7 bizarre facts about the Solar System to stump any scientist

Can you successfully answer all seven?

In 1977, NASA’s Voyager 1 and 2 spacecraft began their pioneering journey across the Solar System to visit the giant outer planets. Now, the Voyagers are hurtling through unexplored territory on their road trip beyond our Solar System. Along the way, they are measuring the interstellar medium, the mysterious environment between stars that is filled with the debris from long-dead stars. Voyager 1 became the most distant spacecraft from Earth in 1998, and no other spacecraft launched, to date, has a chance of catching it.

1.) How many spacecraft have exited the Solar System?

This 1997 artwork shows the planets of the Solar System and the relative trajectories of the first four spacecraft on a course to exit the Solar System. In 1998, Voyager 1 overtook Pioneer 10, and in 2012, it passed the heliopause and entered interstellar space. Voyager 2 entered interstellar space in 2018 and recently surpassed Pioneer 10’s distance in 2023; therefore we strongly suspect that Pioneer 10 is in interstellar space as well, but it is no longer functional, so we cannot make the critical measurements necessary to make such a determination.

Only three have passed the heliopause: Voyager 1, Voyager 2, and Pioneer 10.

There are five spacecraft presently either on their way out of the Solar System or that have already left it. From 1973-1998, Pioneer 10 was the most distant spacecraft from the Sun, but in 1998, Voyager 1 caught and passed it. In 2023, Voyager 2 passed it as well, and eventually New Horizons will pass first Pioneer 11 and later Pioneer 10 as well. In 2098, a gravitational encounter will give the now-defunct Ulysses spacecraft a gravitational kick, meaning that 6 spacecraft are on course to exit the Solar System at present.

Pioneer 11, New Horizons, and eventually Ulysses will join them.

Transits of Venus (top) and Mercury (bottom) across the edge of the Sun. Note how Venus’ atmosphere diffracts sunlight around it, while Mercury’s lack of atmosphere shows no such effects. An airless planet, like Mercury, will have a completely flat transit spectroscopy spectrum, while a planet like Venus will exhibit absorption and/or emission signatures.

2.) Who possesses hotter daytime temperatures: Venus or Mercury?

The Soviet Union’s series of Venera landers are the only spacecraft to ever land and transmit data from the surface of Venus. The longest-lived of all the landers exceeded the two-hour mark before the instruments overheated and contact was lost. To date, no spacecraft has survived for longer on the Venusian surface, where temperatures reach 900 degrees Fahrenheit (482 °C).

Venus, uniformly at 464 °C (867 °F), surpasses Mercury’s peak temperatures.

Although our best-ever views of the planets of Uranus and Neptune still come from the Voyager 2 encounters with these worlds from the late 1980s, the reality is that these two planets are extremely similar in color, composition, and size, with the famous “azure” image of Neptune not representative of its true color. Uranus is just 3% larger than Neptune, but substantially less massive.

3.) Which Solar System planet is coldest?

This 2018 image of Uranus from Hubble shows how the planet changes as it progresses from equinox toward solstice. The bright northern pole experiences a cloud cap, while the clouds and banding features across the rest of the world are decreasing. Uranus will next reach solstice in 2028.

At -224 °C (-372 °F), Uranus possesses the coldest recorded temperatures.

When Voyager 2 flew by Uranus in 1986, the planet was near solstice, with its southern hemisphere facing the Sun and its northern hemisphere facing away. In 2007, Uranus achieved equinox, and now heads toward its next 2028 solstice. It won’t reach equinox again until 2049, when JWST will likely be out of fuel and defunct.

Its cloudy poles experience decades of darkness, getting even colder than Neptune.

This cutaway view of the four terrestrial planets (plus Earth’s moon) shows the relative sizes of the cores, mantles, and crusts of these five worlds. Despite the fact that the Earth is only 5% larger in diameter than Venus, it has more mass than Mercury, Venus, Mars, and the Moon combined. If you could pass through the Earth’s interior as a projectile that didn’t interact electromagnetically with the Earth, you would see your trajectory change slightly as you transitioned across one internal layer to another.

4.) How many non-planets exceed Mercury’s size?

The above image shows an orthographic projection of planet Mercury in this global mosaic centered at 0°N, 0°E. The rayed crater Debussy can be seen toward the bottom of the globe and the peak-ring basin Rachmaninoff can be seen toward the eastern edge. Mercury is the Solar System’s innermost and smallest planet, and was mapped in detail by NASA’s MESSENGER mission.

Only two: Jupiter’s moon Ganymede and Saturn’s Titan.

Although Earth and Venus are the two largest rocky objects in the Solar System, Mars, Mercury, as well as over 100 of the largest moons, asteroids, and Kuiper belt objects have all achieved hydrostatic equilibrium. Ganymede and Titan are larger than Mercury, but Callisto, at 99% of Mercury’s size, has just one-third of Mercury’s mass. All told, all known objects with radii greater than 800 km are in hydrostatic equilibrium, but below that threshold, it isn’t a certainty any longer.

Jupiter’s Callisto, the third-largest non-planet, is 1.2% (58.7 km/36.5 miles) smaller than Mercury.

Of the eight planets in our Solar System, the four gas giant worlds are the least dense, with less than half the density of the least dense rocky planet (Mars), and with Saturn being even less dense than water.

5.) Which planet has the highest density?

When it comes to the large, non-gaseous worlds of the Solar System, Mercury has by far the largest metallic core relative to its size. However, it’s Earth that’s the densest of all these worlds, with no other major body comparing in density, owing to the added factor of gravitational compression. Unlike Venus, Earth, and Mars, Mercury has no separate crustal layer to speak of.

That would be Earth, at 5.51 g/cm³.

The Earth, beneath its thin atmosphere and oceans, transitions from primarily rocky material to a metallic core once you go about 45% of the way down. With core pressures exceeding 3.6 million atmospheres, the atoms in the core are compressed to a fraction of their original size, explaining Earth’s uncharacteristically high density. Recent evidence indicates an innermost core inside the inner core, where a different solid phase of metals exists than in the rest of the inner core. All massive objects, including neutron stars, display this type of pressure gradient.

Although Mercury is ~75-85% metal, gravitational compression gives Earth a 0.08 g/cm³ victory.

The surfaces of six different worlds in our Solar System, from an asteroid to the Moon to Venus, Mars, Titan, and Earth, showcase a wide diversity of properties and histories. While only Earth is known to contain liquid water rainfall and large cumulations of liquid water on its surface, other worlds have other forms of precipitation and surface liquids, both at present and also in the distant past. Perhaps, long ago, Earth was joined by other worlds or even other planets, such as Mars and Venus, in possessing liquid water and perhaps life on its planetary surface.

6.) Which rocky world is the most water-rich?

Although Earth contains the most liquid water on its surface of any of the 8 planets, the most water in any form is found on Jupiter’s moon Ganymede. Next in order is Saturn’s Titan, Jupiter’s Callisto, and Jupiter’s Europa. Planet Earth has only the 5th most water, placing it ahead of Pluto, Dione, Triton, and Enceladus, which land in 6th through 9th place in the Solar System, respectively. Other, more poorly explored Kuiper belt objects, such as Eris, may yet deserve a place on this list.

Jupiter’s moon Ganymede, with 35.4 zetaliters (3.54 × 10²² L), is 46% liquid water.

Beneath a surface consisting largely of hexagonal-ice crystals, a thick saltwater ocean extends downward for an estimated 160 km (~100 miles) below the surface of Ganymede, making it the most water-rich world, by far, in the entire Solar System. It contains more water than all of the other known rocky worlds, combined.

Earth is only fifth, with Titan, Callisto, and Europa all surpassing us.

In Newton’s theory of gravity, orbits make perfect ellipses when they occur around single, large masses. The presence of other masses, like the other planets, cause these elliptical orbits to precess. However, in general relativity, there is an additional precession effect due to both the curvature of spacetime and the fact that the planets are in motion with respect to the Sun, and this causes the orbit to shift over time, in a fashion that is sometimes measurable. Mercury exhibits the largest such effect within our Solar System, precessing at a rate of an extra 43″ (where 1″ is 1/3600th of one degree) per century due to this additional effect.

7.) Which planet contributes most to Mercury’s precession?

This illustration shows the precession of a planet’s orbit around the Sun. A very small amount of precession is due to general relativity in our Solar System; Mercury precesses by 43 arc-seconds per century, the greatest value of all our planets. Although the total rate of precession is 5600 arc-seconds per century, 5025 of them are due to the precession of the equinoxes and 532 are due to the effects of the other planets in our Solar System. Those final 43 arc-seconds per century cannot be explained without general relativity.

Venus (277 arc-seconds/century) leads, followed by Jupiter (150), and then Earth (90).

By size, it’s clear that the gas giant worlds vastly outstrip any of the terrestrial planets, and this is true for mass as well. Although Saturn is nearly the same physical size as Jupiter, it contains just 21% of the planetary mass in the Solar System, compared to Jupiter’s whopping 71%.

An additional, unexplained 43 arc-seconds/century helped demonstrate general relativity’s correctness.

The hypothetical location of the planet Vulcan, presumed to be responsible for the observed precession of Mercury in the 1800s. Exhaustive searches were performed for a planet that could have accounted for the anomalous motions of Mercury in the context of Newtonian gravity, but no such planet exists, falsifying the prediction of an interior planet in our Solar System, general relativity, a different theory of gravity, instead explains this otherwise anomalous precession.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.