Exploring Venus: Earth’s Twin Planet

From The Lecture Series: A Field Guide to the Planets

By Sabine Stanley Ph.D., John Hopkins University

Venus earned the title “Earth’s Twin” due to its diameter, average density, orbit, and distance to the Sun. In all these aspects, Venus is the closest to our Earth. However, these similarities have not created similar situations on the two planets. Venus is many times hotter than Earth, lacks oxygen, has no water, and can crush and destroy with its surface pressure. What causes these huge differences, despite being so similar?

An illustration of Venus in the solar system
Despite the grave differences between the two planets, Venus is commonly called Earth’s twin planet. (Image: NASA images/Shutterstock)

Venus is the brightest object in the sky after the Sun and Moon. Names such as Morning Star and Evening Star were derived from this brightness. The name “Venus” has roots in Roman mythology: Venus is the goddess of love and beauty. This goddess of beauty is so similar to Earth that some even call it Earth’s twin planet.

Earth’s diameter and the average density is only five percent higher than Venus. Venus’s orbit is also closest to Earth’s, and its distance from the Sun is 72% the distance of Earth. That means travel time from Earth to Venus could be less than travel time from Earth to Mars. Maybe the closer distance to the Sun causes a higher temperature, but how much hotter is it than the deserts here? Also, how do we know how hot it is?

Space Missions to Venus

Space missions to Venus began in the 1960s. After 12 unsuccessful attempts and landing failures, the 13th spacecraft was successfully launched. Passing Venus’s atmosphere was especially difficult due to the winds much stronger than our biggest hurricanes and the thick density. Spacecraft 13 was the Venera 4 mission by the Soviet Union in 1967. However, no photo of Venus’s surface could be captured and sent back to Earth, until Venera 9 and 10 landed on Venus in 1975. They captured black and white photos that showed Venus’s surface is covered in rocks, but not much dust. The Venera 13 and 14 missions captured the full-color panorama views in 1982.

A 3D illustration of the surface of Venus. Hot lava flows on Venus.
Spacecrafts do not last long under the extreme heat and pressure of Venus. (Image: Jurik Peter/Shutterstock)

Why did it take so many years and so many missions to capture some images? The first reason is the extreme heat on Venus. The temperature reaches 850oF, due to the greenhouse effect of Venus’s atmosphere. Next, the pressure on Venus’s surface is 92 times higher than sea-level pressure on Earth. Thus, the landers were destroyed easily. Venera 13 returned data for the longest period ever, and that was 127 minutes. The last attempt was Vega 2 in 1985, which transmitted data for almost 60 minutes. Radio waves later helped gather all the information we have today of Venus.

This is a transcript from the video series A Field Guide to the Planets. Watch it now, on The Great Courses Plus.

Venus’s Orbit

Venus orbits the Sun at 72% the distance of Earth but is the only planet that spins opposite the direction that all the planets orbit around the Sun. It is unusual because all planets in the solar system were accreted from a disk of material surrounding the Sun, revolving and rotating in one direction. Hence, all planets spin in the same direction, except for Venus.

Planets spin and orbit at different angles, but Venus has an axial tilt of 177°. This is while Earth, Mars, Saturn, and Neptune have a 20-30° tilt. The reason might be an object hit Venus so strongly that it changed the direction and tilted the planet. Another theory is that the strong winds around Venus’s thick atmosphere pushed the planet in the same direction for so long that they finally flipped it over. Taking the orbit and spinning into account, one might wonder how long one year in Venus is.

Learn more about What the Biggest Exoplanets Reveal.

One Year in Venus

Venus orbits the Sun at a very low rotation rate. In fact, one year in Venus passes quicker than one sidereal day. A sidereal day is the length of time it takes for a planet to complete a full 360-degree rotation, and equals 243 Earth days. A year on Venus is 225 Earth days. The solar day is shorter than the sidereal day: 117 Earth days; thus, one year equals 1.92 Venus solar days. Every 24 years, Venus has a leap year with three days.

Learn more about How Our Sun Defines Our Solar System.

The Atmosphere, Temperature, and Pressure in Venus

Venus’s atmosphere is made up of carbon dioxide (96%), nitrogen (3.5%), and less than 1% carbon monoxide, argon, sulfur dioxide, and water vapor. These gases in the atmosphere turn Venus into a huge greenhouse with a temperature of 850°F, which is too much considering its distance from the Sun and the fact that Venus reflects the light much stronger than Earth. As there are no plants and liquid water on Venus, nothing absorbs the carbon dioxide, and the greenhouse effect is significantly stronger than on Earth.

A 3D illustration of Venus's surface landscape.
The dominance of carbon dioxide turns Venus into a greenhouse giant. (Image: Peter Simoncik/Shutterstock)

The pressure 50 kilometers above Venus’s surface is similar to the pressure on Earth, but on the surface, it is 92 bars. The pressure on Earth at sea level is one bar.

Apparently, the Earth’s twin has much more significant differences than expected.

Learn more about Human Futures in the Solar System.

Common Questions About Venus

Q: Why is Venus called Earth’s sister?

Venus sometimes called Earth’s “sister planet” or Earth’s twin. It is a terrestrial planet because of similar size, mass, proximity to the Sun, and bulk composition to those of the Earth’s. However, in other aspects, it is significantly different and has no living conditions for humans.

Q: Does Venus follow the Sun?

Venus makes a complete orbit around the Sun in a year in Venusian time – equal to 225 Earth days, which is less than two day-night cycles on Venus. Its orbit around the Sun is the most circular of any planet, but Venus orbits the Sun almost upside down. This is one of the biggest dissimilarities with the Earth, despite being Earth’s twin planet.

Q: Which planet is the brightest planet?

Venus, or the Earth’s twin, is often the brightest object in the sky, other than the moon. The reason is the atmosphere of Venus and how much it reflects the Sun’s light and heat. Despite similarities with Earth, Venus can reflect Sun rays much stronger than Earth.

Q: What are the most common elements on Venus?

Venus has an extremely hot surface and very high atmospheric pressure. The most common elements in Venus’s atmosphere are carbon dioxide (96%), nitrogen (3.5%), and less than 1% carbon monoxide, argon, sulfur dioxide, and water vapor. Even though it is Earth’s twin, there is no oxygen in Venus’s atmosphere.

Keep Reading
Great Moons of Our Solar System
Aristotle on Astronomy: Supporters and Challengers
Looped Radio Signal 500 Million Light Years Away Baffles Astronomers