Here in our own Solar System, a single star anchors the system, where inner, rocky planets, an intermediate-distance asteroid belt, and then more distant gas giant planets eventually give way to the Kuiper belt and Oort cloud. This configuration is not universal among stellar and planetary systems, which means our Solar System possesses many properties that are not necessarily common among exoplanetary systems. (Credit: NASA/Dana Berry)

7 bizarre facts about the Solar System to stump any scientist

From the coldest planets to spacecraft that have exited the Solar System, these little-known facts stump even many professional astronomers.

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. (Credit: NASA, ESA, and G. Bacon)

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 and so we cannot make the critical measurements necessary to make such a determination. (Credit: NASA)

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. (Credit: NASA/Johns Hopkins APL/Southwest Research Institute)

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. (Credit: JAXA/NASA/Hinode (top); NASA/TRACE (bottom))

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