Everything in the solar system moves: planets orbit the Sun, moons orbit planets, and every body spins on its axis. Understanding these motions was one of the first great successes of science, and it is still an active field. The laws of motion Precise orbit calculations led to the discovery of Neptune in 1846, predicted … Read more
A planet’s magnetic field is invisible, but it reveals what is happening deep inside and protects the planet from the solar wind. Astronomers study these fields mostly with spacecraft, and also through auroras and radio waves. Where do planetary magnetic fields come from? Most come from a **dynamo**: a flowing, electrically conducting fluid inside a … Read more
The Sun is an ordinary star but the center of everything in our solar system. Its gravity holds the planets in orbit, its light and heat make life on Earth possible, and its activity affects space all the way to the edge of the heliosphere. Gravity: the anchor The Sun contains about **99.8 percent of … Read more
A solar flare is a sudden, intense burst of radiation from the Sun’s surface, released when tangled magnetic fields near sunspots snap and reconnect. A large flare can release as much energy as billions of nuclear bombs in a few minutes. Flare classes Flares are ranked by their peak X-ray brightness, measured by satellites. Each … Read more
No one has drilled into Jupiter or breathed the air of a planet around another star. Yet astronomers know a surprising amount about what planets and their atmospheres are made of, by combining several methods. Density: rock, ice or gas? If you know a planet’s **mass** (from the orbits of its moons, or how it … Read more
The difference comes down to one criterion: whether the object has cleared its orbital neighborhood. This definition was adopted by the International Astronomical Union (IAU) in 2006, and it is why Pluto is no longer counted among the planets. The IAU definition of a planet A planet in our solar system is a body that: … Read more
An exoplanet is a planet orbiting a star other than the Sun. The first ones were confirmed in the 1990s, and the NASA Exoplanet Archive now lists more than 6,000. Most are never seen directly: they are detected through their effects on their star. The transit method When a planet passes in front of its … Read more
A meteor shower happens when Earth passes through a stream of debris left by a comet, or occasionally an asteroid. The particles, mostly smaller than grains of sand, hit the atmosphere at tens of kilometers per second and burn up as streaks of light. Because Earth crosses the same streams every year, showers return on … Read more
Comets and asteroids are both leftovers from the formation of the solar system about 4.6 billion years ago. The main difference is what they are made of, which determines how they behave near the Sun. Asteroids: rock and metal Comets: dirty snowballs **Famous comet**: Halley’s Comet returns about every 76 years and is next expected … Read more
The Moon does not make its own light. It reflects sunlight, and half of it is always lit by the Sun. As the Moon orbits Earth, we see different portions of that lit half. That changing view creates the phases. The eight phases “Waxing” means growing and “waning” means shrinking. How long is the cycle? … Read more
