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

As we look with enthusiasm and anticipation for extraterrestrial life, this uniqueness is one thing that remains in the forefront of our thoughts. Liquid water is the primary factor, although it might exist beneath the frozen crust of exoplanets as well as here in our own solar system.

Astronomers have discovered thousands of planets around nearby stars, but few are close to being even remotely Earth-like.

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

Our moon is the only satellite in the solar system that is even close in size to the planet that it orbits. This causes the moon to exert strong tidal forces, to shield us from impacts with cosmic dust and debris, and to stabilize 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 chance of such a collision with primordial Earth at just the right angle to vaporize Earth’s crustal material that could coalesce to form the moon.

This collision not only created the moon; it also increased the mass and radius of Earth to its present size, also an important criterion for an Earth twin. Earth’s mass, size and distance from a star like 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, also called the Goldilocks 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.

Recent research suggests that certain red giant stars may also be suitable, and these are numerous in our galaxy.

Computer models suggest that a planet of Earth’s size can form at this location only 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 may 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 seafloor opens the possibility of other locales that might harbor life.

In our solar system, Jupiter’s moon Europa has an icy crust that appears to behave much like the rocky crust of Earth. This indicates that a warm ocean might exist beneath the ice the same way that molten rock underlies Earth’s crust.

Before it crashed into Saturn’s atmosphere last year, the Cassini spacecraft imaged wrinkled landscapes and spouting jets on Encel- adus, 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 may be another Earth somewhere, but one that is the right distance from the right kind of star, has a moon of just the right size and is within our reach is about as likely as finding that quarter I flipped overboard for good luck on my first ocean voyage.

Richard Brill is a professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.