A familiar sky, rewritten by probes
For most of history, the planets were wandering points of light. Galileo Galilei turned a small telescope on Jupiter in January 1610 and saw four moons circling it: Io, Europa, Ganymede and Callisto. That single observation broke the idea that everything orbits Earth. Four centuries later, robotic probes have visited every planet, and each visit has delivered surprises that textbooks had to absorb.
Tonight's bulletin walks outward from the Sun. Along the way we mark what has been measured and confirmed, and we flag the questions that remain open. The distinction matters: a confirmed finding has been observed by instruments and checked by independent teams, while an open question is a gap where evidence is thin or competing explanations still fit.
The planet log
Key figures for the eight planets. Distances are averages in astronomical units (AU), where 1 AU is the mean Earth–Sun distance of about 149.6 million kilometres.
| Planet | Mean diameter (km) | Distance from Sun (AU) | Orbital period | Rotation period |
|---|---|---|---|---|
| Mercury | 4,879 | 0.39 | 88 days | 58.6 days |
| Venus | 12,104 | 0.72 | 225 days | 243 days (retrograde) |
| Earth | 12,742 | 1.00 | 365.25 days | 23.9 hours |
| Mars | 6,779 | 1.52 | 687 days | 24.6 hours |
| Jupiter | 139,820 | 5.20 | 11.9 years | 9.9 hours |
| Saturn | 116,460 | 9.54 | 29.4 years | 10.7 hours |
| Uranus | 50,724 | 19.2 | 84 years | 17.2 hours |
| Neptune | 49,244 | 30.1 | 164.8 years | 16.1 hours |
The rocky inner worlds
Venus: the runaway greenhouse
Soviet Venera landers, starting with Venera 7 in 1970, measured surface temperatures around 465 °C and pressure about 90 times Earth's. A thick carbon dioxide atmosphere traps heat so well that Venus is hotter than Mercury, despite being nearly twice as far from the Sun.
Mars: a wet past
Curiosity, which landed in Gale Crater in August 2012, found ancient lakebed sediments. Perseverance is collecting rock cores in Jezero Crater, an old river delta, for a possible future return to Earth.
Phosphine on Venus?
In 2020, a team reported a possible phosphine signal in Venus's clouds, a gas linked on Earth to biology. Reanalyses have disputed the detection. The question remains unsettled and new missions are planned.
Ocean moons of the giants
The biggest surprise of outer solar system exploration was water. Galileo, which orbited Jupiter from 1995 to 2003, detected magnetic signatures pointing to a salty ocean beneath Europa's icy crust. NASA's Europa Clipper, launched on 14 October 2024, will make dozens of close flybys to study that ice shell and the ocean below.
At Saturn, Cassini discovered geysers erupting from the south pole of Enceladus, a moon only about 500 kilometres wide. Flying through the plumes, the spacecraft detected water, salts, silica grains and organic molecules. Titan, Saturn's largest moon, has a thick nitrogen atmosphere and lakes of liquid methane and ethane, mapped by Cassini's radar and visited by the Huygens probe on 14 January 2005.
Open question: these oceans have water, chemistry and likely energy. Whether any of them host life is completely unknown.
Small worlds at the edge
Pluto was reclassified as a dwarf planet by the International Astronomical Union in 2006, the same year New Horizons left Earth. When the probe flew past on 14 July 2015, it found a world far from dead: a heart-shaped plain of nitrogen ice named Sputnik Planitia, mountains of water ice and a hazy blue atmosphere. In 2019 the same spacecraft passed Arrokoth, a snowman-shaped object in the Kuiper Belt that preserves material from the solar system's birth.
Closer in, missions have touched asteroids. Japan's Hayabusa2 returned samples from Ryugu in December 2020, and NASA's OSIRIS-REx delivered material from Bennu in September 2023. In 2022, the DART spacecraft deliberately struck the small moonlet Dimorphos and shortened its orbit, the first test of planetary defence.
Open question: some astronomers argue that clustered orbits of distant objects point to an unseen "Planet Nine", proposed by Konstantin Batygin and Mike Brown in 2016. No one has found it. It may exist, or the clustering may be a sampling effect.
Next in the bulletin: we leave the Sun's family behind. Continue to Deep Space.