## What’s in a name?

How many stars are in our solar system? If you answered hundreds of billions, you are not alone. That is the most common answer I hear. The correct answer is…one. The Sun is the only star in the solar system. This illustrates a widespread confusion over the meaning of three astronomical terms, solar system, galaxy and universe.

The solar system is our star, the Sun, and everything that orbits around it. The Latin name for the Sun is Sol. That is why it is called the solar system. What objects orbit the Sun? At last count, there are 8 planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) and 5 dwarf planets (Ceres, Pluto, Eris, Makemake [pronounced MAH-kay MAH-kay] and Haumea). All of these except Mercury, Venus, Ceres and Makemake have one or more moons orbiting them. There are also innumerable asteroids and comets that orbit the Sun, but no stars.

The solar system is very different from a galaxy. A galaxy is a huge collection of stars bound together by gravity. The Sun is 1 star among 100-400 billion stars in the Milky Way galaxy. Are there other “solar” systems (more properly called stellar systems) in the galaxy? Yes. Astronomers have discovered hundreds of other stars that have planets orbiting them. Most of these newly discovered planets are Jupiter-size, as they are the easiest to find. NASA’s Kepler Mission is now searching for Earth-size and smaller planets orbiting other stars. After its mission concludes, we should have a good estimate of about how many of the stars in our galaxy have planets.

The Universe is everything we know of. It contains hundreds of billions of galaxies.

These are such big numbers that it is hard to comprehend them. Here is an example that might help. If you received \$1,000 per day, how many days would it take until you had \$1 million? The answer is 1,000 days, which is about 2.7 years. Now, for the billion dollar question. If you received \$1,000 per day, how many days would it take until you had \$1 billion? The answer is 1 million days. How many years is 1 million days? Try to figure it out before reading ahead… is it 5.4 years? 27 years?

The answer is 2,738 years! A billion is a REALLY big number. Remember, the Milky Way has over one hundred billion stars. That’s a hundred times bigger than a billion! If you could catalog 1,000 stars a day for 2,700 years, you would have cataloged less than 1% of the stars in our galaxy. That’s why astronomers should have job security.