Astronomy & Astrophysics 101: What Is a Light-Year?

By NASA December 15, 2024

Collected at: https://scitechdaily.com/astronomy-astrophysics-101-what-is-a-light-year/

What Is a Light-Year?

A light-year is the distance light travels in one year. Light moves incredibly fast — about 186,000 miles (300,000 kilometers) per second, covering 5.88 trillion miles (9.46 trillion kilometers) per year.

To measure the vastness of space, we use light-time — the distance light travels in a specific time frame. Since nothing in the universe moves faster than light, it’s a useful standard for astronomical distances.

For example, in one minute, light covers 11,160,000 miles. Sunlight takes about 43.2 minutes to reach Jupiter, located roughly 484 million miles away. In one hour, light travels 671 million miles. Despite light’s incredible speed, the universe’s vast scale makes even such massive distances seem almost unfathomable.

Light-Speed Journeys in the Solar System

Earth is approximately eight light minutes from the Sun. Reaching the edge of the solar system, where the distant Oort Cloud lies, would take about 1.87 years at light speed. Traveling to Proxima Centauri, our nearest neighboring star, would require a 4.25-year journey at light speed.

When considering the immensity of the universe, it’s easy to mention enormous figures — but much harder to truly comprehend just how vast, distant, and numerous celestial objects are.

To get a better sense, for instance, of the true distances to exoplanets – planets around other stars – we might start with the theater in which we find them, the Milky Way galaxy.

The Milky Way and Beyond

Our galaxy is a gravitationally bound collection of stars, swirling in a spiral through space. Based on the deepest images obtained so far, it’s one of about 2 trillion galaxies in the observable universe. Groups of them are bound into clusters of galaxies, and these into superclusters; the superclusters are arranged in immense sheets stretching across the universe, interspersed with dark voids and lending the whole a kind of spiderweb structure. Our galaxy probably contains 100 to 400 billion stars, and is about 100,000 light-years across. That sounds huge, and it is, at least until we start comparing it to other galaxies. Our neighboring Andromeda galaxy, for example, is some 220,000 light-years wide. Another galaxy, IC 1101, spans as much as 4 million light-years.

Based on observations by NASA’s Kepler Space Telescope, we can confidently predict that every star you see in the sky probably hosts at least one planet. Realistically, we’re most likely talking about multi-planet systems rather than just single planets. In our galaxy of hundreds of billions of stars, this pushes the number of planets potentially into the trillions. Confirmed exoplanet detections (made by Kepler and other telescopes, both in space and on the ground) now come to more than 4,000 – and that’s from looking at only tiny slices of our galaxy. Many of these are small, rocky worlds that might be at the right temperature for liquid water to pool on their surfaces.

Nearest Exoplanet: Proxima Centauri

The nearest-known exoplanet is a small, probably rocky planet orbiting Proxima Centauri – the next star over from Earth. A little more than four light-years away, or 24 trillion miles. If an airline offered a flight there by jet, it would take 5 million years. Not much is known about this world; its close orbit and the periodic flaring of its star lower its chances of being habitable.

The TRAPPIST-1 system is seven planets, all roughly in Earth’s size range, orbiting a red dwarf star about 40 light-years away. They are very likely rocky, with four in the “habitable zone” – the orbital distance allowing potential liquid water on the surface. And computer modeling shows some have a good chance of being watery – or icy – worlds. In the next few years, we might learn whether they have atmospheres or oceans, or even signs of habitability.

One of the most distant exoplanets known to us in the Milky Way is Kepler-443 b. Traveling at light speed, it would take 3,000 years to get there. Or 28 billion years, going 60 mph.