Astronomer William Herschel discovered Uranus—and two of its moons—230 years ago. Now a group of astronomers working with data from the telescope that bears his name, the Herschel Space Observatory, have made an unexpected discovery. It looks like Uranus’ moons bear a striking similarity to icy dwarf planets.
The Herschel Space Observatory has been retired since 2013. But all of its data is still of interest to researchers. This discovery was a happy accident, resulting from tests on data from the observatory’s camera detector. Uranus is a very bright infrared energy source, and the team was measuring the influence of very bright infrared objects on the camera.
The images of the moons were discovered by accident.
What’s the most interesting fact you know about Uranus? The fact that its rotational axis is completely out of line with every other planet in the solar system? Or the fact that Uranus’ magnetosphere is asymmetrical, notably tilted relative to its rotational axis, and significantly offset from the center of the planet? Or the fact that it’s moons are all named after characters from Shakespeare or Alexander Pope?
All of those facts (with the exception of the literary references) have come from a very limited dataset. Some of the best data was collected during a Voyager 2 flyby in 1986. Since then, the only new data has come from Earth-based telescopes. While they’ve been steadily increasing in resolution, they have only been able to scratch the surface of what may be lurking in the system surrounding the closest Ice Giant. Hopefully that is about to change, as a team of scientists has published a white paper advocating for a visit from a new Flagship class spacecraft.
During the late 1970s, scientists made a rather interesting discovery about the gas giants of the Solar System. Thanks to ongoing observations using improved optics, it was revealed that gas giants like Uranus – and not just Saturn – have ring systems about them. The main difference is, these ring systems are not easily visible from a distance using conventional optics and require exceptional timing to see light being reflected off of them.
Astronomers think they know how Uranus got flipped onto its side. According to detailed computer simulations, a body about twice the size of Earth slammed into Uranus between 3 to 4 billion years ago. The impact created an oddity in our Solar System: the only planet that rotates on its side.
A study explaining these findings was presented at the American Geophysical Union’s (AGU) Fall Meeting in Washington DC held between December 10th to 14th. It’s led by Jacob Kegerreis, a researcher at Durham University. It builds on previous studies pointing to an impact as the cause of Uranus’ unique orientation. Taken altogether, we’re getting a clearer picture of why Uranus rotates on its side compared to the other planets in our Solar System. The impact also explains why Uranus is unique in other ways. Continue reading “Something Twice the Size of Earth Slammed into Uranus and Knocked it Over on its Side”
The gas/ice giant Uranus has long been a source of mystery to astronomers. In addition to presenting some thermal anomalies and a magnetic field that is off-center, the planet is also unique in that it is the only one in the Solar System to rotate on its side. With an axial tilt of 98°, the planet experiences radical seasons and a day-night cycle at the poles where a single day and night last 42 years each.
Thanks to a new study led by researchers from Durham University, the reason for these mysteries may finally have been found. With the help of NASA researchers and multiple scientific organizations, the team conducted simulations that indicated how Uranus may have suffered a massive impact in its past. Not only would this account for the planet’s extreme tilt and magnetic field, it would also explain why the planet’s outer atmosphere is so cold.
If you’ve got really good eyesight and can find a place where the light pollution is non-existent, you might be able to see Uranus without a telescope. It’s only possible with the right conditions, and if you know exactly where to look. And for thousands of years, scholars and astronomers were doing just that. But given that it was just a tiny pinprick of light, they believed Uranus was a star.
It was not until the late 18th century that the first recorded observation that recognized Uranus as being a planet took place. This occurred on March 13th, 1781, when British astronomer Sir William Herschel observed the planet using a telescope of his own creation. From this point onwards, Uranus would be recognized as the seventh planet and the third gas giant of the Solar System.
Observations pre-18th Century:
The first recorded instance of Uranus being spotted in the night sky is believed to date back to Classical Antiquity. During the 2nd century BCE, Hipparchos – the Greek astronomer, mathematician and founder of trigonometry – apparently recorded the planet as a star in his star catalogue (completed in 129 BCE).
This catalog was later incorporated into Ptolemy’s Almagest,which became the definitive source for Islamic astronomers and for scholars in Medieval Europe for over one-thousand years. During the 17th and 18th centuries, multiple recorded sightings were made by astronomers who also catalogued it as being a star.
This included English astronomer John Flamsteed, who in 1690 observed the star on six occasions and catalogued it as a star in the Taurus constellation (34 Tauri). During the mid-18th century, French astronomer Pierre Lemonnier made twelve recorded sightings, and also recorded it as being a star. It was not until March 13th, 1781, when William Herschel observed it from his garden house in Bath, that Uranus’ true nature began to be revealed.
On the evening in question – March 13th, 1781 – William Herschel was surveying the sky with his telescope, looking for binary stars. His first report on the object was recorded on April 26th, 1781. Initially, he described it as being a “Nebulous star or perhaps a comet”, but later settled on it being a comet since it appeared to have changed its position in the sky.
When he presented his discovery to the Royal Society, he maintained this theory, but also likened it to a planet. As was recorded in the Journal of the Royal Society and Royal Astronomical Society on the occasion of his presentation:
“The power I had on when I first saw the comet was 227. From experience I know that the diameters of the fixed stars are not proportionally magnified with higher powers, as planets are; therefore I now put the powers at 460 and 932, and found that the diameter of the comet increased in proportion to the power, as it ought to be, on the supposition of its not being a fixed star, while the diameters of the stars to which I compared it were not increased in the same ratio. Moreover, the comet being magnified much beyond what its light would admit of, appeared hazy and ill-defined with these great powers, while the stars preserved that lustre and distinctness which from many thousand observations I knew they would retain. The sequel has shown that my surmises were well-founded, this proving to be the Comet we have lately observed.”
While Herschel would continue to maintain that what he observed was a comet, his “discovery” stimulated debate in the astronomical community about what Uranus was. In time, astronomers like Johann Elert Bode would conclude that it was a planet, based on its nearly-circular orbit. By 1783, Herschel himself acknowledged that it was a planet to the Royal Society.
As he lived in England, Herschel originally wanted to name Uranus after his patron, King George III. Specifically, he wanted to call it Georgium Sidus (Latin for “George’s Star”), or the Georgian Planet. Although this was a popular name in Britain, the international astronomy community didn’t think much of it, and wanted to follow the historical precedent of naming the planets after ancient Greek and Roman gods.
Consistent with this, Bode proposed the name Uranus in a 1782 treatise. The Latin form of Ouranos, Uranus was the grandfather of Zeus (Jupiter in the Roman pantheon), the father of Cronos (Saturn), and the king of the Titans in Greek mythology. As it was discovered beyond the orbits of Jupiter and Saturn, the name seemed highly appropriate.
In the following century, Neptune would be discovered, the last of the eight official planets that are currently recognized by the IAU. And by the 20th century, astronomers would discovery Pluto and other minor planets within the Kuiper Belt. The process of discovery has been ongoing, and will likely continue for some time to come.
Earth doesn’t have a corner on auroras. Venus, Mars, Jupiter, Saturn, Uranus and Neptune have their own distinctive versions. Jupiter’s are massive and powerful; Martian auroras patchy and weak.
Auroras are caused by streams of charged particles like electrons that originate with solar winds and in the case of Jupiter, volcanic gases spewed by the moon Io. Whether solar particles or volcanic sulfur, the material gets caught in powerful magnetic fields surrounding a planet and channeled into the upper atmosphere. There, the particles interact with atmospheric gases such as oxygen or nitrogen and spectacular bursts of light result. With Jupiter, Saturn and Uranus excited hydrogen is responsible for the show.
Auroras on Earth, Jupiter and Saturn have been well-studied but not so on the ice-giant planet Uranus. In 2011, the Hubble Space Telescope took the first-ever image of the auroras on Uranus. Then in 2012 and 2014 a team from the Paris Observatory took a second look at the auroras in ultraviolet light using the Space Telescope Imaging Spectrograph (STIS) installed on Hubble.
Two powerful bursts of solar wind traveling from the sun to Uranus stoked the most intense auroras ever observed on the planet in those years. By watching the auroras over time, the team discovered that these powerful shimmering regions rotate with the planet. They also re-discovered Uranus’ long-lost magnetic poles, which were lost shortly after their discovery by Voyager 2 in 1986 due to uncertainties in measurements and the fact that the planet’s surface is practically featureless. Imagine trying to find the north and south poles of a cue ball. Yeah, something like that.
In both photos, the auroras look like glowing dots or patchy spots. Because Uranus’ magnetic field is inclined 59° to its spin axis (remember, this is the planet that rotates on its side!) , the auroral spots appear far from the planet’s north and south geographic poles. They almost look random but of course they’re not. In 2011, the spots lie close to the planet’s north magnetic pole, and in 2012 and 2014, near the south magnetic pole — just like auroras on Earth.
An auroral display can last for hours here on the home planet, but in the case of the 2011 Uranian lights, they pulsed for just minutes before fading away.
Want to know more? Read the team’s findings in detail here.
Uranus is a most unusual planet. Aside from being the seventh planet of our Solar System and the third gas giant, it is also classified sometimes as an “ice giant” (along with Neptune). This is because of its peculiar chemical composition, where water and other volatiles (i.e. ammonia, methane, and other hydrocarbons) in its atmosphere are compressed to the point where they become solid.
In addition to that, it also has a very long orbital period. Basically, it takes Uranus a little over 84 Earth years to complete a single orbit of the Sun. What this means is that a single year on Uranus lasts almost as long as a century here on Earth. On top of that, because of it axial tilt, the planet also experiences extremes of night and day during the course of a year, and some pretty interesting seasonal changes.
Uranus orbits the Sun at an average distance (semi-major axis) of 2.875 billion km (1.786 billion mi), ranging from 2.742 billion km (1.7 mi) at perihelion to 3 billion km (1.86 billion mi) at aphelion. Another way to look at it would be to say that it orbits the Sun at an average distance of 19.2184 AU (over 19 times the distance between the Earth and the Sun), and ranges from 18.33 AU to 20.11 AU.
The difference between its minimum and maximum distance from the Sun is 269.3 million km (167.335 mi) or 1.8 AU, which is the most pronounced of any of the Solar Planets (with the possible exception of Pluto). And with an average orbital speed of 6.8 km/s (4.225 mi/s), Uranus has an orbital period equivalent to 84.0205 Earth years. This means that a single year on Uranus lasts as long as 30,688.5 Earth days.
However, since it takes 17 hours 14 minutes 24 seconds for Uranus to rotate once on its axis (a sidereal day). And because of its immense distance from the Sun, a single solar day on Uranus is about the same. This means that a single year on Uranus lasts 42,718 Uranian solar days. And like Venus, Uranus’ rotates in the direction opposite of its orbit around the Sun (a phenomena known as retrograde rotation).
Another interesting thing about Uranus is the extreme inclination of its axis (97.7°). Whereas all of the Solar Planets are tilted on their axes to some degree, Uranus’s extreme tilt means that the planet’s axis of rotation is approximately parallel with the plane of the Solar System. The reason for this is unknown, but it has been theorized that during the formation of the Solar System, an Earth-sized protoplanet collided with Uranus and tilted it onto its side.
A consequence of this is that when Uranus is nearing its solstice, one pole faces the Sun continuously while the other faces away – leading to a very unusual day-night cycle across the planet. At the poles, one will experience 42 Earth years of day followed by 42 years of night.
This is similar to what is experienced in the Arctic Circle and Antarctica. During the winter season near the poles, a single night will last for more than 24 hours (aka. a “Polar Night”) while during the summer, a single day will last longer than 24 hours (a “Polar Day”, or “Midnight Sun”).
Meanwhile, near the time of the equinoxes, the Sun faces Uranus’ equator and gives it a period of day-night cycles that are similar to those seen on most of the other planets. Uranus reached its most recent equinox on December 7th, 2007. During the Voyager 2 probe’s historic flyby in 1986, Uranus’s south pole was pointed almost directly at the Sun.
Uranus’ long orbital period and extreme axial tilt also lead to some extreme seasonal variations in terms of its weather. Determining the full extent of these changes is difficult because astronomers have yet to observe Uranus for a full Uranian year. However, data obtained from the mid-20th century onward has showed regular changes in terms of brightness, temperature and microwave radiation between the solstices and equinoxes.
These changes are believed to be related to visibility in the atmosphere, where the sunlit hemisphere is thought to experience a local thickening of methane clouds which produce strong hazes. Increases in cloud formation have also been observed, with very bright cloud features being spotted in 1999, 2004, and 2005. Changes in wind speed have also been noted that appeared to be related to seasonal increases in temperature.
Uranus’ “Great Dark Spot” and its smaller dark spot are also thought to be related to seasonal changes. Much like Jupiter’s Great Red Spot, this feature is a giant cloud vortex that is created by winds – which in this case are estimated to reach speeds of up to 900 km/h (560 mph). In 2006, researchers at the Space Science Institute and the University of Wisconsin observed a storm that measured 1,700 by 3,000 kilometers (1,100 miles by 1,900 miles).
Interestingly enough, while Uranus’ polar regions receive more energy on average over the course of a year than the equatorial regions, the equatorial regions have been found to be hotter than the poles. The exact cause of this remains unknown, but is certainly believed to be due to something endogenic.
Yep, Uranus is a pretty weird place! On this planet, a single year lasts almost a century, and the seasons are characterized by extreme versions of Polar Nights and Midnight Suns. And of course, an average year brings all kinds of seasonal changes, complete with extreme winds, massive storms, and thickening methane clouds.
Clear night ahead? Let’s see what’s up. We’ll start close to home with the Moon, zoom out to lonely Fomalhaut 25 light years away and then return to our own Solar System to track down the 7th planet. Even before the sky is dark, you can’t miss the 4-day-old crescent Moon reclining in the southwestern sky. Watch for it to wax to a half-moon by Thursday as it circles Earth at an average speed of 2,200 mph (3,600 km/hr). That fact that it orbits Earth means that the angle the Moon makes with the sun and our planet constantly varies, the reason for its ever-changing phase.
With the naked eye you’ll be able to make two prominent dark patches within the crescent — Mare Crisium (Sea of Crises) and Mare Fecunditatis (Sea of Fecundity). Each is a vast, lava-flooded plain peppered with thousands of craters , most of which require a telescope to see. Not so Janssen. This large, 118-mile-wide (190-km) ring will be easy to pick out in a pair of seven to 10 power binoculars. Janssen is named for 19th century French astronomer Pierre Janssen, who was the first to see the bright yellow line of helium in the sun’s spectrum while observing August 1868 total solar eclipse.
English scientist Norman Lockyer also observed the line later in 1868 and concluded it represented a new solar element which he named “helium” after “helios”, the Greek word for sun. Helium on Earth wouldn’t be discovered for another 10 years, making this party-balloon gas the only element first discovered off-planet!
Directing your gaze south around 7 o’clock, you’ll see a single bright star low in the southern sky. This is Fomalhaut in the dim constellation of Piscis Austrinus, the Southern Fish. The Arabic name means “mouth of the fish”. If live under a dark, light-pollution-free sky, you’ll be able to make out a loop of faint stars vaguely fish-like in form. Aside from being the only first magnitude star among the seasonal fall constellations, Fomalhaut stands out in another way — the star is ringed by a planet-forming disk of dust and rock much as our own Solar System was more than 4 billion years ago.
Within that disk is a new planet, Fomalhaut b, with less than twice Jupiter’s mass and enshrouded either by a cloud of dusty debris or a ring system like Saturn. Fomalhaut b has the distinction of being the first extrasolar planet ever photographed in visible light. The plodding planet takes an estimated 1,700 years to make one loop around Fomalhaut, with its distance from its parent star varying from about 50 times Earth’s distance from the sun at closest to 300 times that distance at farthest.
Next, we move on to one of the more remote planets in our own solar system, Uranus. The 7th planet from the sun, Uranus reached opposition — its closest to Earth and brightest appearance for the year — only a month ago. It’s well-placed for viewing in Pisces the Fish after nightfall high in the southeastern sky below the prominent sky asterism, the Great Square of Pegasus.
A telescope will tease out its tiny, greenish disk, but almost any pair of binoculars will easily show the planet as a star-like point of light slowly marching westward against the starry backdrop in the coming weeks. Check in every few weeks to watch it move first west, in retrograde motion, and then turn back east around Christmas. For those with 8-inch and larger telescopes who love a challenge, use this Uranian Moon Finder to track the planet’s two brightest moons, Titania and Oberon, which glimmer weakly around 14th magnitude.
We’ve barely scratched the surface of the vacuum with these offerings; they’re just a few of the many highlights of mid-November nights that also include the annual Leonid meteor shower, which peaks Tuesday and Wednesday mornings (Nov. 17-18). So much to see!