Earth is a unique planet in the solar system, and maybe the only one of its kind anywhere.

As we look with enthusiasm and anticipation for extraterrestrial life, this is one thing that remains in the forefront of our thoughts.

Astronomers have discovered dozens of planets around nearby stars, but none are close to being truly like Earth.

To be truly Earth-like there would have to be a moon. The moon has played a subtle but necessary role in the evolution of life here on Earth.

Being the only satellite in the solar system that is close in size to the planet that it orbits, the moon exerts strong tidal forces, shields us from impacts with cosmic dust and debris, and stabilizes the tilt of Earth's rotational axis to keep the seasons constant.

The moon exists due to an improbable collision with a Mars-size object 4 billion years ago near the end of the period when planetessimals had reached planetary size. It has been estimated that there is at best a 5 to 10 percent probability of a collision with primordial Earth at the right angle to vaporize Earth's crustal material that could coalesce to form the moon.

This collision not only created the moon, but also increased the mass and radius of Earth to its present size. Earth's mass, size and distance from the sun allow it to maintain an atmosphere within a range of pressure and temperature that allows water to exist in all three phases.

The habitable zone for our sun starts beyond Venus and ends before Mars, a mere 70 million-mile zone in a solar system that extends more than 1 billion miles.

The sun's radiant energy is just in the spectral range where lightweight atoms like carbon, hydrogen, oxygen and nitrogen bonds can form organic chemicals and gases such as ozone.

Computer models suggest that a planet of Earth's size can only form at this location if a Jupiter-size planet has formed within a certain range of distances from the sun.

With every new exploration of Mars, it becomes more likely that the red planet might have had the conditions necessary for some form of primitive life to begin before its surface water disappeared.

The discovery of chemosynthetic life nourished by volcanic vents on the sea floor open the possibility that there are other locales within the solar system that might harbor life.

Europa, a moon of Jupiter, has an icy crust that appears to behave much like the rocky crust of Earth. This leads to speculation that a warm ocean could exist beneath the ice the way molten rock underlies Earth's crust.

The Cassini spacecraft has imaged wrinkled landscapes and spouting jets on Enceladus, Saturn's sixth-largest moon, that hint at subsurface water and open the doors of speculation about the conditions for life to exist there.

The discovery of extraterrestrial life of any kind would force us to rethink and redefine our own origins and existence, but it is highly unlikely that we will ever encounter extraterrestrial beings anything like ourselves.

There might be an Earth-like planet somewhere, but one that is the right distance from the right kind of star, that has a moon of just the right size and is within our reach is about as likely as finding that penny I threw overboard for good luck on my first ocean voyage.