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The vermilion, amethyst and jade ribbons of the northern and southern lights are some of Earth\u2019s most distinctive features. But our planet doesn\u2019t have a monopoly on auroras. Scientists have spied them throughout the solar system, weaving through the skies of Mars, Saturn, Jupiter and even on some of Jupiter\u2019s fiery and icy moons.<\/p>\n
Lights glow in the skies of Uranus, too. But auroras around our sun\u2019s most distant planet, Neptune, have long eluded astronomers.<\/p>\n
That has changed with the powerful infrared instruments aboard the James Webb Space Telescope. In a study published on Wednesday in the journal Nature Astronomy, scientists reveal unique auroras that spill over either side of Neptune\u2019s equator, a contrast with the glowing gossamer seen arcing over other worlds\u2019 poles.<\/p>\n
Astronomers are thrilled to see the completion of an aurora-hunting quest decades in the making. \u201cEveryone is very excited to prove that it\u2019s there, just like we thought,\u201d said Rosie Johnson, a space physics researcher at Aberystwyth University in Wales who wasn\u2019t involved with the new study.<\/p>\n<\/div>\n<\/div>\n
This discovery will also allow scientists to study aspects of Neptune that have previously been out of reach. \u201cThey\u2019re using aurora to understand the shape of the planet\u2019s magnetic field, which is seeing the unseen,\u201d said Carl Schmidt, a planetary astronomer at Boston University who wasn\u2019t involved with the new study.<\/p>\n
Each world generates auroras differently, but the basics are the same. Energetic particles (often from the sun, but sometimes from a moon\u2019s volcanic eruptions) slam into an atmosphere and bounce off gases. That particle collision briefly causes flashes of light. And if a world has a magnetic field, that guides the location of the auroras.<\/p>\n
Auroras don\u2019t always glow in visible light; Saturn, for example, emits mostly ultraviolet auroras. But they can be observed with the right telescopes.<\/p>\n
It hasn\u2019t been possible until now to spot Neptune\u2019s atmospheric lights.<\/p>\n
\u201cAstronomers have been trying to detect the aurora of Neptune for decades, and each attempt has failed,\u201d said Henrik Melin, a planetary scientist at Northumbria University in England and one of the study\u2019s authors.<\/p>\n
Voyager 2, the only spacecraft to fly by Neptune (in 1989), found hints of an aurora. But all follow-up observations \u2014 even with the Hubble Space Telescope \u2014 failed to spy telltale shimmering.<\/p>\n<\/div>\n<\/div>\n
Fortunately, the Webb telescope, launched in 2021, has come to the rescue.<\/p>\n
Heidi Hammel, an astronomer at the Association of Universities for Research in Astronomy and another of the study\u2019s authors, has been studying Neptune since the 1980s. She thought that if Webb \u201cwas powerful enough to see the earliest galaxies in the universe, it\u2019d better be powerful enough to see things like aurorae on Neptune,\u201d she said. \u201cAnd by golly, it was.\u201d<\/p>\n
Using the telescope\u2019s Near-Infrared Spectrograph, astronomers caught Neptune\u2019s infrared auroras in June 2023. And unlike Earth\u2019s, they dance not above the poles, but its mid-latitudes. That\u2019s because Neptune has a wonky magnetic field that is tilted by 47 degrees from the planet\u2019s spin axis.<\/p>\n
The new Webb observations also reveal why Neptune\u2019s auroras have been invisible until now. Nearly 40 years ago, Voyager 2 recorded a temperature of around 900 degrees Fahrenheit for Neptune\u2019s upper atmosphere. But the Webb telescope shows that the temperature has dropped, to close to 200 degrees. That lower temperature means the auroras are dimmer.<\/p>\n
In fact, Neptune\u2019s aurora is glowing \u201cwith less than 1 percent of the brightness we expected, explaining why we haven\u2019t seen it,\u201d said James O\u2019Donoghue, a planetary astronomer at the University of Reading in England and one of the study\u2019s authors. \u201cHowever, that means we now have a new mystery on our hands: How has Neptune cooled down so much?\u201d<\/p>\n
With the detection of Neptune\u2019s strange light show, answers may be forthcoming.<\/p>\n
\u201cAuroras are like a TV screen,\u201d said Leigh Fletcher, a planetary scientist at the University of Leicester in England and one of the study\u2019s authors. They are \u201callowing us to watch the delicate dance of processes in the magnetosphere \u2014 all without actually being there.\u201d<\/p>\n<\/div>\n<\/div>\n