The majestic Northern and Southern Lights have mystified humans for millennia. During the long, dark winter nights – when the space weather is good – green and purple clouds can cover the polar sky from horizon to horizon. The glowing lights dance and move in curtains and arcs, forming a crescendo of color before disappearing.
The terrestrial auroras are not alone in our solar system either. Telescope observations and visiting spacecraft have found evidence of auroras on Saturn, Jupiter, Mars, Venus – and even exoplanets. And with a recent discovery on Jupiter’s mysterious “dawn of dawn” published in the journal AGU Advances, astronomers continue to unravel the mysteries of these lights throughout our solar system.
(Credit: Roen Kelly / Discover)
Aurora on Earth
Auroras on Earth are created when our planet is bombarded by particles from the Sun, mainly electrons and protons. These particles travel down Earth’s magnetic field lines, where they collide with nitrogen and oxygen and excite molecules until they glow. The basic mechanics are not too different from a simple neon.
And yet the Northern Lights and the Northern Lights continue to confuse astronomers’ attempts to predict them and explain their many nuances. For example, just recently a group of citizen scientists discovered a whole new type of aurora, which they dubbed “Steve”. And some researchers are now even trying to find out whether auroras can make sounds – an idea that recurs regularly in folklore but has generally been rejected by science.
More and more, astronomers are also discovering that our planet is not the only one with auroras. Yet not all of them function quite the same as the Earth.
Venus does not have a strong magnetic field, but its proximity to the Sun means that the planet is hit by solar storms so strong that they cause a kind of aurora. (Credit: ESA / C. Carreau)
Aurora on Venus
Venus completely lacks a magnetic field. So if Earth’s auroras are caused by interactions with our planet’s magnetic field, then surely Venus shouldn’t have any Northern Lights at all. Right?
And yet, for forty years, astronomers have wondered about the strange green signals observed on images of the planet.
Scientists moved closer to solving the mystery in 2014. A team of researchers using a telescope in New Mexico observed that Venus was repeatedly struck by solar storms. Each time, green excited oxygen signals appeared.
This evidence, combined with observations collected by the European Space Agency’s Venus Express spacecraft, suggests that the Sun’s own magnetic field could be extended to the planet by solar wind. This process is sufficient to create auroras.
A localized aurora dances above Mars in this artists illustration. (Credit: ESA / C. Carreau)
Aurora on Mars
Like Venus, Mars also has auroras despite its extremely weak magnetic field. And the auroras of Mars are made even more incredible because the red planet orbits twice as far from the Sun as Venus.
Nearly two decades ago, ESA’s Mars Express spacecraft caught a startling glow in the planet’s upper atmosphere. It turns out that the aurorae of Mars are unlike anything else in the solar system.
While the Red Planet lost its magnetic field a long time ago, observations from spacecraft show that magnetic anomalies still persist today, scattered across the planet’s crust. And these regions correspond to the areas where the residual magnetic field of Mars is strongest. So when charged solar particles hit the planet, they interact with this patchwork of magnetism and produce faint, scattered auroras.
A new study helps explain the existence of what is called “the dawn of dawn” on Jupiter. (Credit: NASA / JPL-Caltech / SwRI / UVS / STScI / MODIS / WIC / IMAGE / ULiège / Bonfond)
Aurora over Jupiter
In the far reaches of our solar system, astronomers have seen auroras on Jupiter, Saturn, Uranus and Neptune. Auroras on these gas giant planets likely have similar mechanisms, although Uranus and Neptune have only been visited once by spaceships.
Of all, Jupiter is home to the most spectacular light show in the solar system, as the Hubble Space Telescope has shown us in great detail. Its aurorae are absolutely massive in size thanks to a magnetosphere about 20,000 times more powerful than that of Earth.
They never stop either. While Earth’s auroras fired their spark only from the Sun, Jupiter also receives a constant dose of charged particles from its volcanic moon, Io.
Hubble observed the auroras for an extended and particularly active period in 2016 that coincided with the arrival of orbiter Juno to Jupiter. As Hubble observed the planet, the arriving probe measured the solar wind. Together, the spacecraft has provided new insight into how Jupiter’s auroras react to charged particles from the Sun.
A new study published in March 2021 provided additional information. Before the arrival of the Juno spacecraft, astronomers could not observe the auroras on the day side of Jupiter. And as the spacecraft scanned the night side of Jupiter, it spotted the emergence of extremely bright auroras called “dawn storms.” These storms produce hundreds of times more energy than a nuclear reactor on Earth. And for the first time, this new study has followed storms from their origins on the night side of the planet to their full evolution to the day side.
It turns out that they form much like a kind of aurora on Earth called an auroral sub-storm. Here they are caused by sudden and “explosive” reconfigurations of our planet’s magnetosphere as the solar wind varies. But on Jupiter, the process is probably related to changes in the plasma from Io.
SIMP j01365663 + 0933473, a rogue planet with intense auroras. (Credit: Chuck Carter, Caltech, NRAO / AUI / NSF)
Aurora on a rogue planet
In 2018, astronomers discovered a huge gas giant planet about 20 light years away, dubbed SIMP j01365663 + 0933473. With a mass 13 times that of Jupiter, it traverses our original starless galaxy, making it a so-called “rogue planet”. These worlds formed without stars or were thrown out of their solar systems.
SIMP j01365663 + 0933473 also appears to have a magnetic field millions of times stronger than that of Earth with auroras that can put our northern lights to shame. According to a 2018 study in The Journal of Astrophysics, its auroras occur through a process totally different from anything that happens in our solar system.
Without a star to bombard it with charged particles, SIMP j01365663 + 0933473 must have another source. This could mean that this rogue planet does not wander the galaxy completely on its own. Like Jupiter and Io, it may have a volcanically active moon that feeds it with charged particles. Or, alternatively, like so many different aurora mechanisms that astronomers have discovered over the years, SIMP j01365663 + 0933473 may hold some surprises as well.