The first planet to be discovered in the Solar System is Mercury. Its existence was recorded as early as 3,000 years BC. On November 7, 1963, French astronomer Pierre Gassendi observed Mercury for the first time through a telescope.
On March 29, 1974, the Mariner 10 spacecraft made its first flyby of Mercury and captured images of the planet in a crescent shape. Following the Mariner 10 mission, NASA launched the Messenger spacecraft on August 3, 2004, to further explore Mercury.
Mercury, also known as the Swift Planet, was named by the Romans after the messenger of the gods, known for his quick news delivery. In Greek mythology, the same god was named Hermes. The Vietnamese name for Mercury is based on the “water” element in the Chinese Five Elements system.
Mercury is slightly larger than Earth’s Moon. It is the planet closest to the Sun, with an orbital period of approximately 88 Earth days. It has a radius of 2,347.7 km, a mass of 3.3022 x 10^23 kg, and a flattened spherical shape.
The planet’s lack of atmosphere makes its surface resemble the Moon, with large craters and rugged terrain. Unlike other planets in the Solar System, Mercury does not experience seasonal weather variations.
From Earth’s perspective, Mercury has an approximate synodic period of 116 days, which is faster compared to other planets. Its axial tilt is among the smallest, about 1/30th of a degree, but it has the largest orbital eccentricity.
Mercury is notorious for its extreme temperatures due to its proximity to the Sun. It has an orbital range of 46 to 70 million km (28 to 43 million miles) from the Sun, experiencing the most intense solar radiation among the planets.
According to NASA, this small planet endures the most severe temperature conditions compared to other planets in the Solar System. During the day, surface temperatures can reach up to 427ºC (800ºF), while at night, they can drop to around -180ºC (-290ºF). The average temperature on Mercury is 167ºC (332ºF).
Scientists initially believed that only one side of Mercury faced the Sun, a phenomenon known as tidal locking. Due to its close proximity to the Sun, the planet’s surface is uneven, with rocky features facing different points in its orbit.
However, further studies revealed that Mercury rotates very slowly. When using fixed stars as a reference frame, Mercury completes only three rotations around its axis for every two orbits around the Sun. This motion is known as a “resonance rotation.”
During a year on Mercury, the Sun appears to move from the eastern horizon to the western horizon at a significantly slower pace. Meanwhile, Mercury completes one orbit around the Sun, passing through both perihelion and aphelion. At perihelion, the intensity of sunlight on Mercury’s surface is twice as high as at aphelion. Certain locations on the surface experience continuous “daylight” at perihelion, resulting in extremely high temperatures.
The temperature difference between perihelion and aphelion increases with the changing speed of the Sun’s motion across Mercury’s sky. As the orbital velocity of Mercury increases near perihelion and decreases near aphelion, observers from the Sun would see one hemisphere of Mercury facing the Sun, similar to how the Moon always faces Earth. The hemisphere facing the Sun stays illuminated for longer periods, leading to higher radiation exposure when near perihelion.
Due to its small mass, Mercury is unable to maintain its own atmosphere. The atoms in its atmosphere are constantly lost into space due to the weak gravitational force. Furthermore, Mercury is a hot planet and has the thinnest atmosphere. The atmosphere acts as a protective shield for a planet, keeping the heat from leaking into space. Without a sufficient atmosphere or if the atmosphere is too thin, Mercury or any other planet would lose heat to space instead of sharing temperatures with the night side.
This phenomenon on Mercury is similar to the “Greenhouse Effect” on Earth. The “Greenhouse Effect” occurs when shortwave radiation from the Sun passes through the atmosphere and reaches the Earth’s surface. After absorbing the radiation, the Earth’s surface becomes heated and emits longwave radiation back into the atmosphere, where CO₂ absorbs it, causing the air temperature to rise.
Through this article, you have surely learned that the first planet discovered in the Solar System is Mercury. People often believe that Mercury is the hottest planet. Is this true or not? Stay tuned for our next article to explore this issue further!