells across both the land and oceans to a thickness of fifteen feet, the planet would be a vast teeming mass of biology–literally, life as far as the eye could see. The thickness of fifteen feet, while extreme overpopulation on the land, is likely an underestimate given the depth of the more three-dimensional ocean biosphere or the realms of winged species.

In this way, the ceiling on the carrying capacity of Earth for cellular life is vast, since about ten million times the number of plant or animal cells could pack the planet than the number of stars in the visible universe. Compared to 70 sextillion, the cellular capacity terrestrially is estimated to be what can be called one undecillion, or ten raised to the power of 30.

Q: How many stars have scientists examined so far?

A: In any great detail, perhaps about 10,000 stars. It depends on if the question refers to whether stars are looked at in visual light or as radio sources. It also is not so much the stars themselves, but the number of habitable planets that is important.

Using terrestrial radio telescopes, Berkeley’s SETI@home project stores about 100 million candidate radio signals, and classifies about 200 of these candidates as "interesting".

For direct observations, the 2004 French COROT mission will look at 50,000 to 60,000 stars and should find a few dozen terrestrial planets and several hundred close-in gas-giant planets during a two- to three-year mission. In 2006, the Kepler mission, or Extrasolar Terrestrial Planet Detection Mission, is designed to look for transiting or earth-size planets that eclipse their parent stars [in a sky survey of 100,000 stars]. Scientists expect to find thousands of planets, and perhaps 50 Earth-like candidates.

One goal of the Terrestrial Planet Finder (TPF) will be to find and characterize any Earth-like planets orbiting 250 of the closest stars. This search will focus on the habitable zone, which is defined by the range of temperatures where liquid water, and thus the conditions for the formation of life, might be present. TPF will make detailed observations of the atmospheres of the most promising candidates to search for the spectral signatures of habitability and of life. Next steps beyond TPF may include a "Planet Imager" to provide more detailed images and/or spectroscopy of any planets found by TPF.

The challenge to astrobiologists is to determine what biosignatures can be expected on any living planet.

Knowing what to look for on a living planet is not trivial. When the Galileo spacecraft flew by Earth on its way to Jupiter, the spacecraft turned its instruments toward Earth to look for signs of life. Other than the radio signals and the lights being on at night, the signs of life from Earth were surprisingly subtle. There was a complex green color on the continents (which wa known as terrestrial plants) and chemicals like carbon dioxide, oxygen, methane, and nitrites coexisting in the atmosphere – a chemical impossibility unless maintained by something like life.

Q: In doing a radio search, what kinds of signals are looked for?

A: SETI radio search involves a frequency that is relatively quiet, between 1000 and 10,000 MHz–just above the frequencies used by electronic pagers and some wireless cell phones at 900 MHz. The most abundant molecules are hydrogen, either neutral gas at 1420 MHz or combined with oxygen at 1640 MHz. In the spectrum of background that rains on our planet from interstellar space, this quiet region is called the ‘water-hole ‘, because water molecules (necessary for life as we know it terrestrially), has this vibration.

Modern SETI efforts began with a paper written by physicists Giuseppe Cocconi and Philip Morrison. They published in the science press in 1959. Cocconi and Morrison suggested that the microwave frequencies between 1000 and 10,000 megahertz would be best suited for interstellar communications.

Related Web Pages

Terrestrial Planet Finder Home Page
What Does ET Look Like from 40 Light Years Away?
Anybody Out There? Part I
Anybody Out There? Part II
Search for Life in the Universe: Neil deGrasse Tyson Interview
Aliens Depend on Time to Grow Brains
Darwin Mission
Rare Earth? Are we so special
Stellar Countdown Yields Skymap
Berkeley: SETI