What Is a Light-Year?

A light-year is the distance that light travels in one year. Since light moves at an incredible speed of about 186,000 miles (300,000 kilometers) per second, it covers approximately 5.88 trillion miles (9.46 trillion kilometers) in a year.

To measure the vastness of space, we use light-time — the distance light travels within a specific time frame. Since nothing in the universe moves faster than light, it serves as an effective standard for measuring astronomical distances.

Figure 1. Light-Year: Measuring Space's Vastness

For instance, in one minute, light travels 11,160,000 miles. Sunlight takes about 43.2 minutes to reach Jupiter, which is approximately 484 million miles away. In one hour, light covers 671 million miles. Despite light’s incredible speed, the immense scale of the universe makes even these vast distances seem nearly unimaginable. Figure 1 shows Light-Year: Measuring Space's Vastness.

Light-Speed Travels Across the Solar System

Earth is roughly eight light minutes away from the Sun. Reaching the outer edge of the solar system, where the Oort Cloud resides, would take about 1.87 years at light speed. A journey to Proxima Centauri, the nearest star to us, would span 4.25 years at light speed.

While the vastness of the universe is often expressed in huge numbers, truly grasping the immense distances and countless celestial objects can be challenging.

To better understand the distances to exoplanets—planets orbiting other stars—it helps to first consider the scale of the Milky Way galaxy, the "stage" where these planets exist.

The Milky Way and the Vastness Beyond

Our galaxy, the Milky Way, is a vast, gravitationally bound collection of stars, swirling in a spiral through space. It is one of approximately 2 trillion galaxies in the observable universe. These galaxies are grouped into clusters, which in turn form superclusters, arranged in enormous sheets across the universe, separated by dark voids. This structure gives the universe a spiderweb-like appearance. The Milky Way is estimated to contain between 100 and 400 billion stars, and it spans about 100,000 light-years. While this sounds enormous, it’s small compared to other galaxies. For instance, the Andromeda galaxy, our nearest neighbor, is about 220,000 light-years wide, and the IC 1101 galaxy stretches up to 4 million light-years across.

Based on observations from NASA’s Kepler Space Telescope, we can confidently predict that almost every star visible in the night sky likely has at least one planet orbiting it, and many of these stars likely have multiple planets. With hundreds of billions of stars in our galaxy alone, this suggests the number of planets could reach into the trillions. To date, over 4,000 exoplanets have been confirmed by Kepler and other telescopes, many of which are small, rocky planets that could potentially support liquid water on their surfaces. These discoveries come from observing only tiny portions of the galaxy.

Closest Exoplanet: Proxima Centauri

The closest known exoplanet is a small, likely rocky planet orbiting Proxima Centauri, the star nearest to Earth. Located just over four light-years away, or about 24 trillion miles, it would take 5 million years to reach it by jet. While not much is known about this planet, its close orbit and the periodic flares from its star make it unlikely to be habitable.

The TRAPPIST-1 system, about 40 light-years away, contains seven planets roughly the size of Earth, all orbiting a red dwarf star. Four of these planets lie in the "habitable zone," where conditions might allow for liquid water. Computer models suggest that some of them could be watery or icy worlds. In the coming years, we may learn more about their atmospheres, oceans, or potential for habitability.

One of the farthest exoplanets discovered, Kepler-443 b, is located so far away that traveling there at light speed would take 3,000 years. At a speed of 60 mph, it would take 28 billion years to reach.

Source: NASA

Cite this article:

Janani R (2024), Astronomy & Astrophysics 101: Understanding a Light-Year, AnaTechmaz, pp. 139