The Sky's Illuminators: A closer Look at the Brightest Planets
Last Updated: February 15, 2023
Have you ever looked up at the night sky and been awed by the twinkling stars and glowing planets? While every celestial object in the sky has its own unique beauty, some stand out more than others. In this article, we’ll take a closer look at the brightest planets in the night sky and explore what makes them so luminous.
From the reflective properties of their surfaces to their proximity to the sun, we’ll delve into the many factors that contribute to a planet’s brightness. Join us on this journey to discover which planets shine the brightest, and why one, in particular, stands out from the rest.
While the stars track predictable paths across the night sky, early humans soon noticed 5 bright “wanderers” who didn’t follow these set tracks. These planets as we soon called them, from the word Greek word for “wanderer”, had their own paths, which we were eventually able to track with careful observation, and eventually use these calculations to realize that they are similar to Earth, relatively close by compared to stars, and also orbit our Sun.
Together, let’s explore what causes planets to appear bright, which ones are the brightest, and focus on the brightest of the five.
The Brightness of Planets
Stars create their own light from nuclear reactions, sending it out into the universe to be viewed by observers, like us. Planets, however, don’t produce their own light. They simply reflect the light from their star.
This is why stars twinkle since their light travels so far while the light from planets (and moons) remains constant since they are simply reflecting the light from our nearby Sun.
Albedo is the measure of how much light a surface reflects from a star. Light-colored surfaces reflect a lot of light, meaning they have high albedo while darker colors absorb light, reflecting less and therefore having a low albedo. Albedo is measured on a scale of zero to one with zero being hypothetical in which no sunlight is reflected back (even a planet completely covered in soot would have an albedo of 0.04, meaning 4% would be reflected) and a planet covered in white snow would have an albedo of 0.9.
When we are studying brightness, we need to consider how much light is produced from the source and how far the light travels, but brightness of reflected light includes a few more factors. How bright a planet appears to us here on Earth depends on how far it is from the Sun, how high its albedo is, how big it is (as a bigger planet has more surface area to reflect received light), and how far away it is from us here on Earth.
In addition, it is important to consider the proportion of the planet that is facing towards us and therefore reflecting light in our direction as planets also experience phases like our moon from our location here on Earth. Planets outside of Earth’s orbit are at their brightest when they are at full phase and closest to Earth (known as opposition), while planets closer to the Sun than us become a little more complicated.
The Brightest Planets in the Solar System
The five brightest planets in the sky have been known since ancient times: Venus, Mars, Mercury, Jupiter, and Saturn.
As we are observing from here on Earth we need to consider their distance and position relative to the Sun and to Earth. The closer they are to the Sun, the more light they will receive, and the closer they are to us, the more reflected light we will see. We use an average number to calculate the distance between Earth and the planet, but there are sections of their orbits in relation to ours in which they are closer or further away than the average and the best viewing (when they are at their brightest) will typically occur when they are at closest approach while the worst viewing (when they will be at their dimmest) will be when they are the furthest away.
Mercury is very close to the Sun meaning it receives a lot more light, but it can be hard to spot as the Sun easily overpowers it. In addition, it is small, dark, and airless with an albedo of 0.1, making it often hard to spot at 56,974,146 miles (91,691,000 km) away and as the smallest planet with only 9,525.1 miles (15,32.1 km) around its circumference. We often use Astronomical Units (AU) for distance which is equivalent to the distance from the Sun to Earth, about 93 million miles (150,000 km) and compare the size of the target planet to the size of Earth. Mercury is about 0.61 AU from us and a little more than a third of our size.
Saturn and Jupiter, while very large, fitting 10 to 11 Earths across their equators respectively, are much further away from Earth sitting at 8.52 AU and 4.2 AU from us respectively. So, while they have a much larger surface area to reflect more light, that light also has to travel much farther to reach us. In addition, they both reflect about half of the light they receive from the Sun with Saturn at about 0.47 and Jupiter at about 0.52.
Mars is only 0.52 AU from us and almost 2 times smaller than us with an albedo of 0.25. What helps Mars more noticeable in the night sky is the fact that its red surface provides a reddish tinge to its light, making it distinct from the generally white or blue-white stars in the night sky.
Venus is the closest to Earth at only 0.28 AU (about 41,500,000 miles or 25,725,000 km) and just a little smaller than Earth (about 1,000 miles or 2,000 miles less around its equator). This proximity to us and large relative size help to boost its brightness, in addition to other factors that impact its albedo which we will get into in the next section.
So, how can we spot planets in the night sky?
You can actually see all five of the brightest planets without a telescope, though a telescope provides amazing detail of the planets. When visible, planets move across the night sky, east to west. In the northern hemisphere, you will want to look south, tracking the planets from east to southeast to south. In the southern hemisphere, you will want to look north, toward the Eastern to the northeastern horizon. The planets follow the path of the ecliptic, which is the apparent path of the Sun as it is actually the plane of our orbit around it. Within the ecliptic, we find all the objects in the solar system that follow this general path, which includes our Moon and the planets. This makes spotting multiple objects, when visible, more manageable since if you find one, you can trace up or down the ecliptic to the next.
Mercury can often be the hardest planet to spot as it is very close to the Sun, so when it is visible, it is low on the horizon and very close to the Sun, around sunrise or sunset. Venus also stays close to the Sun and is often found in the early morning and early evening. You can even view Venus during the day, especially with a telescope.
It is important to note that although finding it provides a wonderful hunt, Venus through a telescope, won’t reveal many details as it is covered in clouds. Mars often looks like a reddish star though it won’t twinkle and a large telescope can help you observe the Martian landscape, including the polar ice caps.
While visible with the naked eye, Jupiter is gorgeous in a small telescope such as a 4-inch aperture though an 8-inch aperture will bring its beautiful stripes and even Great Red Spot into beautiful detail. A bit more difficult, but still visible with the naked eye is Saturn, whose magnificent rings become visible with a telescope, especially with a longer focal length.
There are also atmospheric concerns when considering the brightness of planets. Our atmosphere consists of gases that scatter light, making astronomical objects appear slightly distorted, wobbly, shimmery, or dimmer. This atmosphere can be unpredictable based on air pressure and more, affecting our view even with otherwise ideal viewing situations such as closest approaches.
The Brightest Planet in the Sky
Combining these factors, Venus is the brightest in the night sky, followed by Mars, Jupiter, Mercury, and Saturn. In fact, Venus can appear almost 200 times brighter than Mars. Uranus and Neptune are both so far away at 18.21 and 29.09 AU respectively, and only about four times bigger than Earth, making them essentially impossible to spot with the naked eye. (If you have 20/20 vision and know exactly where to look, you could see Uranus with your naked eye.)
To better understand how bright each planet is, we quantify it using the scale of magnitude developed by the ancient Greeks, with the brightest stars in the sky having a magnitude of 1 and the faintest receiving a magnitude of 6. A magnitude 1 star is 100 times brighter than a magnitude six star. Since this scale was developed with interstellar stars as the basis, many of the brighter objects within our solar system will have a negative magnitude.
Our Sun has a magnitude of -26.7 with our Moon coming in second at -12.7 (in full phase), and Venus is third with a magnitude of about -4.5 when it is brightest. Mars and Jupiter actually come in at approximately the same magnitude at their brightest with Mars being about -2.9 and Jupiter at about -2.8; Mercury can reach about -1.9 magnitude and Saturn at its brightest is -0.92, making it on par with most of the brightest stars. Uranus comes in at a magnitude of 5.6.
In addition to Venus’s proximity to us and large relative size, it also has a very thick atmosphere and high albedo. Venus is often described as Earth’s evil twin due to its thick atmosphere composed primarily of carbon dioxide and sulfuric acid. These clouds are highly reflective and cover the entirety of the planet, making Venus the most reflective planet in the solar system with an albedo of 0.77. Only a few icy moons such as Enceladus of Saturn have a higher albedo.
Venus’s brightness and visibility are dependent on its position in its orbit around the Sun inside our orbit. When Venus is closest to Earth, its sunlit side will be away from us (since Venus will be between us and the Sun) and it is fully illuminated when it is on the other side of the Sun and therefore furthest away.
When Venus is in its crescent phase near its closest approach to us and therefore at its brightest (with 26% illumination) due to proximity, it’s only slightly fainter than when it is the furthest away from Earth because it is in its full phase. It’s brighter when it’s closer to us, but less of its face will be reflecting toward us and it’s dimmer when further away, but it will be reflecting more light toward us as it will be in a fuller phase.
There are many beautiful objects in the night sky ranging in brightness with the brightest stars having a magnitude of 1 and the faintest stars having a magnitude of 6. Within this sea of stars, we also see objects within our solar system including the “wanderer” planets.
Brightness is determined by how much light is being put out and how far away the observer is. When observing planets which reflect the light of the Sun, we also have to consider how far away it is from the Sun, how far away it is from us, how big it is, how much of its surface is facing us, and how reflective it is (how high its albedo is).
Due to its large relative size to us, close proximity, and unusually high albedo (reflectivity) from its thick atmosphere, Venus is the brightest planet in the sky and the second brightest object in the night sky after our Moon. Next, we have Mars, Jupiter, Mercury, and Saturn, with Uranus and Neptune being exceptionally faint.
Knowing how bright planets are in relation to other objects in the night sky helps us to locate them in the night sky. This brightness is impacted by where they are in their orbit, including how close or far away they are from us and what phase they are in.
Knowing these factors can help us navigate the night sky better, spotting both the planets and other objects in the night sky in relation to them.
Sarah Hoffschwelle is a freelance writer who covers a combination of topics including astronomy, general science and STEM, self-development, art, and societal commentary. In the past, Sarah worked in educational nonprofits providing free-choice learning experiences for audiences ages 2-99. As a lifelong space nerd, she loves sharing the universe with others through her words. She currently writes on Medium at https://medium.com/@sarah-marie and authors self-help and children’s books.