Over millenia the Moon has been the focus of countless songs, poems, stories – romantic, scary, and everything in between. This bright silvery orb has fired the human imagination since the very beginning. Today we marvel at eclipses, blood moons, supermoons, and the like. Early this year we experienced a so-called ‘blue moon’ (not blue at all) and some will recall the rare selenelion that occurred in July.
Prior to the acceptance of Copernicus’ heliocentric model of the Solar System, the Moon was considered to be a planet. Modern theory posits that, in a way, it was, having been formed in the aftermath of a collision between a Mars-sized object and the early Earth.
For the longest time there was just one Moon but in the year 1610 Galileo changed our understanding when he directly observed the four largest moons of Jupiter (later called the Galilean moons). The first Saturnian moon to be discovered was Titan by Christiaan Huygens in 1655. A trickle of discoveries followed over the next couple of centuries or so. This turned into a rain of discoveries in the 20th century followed by a deluge in the 21st! Check out the timeline of Solar System discoveries here.
Moons are natural satellites. Many orbit planets, some orbit dwarf planets (Pluto has five), and many more tiny ones circle asteroids. Of the planets, Mercury and Venus are the only ones devoid of moons. Most major moons orbit their host planets more or less along their planets’ equatorial planes, but many of the smaller moons orbit at very different angles. Most travel in the same direction as their planets’ rotation (prograde) while some, mostly smaller moons, orbit backwards (retrograde). A notable exception is Neptune’s large moon Triton which is retrograde. All retrograde satellites are thought to have formed separately before being captured by their parent planets.
Some moons are volcanic (Jupiter’s Io and Neptune’s Triton are volcanically active but in very different ways). Some harbour sub-surface oceans of liquid water and are considered possible candidates for indigenous life! Of all the Solar System moons, only four are known to have collisional atmospheres, and only one of these, Saturn’s Titan, has an atmosphere of significant density.
Credit: NASA / JPL / SSI / Ted Stryk / Jason Perry / Emily Lakdawalla
Moons with atmospheres
In order of decreasing atmospheric column density, they are: Saturn’s Titan, Neptune’s Triton, and Jupiter’s Io and Callisto. In addition to its thick atmosphere, Titan is the only place in the Solar System, aside from Earth, confirmed to have bodies of surface liquid. It also remains the only moon, other than Earths’ own, to have been physically touched by human technology. The Cassini Huygens spacecaft made landfall in 2005.
We are still discovering moons!
Jupiter harbours the most recent Solar System discoveries. By 2015, 67 moons were known, but since then 12 more were added by a team led by Scott S Sheppard at the Carnegie Institution for Science, to bring the tally up to 79 – the largest planetary quota of moons with reasonably stable orbits. When I went to school this planet had just 12 moons in total! This recent discovery came about as a sideline to Sheppard’s main quest which continues to be the search for Planet Nine – a massive planet predicted to exist in the outer reaches of the Solar System. In 2017 it just so happened that Jupiter was in the area of sky being studied and Sheppard, having been involved in 48 other moon discoveries, took the opportunity to examine Jupiter one more time.
The orbits of the twelve newly discovered moons of Jupiter are shown here in bold. One moon is located in the outer group but orbits in the opposite direction. Credit: Roberto Molar-Candanosa/Carnegie Institution for Science
To wrap up Solar System moons, take a look at the following sites for some very cool information:
- NASA moon overview here.
- The 10 most incredible moons in the Solar System – ‘Hybrid Librarians’ video. Well worth a watch here!
And now for a discovery which is the icing on the cake…
In the weeks prior to writing, astronomers Alex Teachey and David Kipping of Columbia University revealed the likely discovery of the first exomoon – i.e. a moon orbiting a planet orbiting another star. Combing through data from the Kepler Space Telescope they have uncovered evidence for a Neptune-sized gas moon orbiting a Jupiter mass planet in the Kepler 1625 system 8,000 light years away. If confirmed, ‘Kepler 1625b I’ as it is called, will be unlike any moon existing in our Solar System. Watch this space.