Children usually meet the solar system as a list of names: planets in order, one after another. It gets more fun when they can see how different the planets are, and hold a model. Here are the facts from NASA for a solar system for kids, a note on the “nine planets” question, and what research says about learning size and scale, with sources.
The short version
- NASA says “our solar system has eight planets, and five dwarf planets.” Pluto was long counted as the ninth planet, until the International Astronomical Union reclassified it as a dwarf planet in 2006.
- The planets differ hugely: NASA lists Jupiter at 142,984 km across and Mercury at 4,879 km, and days run from about 10 hours on Jupiter to months on Mercury and Venus.
- Sizes and distances are hard to picture. In a 2018 study, nearly half of the Norwegian middle schoolers tested thought planets are bigger than stars.
The planets in order
NASA lists the planets in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The first four are the inner planets. NASA says they are “also are known as terrestrial planets because they have solid surfaces.” The outer four are giants without a hard surface. Jupiter and Saturn are gas giants, and Uranus and Neptune are ice giants.
The Sun is by far the biggest thing in the family. Wikipedia says it “accounts for 99.86% of the Solar System’s total mass.” The planets are small by comparison.
How big is each planet?
These are NASA’s figures for each planet’s diameter, from its planetary fact sheet. The bars use the same scale, so you can see the gap between the small rocky planets and the giants.
- Mercury4,879 km
- Venus12,104 km
- Earth12,756 km
- Mars6,792 km
- Jupiter142,984 km
- Saturn120,536 km
- Uranus51,118 km
- Neptune49,528 km
Source: NASA Goddard Space Flight Center, Planetary Fact Sheet.
A day on each planet
Kids love this one. NASA’s fact sheet gives the “length of day” for each world. It defines it as the average time for the Sun to go from noon to noon at the equator, so it is a day as a person standing there would see it.
| Planet | Length of day (hours) | Distance from the Sun (million km) |
|---|---|---|
| Mercury | 4,222.6 | 57.9 |
| Venus | 2,802.0 | 108.2 |
| Earth | 24.0 | 149.6 |
| Mars | 24.7 | 228.0 |
| Jupiter | 9.9 | 778.5 |
| Saturn | 10.7 | 1,432.0 |
| Uranus | 17.2 | 2,867.0 |
| Neptune | 16.1 | 4,515.0 |
Our arithmetic, not NASA’s: 4,222.6 hours is about 176 Earth days for one day on Mercury, and 2,802 hours is about 117 for Venus. Jupiter, the biggest planet, has the shortest day at 9.9 hours. NASA also notes that Venus spins backwards compared with Earth.
Nine planets or eight? What happened to Pluto
Many of us learned nine. NASA says Clyde Tombaugh discovered Pluto in 1930 and that it was long considered the ninth planet. Then astronomers found similar worlds in the distant Kuiper Belt.
In 2006 the International Astronomical Union (IAU), the world organization of astronomers, created the official definition of a planet, and NASA says it reclassified Pluto as a dwarf planet that year. Under NASA’s summary, a planet must orbit the Sun, be mostly round, and have enough gravity to clear other similar-sized objects from its orbit. Pluto does not meet the third rule. Wikipedia dates the decision to 24 August 2006 (Resolution 6A). NASA now counts five dwarf planets: Ceres, Pluto, Haumea, Makemake and Eris.
So when someone says “nine planets,” they are either using the pre-2006 count or counting Pluto as an extra world. NASA’s table still lists Pluto: a day of 153.3 hours and a diameter of 2,376 km, much smaller than any of the eight.
Why a physical model helps picture scale
Size and distance are the hard part. Rajpaul and colleagues tested Norwegian middle school students and trainee teachers in Physical Review Physics Education Research (2018). Their abstract reports that, even after instruction, many people “held erroneous views and conceptions regarding sizes, distances, and the nature of basic astronomical entities.” The related Physics magazine article adds that nearly half of the middle schoolers thought planets are bigger than stars, and that showing the Sun and planets to proper scale in pictures is not easy.
Does holding a model fix this? We cannot say that. No study we opened tests a solar system box. The closest we found is a 2015 Psychological Science paper by Kontra and colleagues: in a randomized class experiment, a brief physical experience with the forces behind angular momentum “significantly improved quiz scores” compared with watching. That was college students and a physics topic, so it is a hint about hands-on learning in general, not a promise.
Space toys for kids: what to look for
A good model gives a child something to do: name each planet, put them in order, handle them. If scale is the goal, check whether a toy says it is to scale. Many do not, and ours does not say so either.
What we would pick
Our Nine Planets Crystal Solar System box is made for exactly that hands-on part. The product page says: open the wooden box and the planets lie in a bed of black foam under a lid printed with a night sky and every planet’s name. Lift one out, roll it in your palm, find its name on the lid, and put it back in its place. The planets are colourful crystal stones, each with its own colour and pattern. The box has a hinged lid and a brass clasp, and a foam bed with a place for each planet.
Two honest notes. The page does not list which nine bodies the stones stand for, and it does not say the stones are in proportion to the real planets, so we do not claim either. Also, it contains small stones: keep away from children under 3.
If your child likes real stones, our Gemstone Advent Calendar has a tumbled gemstone or mineral behind each of its 24 doors (recommended age 6+, small parts, not for children under 3).
Sources
- Planets, NASA Science: eight planets in order from the Sun, inner (terrestrial) and giant planets, five dwarf planets.
- Planetary Fact Sheet, NASA Goddard Space Flight Center: diameters, length of day and distance from the Sun for each planet, and for Pluto.
- Planetary Fact Sheet notes, NASA Goddard: what length of day and negative rotation mean.
- Dwarf Planets, NASA Science: the 2006 IAU definition, the three rules, Pluto reclassified, Tombaugh 1930.
- Dwarf planet, Wikipedia: Resolution 6A, 24 August 2006.
- Solar System, Wikipedia: the Sun holds 99.86% of the system’s mass.
- Rajpaul et al., Physical Review Physics Education Research 14, 020108 (2018) and Astronomy Students Not Learning the Basics, Physics magazine: errors about sizes and distances even after instruction; nearly half of middle schoolers thought planets are bigger than stars.
- Physical experience enhances science learning, Kontra et al., Psychological Science, 2015 (PubMed): hands-on experience improved quiz scores in a college physics class.
