When asked to name the planet closest to earth in size, most will cite Venus. That is because Venus is the smallest of the rocky planets and passes close by Earth every time it orbits our Sun.
But Mercury is actually our closest neighbor in terms of size – continue reading to discover why.
Mercury
Mercury, commonly referred to as both morning and evening star, shares an orbit with the Sun that’s extremely close, taking just under 8 days per rotation around its star. Due to this closeness, its orbit brings it within 50 degrees of Earth at any given time; with half its time spent facing towards us and half away. Mercury boasts extremely hot surface temperatures without an atmosphere to hold onto that heat after dark; complete one rotation takes an estimated 59 days!
Mercury boasts an enormous iron core that dominates its interior structure. While we don’t yet fully understand its formation, one theory suggests it was likely caused by processes that concentrated metal relative to silicate in the protoplanetary disk from which our Solar System formed.
Most of Earth’s surface appears greyish-brown and smooth, yet there are bright streaks extending from many craters. These are known as “crater rays,” composed of finely crushed rock that’s more reflective than whole rocks; over time these rays become darkened due to space debris and dust accumulation.
Mercury differs from its terrestrial planet neighbors in that it does not feature any tectonic plates, instead having a crust that’s relatively thin and brittle. There is also a tenuous exosphere surrounding Mercury composed of hydrogen, helium, sodium, calcium, oxygen, water vapor as well as trace amounts of nitrogen carbon dioxide argon xenon neon produced through interaction between solar radiation, wind flow and micrometeoroid impacts forming this exosphere.
Venus
Venus is the closest in size and composition to Earth; its surface temperatures reach hot enough to melt lead. A thick blanket of poisonous carbon dioxide traps heat from the Sun, creating an out-of-control greenhouse effect.
Because it orbits close to the Sun, Venus is one of the brightest planets visible at sunset or sunrise and often makes an appearance first thing in the evening sky and last thing at night, earning ancient Greeks such as Phosphorus the title “light-bringer”, while Romans used Lucifer (“Light-Bringer”).
Landing on Venus would require spacecraft equipped with powerful air brakes; its atmosphere is so thick that escaping its gravity requires 23300 miles per hour compared to only 24,000 on Earth.
Scientists speculate that Venus once enjoyed a temperate climate similar to our own planet. Unfortunately, due to greenhouse effects and toxic gases present on Venus today, its atmosphere is now almost uninhabitable due to carbon dioxide absorption which heats the planet up further.
Venus can be seen for approximately 263 days each year in our morning sky, reaching its farthest distance from the Sun early each morning – known as its greatest elongation west – before reappearing at sunset or sunrise for another 263 days.
Mars
Astronomers are excited by Mars’ close approach to Earth this week. When perigee occurs, Mars appears much brighter and closer to the horizon than usual – one or two times each year and it offers one of the best times to view Mars. You may find Mars either at sunrise or sunset depending on where you reside – on April 29, there will even be a close conjunction between Neptune and Mars that day!
Mars, located four planets from the Sun and two in size after Earth, has only slightly larger dimensions than those of Earth and features a dry rocky surface with an axial tilt of 25 degrees causing seasons in both hemispheres of its orbital plane – summer in the northern hemisphere and winter in its southern counterpart.
Mars hosts two small moons named Phobos and Deimos that may have formed from captured asteroids. Given their low mass, these potato-shaped satellites orbit relatively closely to Mars; Phobos takes approximately 13 hours to complete one rotation while Deimos follows an orbit similar to Earth’s moon’s. Spacecraft are often launched towards Mars by using this close approach as an ideal launch window.
Jupiter
Jupiter is the largest and heaviest gas planet in our solar system, serving as an excellent example of star formation by nuclear fusion. Its thick atmosphere consists mainly of hydrogen (approximately 90%) and helium (10%) with small quantities of methane, ammonia, and other gases present as well.
Like Earth, Jupiter is known for its extreme weather patterns and storms – including one known as the Great Red Spot that has been raging for centuries and can reach speeds of 100 meters per second (360 kilometers per hour). These events are driven by extreme temperatures and pressures.
Jupiter, with an approximate radius of 43,440.7 miles (69,911 kilometers), is 11 times larger than Earth. Its equatorial bulge results from its rotation which takes about 10 hours per day.
Jupiter has fainter rings than Saturn, including one main ring, a halo and two gossamer rings that are replenished with dust particles from its inner moons – another example of how planets maintain their structure even without solid surfaces. Jupiter also boasts a powerful magnetic field 14 times stronger than that of Earth; its influence stretches throughout the Solar System as it forms an enormous magnetosphere which swells toward its star before shrinking back and creating a 600 million mile (1 billion km) tail around itself extending back towards Jupiter.
Saturn
Saturn dwarfs its many moons with its diameter of 72,400 miles (116,500 kilometers). Cassini’s wide-angle camera captured this view from 8 degrees above ring plane on March 7, 2015 using a filter which prioritizes near infrared wavelengths centered at 752 nanometers.
This filter enhances the contrast between dark features in Saturn’s rings and bright clouds in its atmosphere. Like Jupiter, Saturn features cloud layers with lighter and darker spots, bands, eddies, and vortices; however, atmospheric circulation on Saturn is less active, giving its overall appearance more of a uniform look.
Saturn is less dense than Jupiter due to its oblate shape; with less of its equatorial gravity canceling out polar gravitational pull being counteracted at any one time by equatorial gravity. As such, its mean density only accounts for roughly 12 percent of that of Earth.
Saturn’s rings are composed largely of dust, but also contain some ice and rock particles. Their three rings – A, B and C – can be divided into the A, B and C rings with a 64 km wide gap known as the Maxwell gap between B and C rings. Saturn also boasts 53 named satellites as well as many more that have yet to be discovered, such as Titan and Enceladus which possess thick nitrogen-rich atmospheres similar to Earth at one point during its history.
Uranus
Uranus, a gas giant and the coldest planet in our Solar System, features an atmosphere composed of hydrogen, helium and trace amounts of water, methane, carbon monoxide and carbon dioxide – its blue-green hue is due to methane absorbing red wavelengths of sunlight – giving rise to its signature blue-green hue. Uranus holds third largest diameter in terms of diameter in our Solar System while fourth in terms of mass.
Atmospheric gases in Earth’s atmosphere can be divided into three layers, as follows: 1) Troposphere: From altitudes between 300 km (186 mi) and 50 km (31.5 miles), with pressures ranging from 0 to 100 bar (10 kPa to 10 mpa); 2) Stratosphere: Extending between 50 and 4,000 km (3,175 and 2,4885 mi), with pressures from 0.1 bar to 1 bar (1 kPa to 9.5 mpa); and finally; thermosphere, which extends all the way up to 20,000 km (12,400 mi).
Like Jupiter and Saturn, Uranus features an uneven magnetosphere that produces aurorae near its poles rather than at the planet’s equator – this phenomenon stems from its magnetic axis being off center by one-third of its radius, creating an corkscrew-shaped magnetic field.
Uranus boasts 27 moons, the largest being Titania with an approximate diameter of 981 miles (1,587 kilometers), which makes up nearly half the size of Earth’s Moon. Other major Uranian moons include Miranda, Ariel, Umbriel and Oberon.