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.
Ganymede’s surface is a bit of a puzzle for planetary scientists. About two-thirds of its surface is covered in lighter terrain, while the remainder is darker. Both types of terrain are ancient, with the lighter portion being slightly younger. The two types of terrain are spread around the moon, and the darker terrain contains concurrent furrows.
For the most part, scientists think that the furrows were caused by tectonic activity, possibly related to tidal heating as the moon went through unstable orbital resonances in the past.
But a new study says that a massive impact might be responsible for all those furrows.
With the excitement and interest in the newly discovered ‘mini-moon’ found orbiting Earth, astronomers quickly set their sights on trying to get more details, to determine what this object actually is.
Using the Gemini Observatory in Hawaii, a group of astronomers captured a clearer view of this so-called Temporarily Captured Object (TCO), named 2020 CD3. The image, above, was obtained on February 24, 2020. It shows a tiny pinpoint of light against trailing stars.
We can’t seem to get enough of Saturn. It’s the most visually distinct object in our Solar System (other than the Sun, of course, but it’s kind of hard to gaze at). The Cassini mission to Saturn wrapped up about a year ago, and since then we’re relying on the venerable Hubble telescope to satisfy our appetite for images of the ringed planet.
For millennia, human beings stared up at the night sky and were held in awe by the Moon. To many ancient cultures, it represented a deity, and its cycles were accorded divine significance. By the time of Classical Antiquity and the Middle Ages, the Moon was considered to be a heavenly body that orbited Earth, much like the other known planets of the day (Mercury, Venus, Mars, Jupiter, and Saturn).
However, our understanding of moons was revolutionized when in 1610, astronomer Galileo Galilei pointed his telescope to Jupiter and noticed ” four wandering stars” around Jupiter. From this point onward, astronomers have come to understand that planets other than Earth can have their own moons – in some cases, several dozen or more. So just how many moons are there in the Solar System?
In truth, answering that question requires a bit of a clarification first. If we are talking about confirmed moons that orbit any of the planets of the Solar System (i.e. those that are consistent with the definition adopted by the IAU in 2006), then we can say that there are currently 173 known moons. If, however, we open the floor to dwarf planets that have objects orbiting them, the number climbs to 182.
However, over 200 minor-planet moons have also been observed in the Solar System (as of Jan. 2012). This includes the 76 known objects in the asteroid belt with satellites, four Jupiter Trojans, 39 near-Earth objects (two with two satellites each), 14 Mars-crossers, and 84 natural satellites of Trans-Neptunian Objects. And some 150 additional small bodies have been observed within the rings of Saturn. If we include all these, then we can say that the Solar System has 545 known satellites.
Inner Solar System:
The planets of the Inner Solar system – Mercury, Venus, Earth and Mars – are all terrestrial planets, which means that they are composed of silicate rock and minerals that are differentiated between a metallic core and a silicate mantle and crust. For a number of reasons, few satellites exist within this region of the Solar System.
All told, only three natural satellites exist orbiting planetary bodies in the Inner Solar System – Earth and Mars. While scientist theorize that there were moons around Mercury and Venus in the past, it is believed that these moons impacted on the surface a long time ago. The reason for this sparseness of satellites has a lot to do with the gravitational influence of the Sun.
Both Mercury and Venus are too close to the Sun (and in Mercury’s case, too weak in terms of its own gravitational pull) to have grabbed onto a passing object, or held onto rings of debris in orbit that could have coalesced to form a satellite over time. Earth and Mars were able to retain satellites, but mainly because they are the outermost of the Inner planets.
Earth has only the one natural satellite, which we are familiar with – the Moon. With a mean radius of 1737 km and a mass of 7.3477 x 10²² kg, the Moon is 0.273 times the size of Earth and 0.0123 as massive, which is quite large for a satellite. It is also the second densest moon in our Solar System (after Io), with a mean density of 3.3464 g/cm³.
Several theories have been proposed for the formation of the Moon. The prevailing hypothesis today is that the Earth-Moon system formed as a result of an impact between the newly-formed proto-Earth and a Mars-sized object (named Theia) roughly 4.5 billion years ago. This impact would have blasted material from both objects into orbit, where it eventually accreted to form the Moon.
Mars, meanwhile, has two moons – Phobos and Deimos. Like our own Moon, both of the Martian moons are tidally locked to Mars, so they always present the same face to the planet. Compared to our Moon, they are rough and asteroid-like in appearance, and also much smaller. Hence the prevailing theory that they were once asteroids that were kicked out of the Main Belt by Jupiter’s gravity, and were then acquired by Mars.
The larger moon is Phobos, whose name comes from the Greek word which means “fear” (i.e. phobia). Phobos measures just 22.7 km across and has an orbit that places it closer to Mars than Deimos. Compared to Earth’s own Moon — which orbits at a distance of 384,403 km away from our planet — Phobos orbits at an average distance of only 9,377 km above Mars.
Mars’ second moon is Deimos, which takes its name from the Greek word for panic. It is even smaller, measuring just 12.6 km across, and is also less irregular in shape. Its orbit places it much farther away from Mars, at a distance of 23,460 km, which means that Deimos takes 30.35 hours to complete an orbit around Mars.
These three moons are the sum total of moons to be found within the Inner Solar System (at least, by the conventional definition). But looking further abroad, we see that this is really just the tip of the iceberg. To think we once believed that the Moon was the only one of its kind!
Outer Solar System:
Beyond the Asteroid Belt (and Frost Line), things become quite different. In this region of the Solar System, every planet has a substantial system of Moons; in the case of Jupiter and Saturn, reaching perhaps even into the hundreds. So far, a total of 170 moons have been confirmed orbiting the Outer Planets, while several hundred more orbit minor bodies and asteroids.
Due to its immense size, mass, and gravitational pull, Jupiter has the most satellites of any planet in the Solar System. At present, the Jovian system includes 67 known moons, though it is estimated that it may have up to 200 moons and moonlets (the majority of which are yet to been confirmed and classified).
The four largest Jovian moons are known as the Galilean Moons (named after their discoverer, Galileo Galilei). They include: Io, the most volcanically active body in our Solar System; Europa, which is suspected of having a massive subsurface ocean; Ganymede, the largest moon in our Solar System; and Callisto, which is also thought to have a subsurface ocean and features some of the oldest surface material in the Solar System.
Then there’s the Inner Group (or Amalthea group), which is made up of four small moons that have diameters of less than 200 km, orbit at radii less than 200,000 km, and have orbital inclinations of less than half a degree. This groups includes the moons of Metis, Adrastea, Amalthea, and Thebe. Along with a number of as-yet-unseen inner moonlets, these moons replenish and maintain Jupiter’s faint ring system.
Jupiter also has an array of Irregular Satellites, which are substantially smaller and have more distant and eccentric orbits than the others. These moons are broken down into families that have similarities in orbit and composition, and are believed to be largely the result of collisions from large objects that were captured by Jupiter’s gravity.
Similar to Jupiter, it is estimated that Saturn has at least 150 moons and moonlets, but only 53 of these moons have been given official names. Of these, 34 are less than 10 km in diameter and another 14 are between 10 and 50 km in diameter. However, some of its inner and outer moons are rather large, ranging from 250 to over 5000 km.
Traditionally, most of Saturn’s moons have been named after the Titans of Greek mythology, and are grouped based on their size, orbits, and proximity to Saturn. The innermost moons and regular moons all have small orbital inclinations and eccentricities and prograde orbits. Meanwhile, the irregular moons in the outermost regions have orbital radii of millions of kilometers, orbital periods lasting several years, and move in retrograde orbits.
The Inner Large Moons, which orbit within the E Ring, includes the larger satellites MimasEnceladus, Tethys, and Dione. These moons are all composed primarily of water ice, and are believed to be differentiated into a rocky core and an icy mantle and crust. The Large Outer Moons, which orbit outside of the Saturn’s E Ring, are similar in composition to the Inner Moons – i.e. composed primarily of water ice and rock.
At 5150 km in diameter, and 1,350×1020 kg in mass, Titan is Saturn’s largest moon and comprises more than 96% of the mass in orbit around the planet. Titan is also the only large moon to have its own atmosphere, which is cold, dense, and composed primarily of nitrogen with a small fraction of methane. Scientists have also noted the presence of polycyclic aromatic hydrocarbons in the upper atmosphere, as well as methane ice crystals.
The surface of Titan, which is difficult to observe due to persistent atmospheric haze, shows only a few impact craters, evidence of cryo-volcanoes, and longitudinal dune fields that were apparently shaped by tidal winds. Titan is also the only body in the Solar System beside Earth with bodies of liquid on its surface, in the form of methane–ethane lakes in Titan’s north and south polar regions.
Uranus has 27 known satellites, which are divided into the categories of larger moons, inner moons, and irregular moons (similar to other gas giants). The largest moons of Uranus are, in order of size, Miranda, Ariel, Umbriel, Oberon and Titania. These moons range in diameter and mass from 472 km and 6.7 × 1019 kg for Miranda to 1578 km and 3.5 × 1021 kg for Titania. Each of these moons is particularly dark, with low bond and geometric albedos. Ariel is the brightest while Umbriel is the darkest.
All of the large moons of Uranus are believed to have formed in the accretion disc, which existed around Uranus for some time after its formation, or resulted from the large impact suffered by Uranus early in its history. Each one is comprised of roughly equal amounts of rock and ice, except for Miranda which is made primarily of ice.
The ice component may include ammonia and carbon dioxide, while the rocky material is believed to be composed of carbonaceous material, including organic compounds (similar to asteroids and comets). Their compositions are believed to be differentiated, with an icy mantle surrounding a rocky core.
Neptune has 14 known satellites, all but one of which are named after Greek and Roman deities of the sea (except for S/2004 N 1, which is currently unnamed). These moons are divided into two groups – the regular and irregular moons – based on their orbit and proximity to Neptune. Neptune’s Regular Moons – Naiad, Thalassa, Despina, Galatea, Larissa, S/2004 N 1, and Proteus – are those that are closest to the planet and which follow circular, prograde orbits that lie in the planet’s equatorial plane.
Neptune’s irregular moons consist of the planet’s remaining satellites (including Triton). They generally follow inclined eccentric and often retrograde orbits far from Neptune. The only exception is Triton, which orbits close to the planet, following a circular orbit, though retrograde and inclined.
In order of their distance from the planet, the irregular moons are Triton, Nereid, Halimede, Sao, Laomedeia, Neso and Psamathe – a group that includes both prograde and retrograde objects. With the exception of Triton and Nereid, Neptune’s irregular moons are similar to those of other giant planets and are believed to have been gravitationally captured by Neptune.
With a mean diameter of around 2700 km ( mi) and a mass of 214080 ± 520 x 1017 kg, Triton is the largest of Neptune’s moons, and the only one large enough to achieve hydrostatic equilibrium (i.e. is spherical in shape). At a distance of 354,759 km from Neptune, it also sits between the planet’s inner and outer moons.
These moons make up the lion’s share of natural satellites found in the Solar System. However, thanks to ongoing exploration and improvements made in our instrumentation, satellites are being discovered in orbit around minor bodies as well.
Dwarf Planets and Other Bodies:
As already noted, there are several dwarf planets, TNOs, and other bodies in the Solar System that also have their own moons. These consist mainly of the natural satellites that have been confirmed orbiting Pluto, Eris, Haumea and Makemake. With five orbiting satellites, Pluto has the most confirmed moons (though that may change with further observation).
The largest, and closest in orbit to Pluto, is Charon. This moon was first identified in 1978 by astronomer James Christy using photographic plates from the United States Naval Observatory (USNO) in Washington, D.C. Beyond Charon lies the four other circumbinary moons – Styx, Nix, Kerberos, and Hydra, respectively.
Nix and Hydra were discovered simultaneously in 2005 by the Pluto Companion Search Team using the Hubble Space Telescope. The same team discovered Kerberos in 2011. The fifth and final satellite, Styx, was discovered by the New Horizons spacecraft in 2012 while capturing images of Pluto and Charon.
Charon, Styx and Kerberos are all massive enough to have collapsed into a spheroid shape under their own gravity. Nix and Hydra, meanwhile, are oblong in shape. The Pluto-Charon system is unusual, since it is one of the few systems in the Solar System whose barycenter lies above the primary’s surface. In short, Pluto and Charon orbit each other, causing some scientists to claim that it is a “double-dwarf system” instead of a dwarf planet and an orbiting moon.
In addition, it is unusual in that each body is tidally locked to the other. Charon and Pluto always present the same face to each other; and from any position on either body, the other is always at the same position in the sky, or always obscured. This also means that the rotation period of each is equal to the time it takes the entire system to rotate around its common center of gravity.
In 2007, observations by the Gemini Observatory of patches of ammonia hydrates and water crystals on the surface of Charon suggested the presence of active cryo-geysers. This would seem indicate that Pluto does have a subsurface ocean that is warm in temperature, and that the core is geologically active. Pluto’s moons are believed to have been formed by a collision between Pluto and a similar-sized body early in the history of the Solar System. The collision released material that consolidated into the moons around Pluto.
Coming in second is Haumea, which has two known moons – Hi’iaka and Namaka – which are named after the daughters of the Hawaiian goddess. Both were discovered in 2005 by Brown’s team while conducting observations of Haumea at the W.M. Keck Observatory. Hi’iaka, which was initially nicknamed “Rudolph” by the Caltech team, was discovered January 26th, 2005.
It is the outer and – at roughly 310 km in diameter – the larger and brighter of the two, and orbits Haumea in a nearly circular path every 49 days. Infrared observations indicate that its surface is almost entirely covered by pure crystalline water ice. Because of this, Brown and his team have speculated that the moon is a fragment of Haumea that broke off during a collision.
Namaka, the smaller and innermost of the two, was discovered on June 30th, 2005, and nicknamed “Blitzen”. It is a tenth the mass of Hi‘iaka and orbits Haumea in 18 days in a highly elliptical orbit. Both moons circle Haumea is highly eccentric orbits. No estimates have been made yet as to their mass.
Eris has one moon called Dysnomia, which is named after the daughter of Eris in Greek mythology, which was first observed on September 10th, 2005 – a few months after the discovery of Eris. The moon was spotted by a team using the Keck telescopes in Hawaii, who were busy carrying out observations of the four brightest TNOs (Pluto, Makemake, Haumea, and Eris) at the time.
In April of 2016, observations using the Hubble Space Telescope‘s Wide Field Camera 3 revealed that Makemake had a natural satellite – which was designated S/2015 (136472) 1 (nicknamed MK 2 by the discovery team). It is estimated to be 175 km (110 mi) km in diameter and has a semi-major axis at least 21,000 km (13,000 mi) from Makemake.
Largest and Smallest Moons:
The title for largest moon in the Solar System goes to Ganymede, which measures 5262.4 kilometers (3270 mi) in diameter. This not only makes it larger than Earth’s Moon, but larger even than the planet Mercury – though it has only half of Mercury’s mass. As for the smallest satellite, that is a tie between S/2003 J 9 and S/2003 J 12. These two satellites, both of which orbit Jupiter, measure about 1 km (0.6 mi) in diameter.
An important thing to note when discussing the number of known moons in the Solar System is that the key word here is “known”. With every passing year, more satellites are being confirmed, and the vast majority of those we now know about were only discovered in the past few decades. As our exploration efforts continue, and our instruments improve, we may find that there are hundreds more lurking around out there!
The dwarf planets in our Solar System are some of the most interesting objects around. Of course, all of the Solar System objects–and anything in nature, really–are fascinating when you really focus on them. Now, a new study puts the focus squarely on the dwarf planet Haumea, and deepens the mystery surrounding its origins.
Dwarf planets Pluto and Haumea are considered cousins. Both of them, and their respective moons, are thought to be collisional families. This means they have a common origin in the form of an impact event. But the study, from Luke D. Burkhart, Darin Ragozzine, and Michael E. Brown, shows that Haumea doesn’t have the same kinds of moons as Pluto, which has astronomers puzzling over Haumea’s origins.
Pluto and Haumea are the only two bodies in the outer Solar System that have more than one Moon. Pluto has five moons (Charon, Styx, Nix, Kerberos, and Hydra) while Haumea has two moons, Hi’iaka and Namaka. Haumea is also the parent of a number of icy bodies which were parts of its surface, but now orbit the Sun on their own. The two other dwarf planets in the Kuiper Belt, Eris and Makemake, each have only one moon.
One thing that differentiates Haumea from Pluto is Haumea’s family of small icy bodies that came from its surface. While Pluto has a number of small icy moons, Haumea’s icy bodies orbit the Sun independently, and are not moons. Other properties of Haumea, like its inordinately high rate of spin, make Haumea a very interesting object to study. They also differentiate Haumea from Pluto, and are leading to questions about the cousin relationship between the two. If they are indeed cousins, then shouldn’t they share the same formation method?
Haumea’s lack of icy moons similar to Pluto’s was noted by researcher Darin Ragozzine. “While we’ve known about Pluto’s and Haumea’s moons for years, we now know that Haumea does not share tiny moons like Pluto’s, increasing our understanding of this intriguing object,” Ragozzine said.
There are definite similarities between Pluto and Haumea, but this study suggests that the satellite systems of the icy cousins, or former cousins, formed differently. “There is no self-consistent formation hypothesis for either set of satellites,” said Ragozzine.
Two things were at the heart of this new study. The first is the workhorse Hubble Space Telescope. In 2010, the Hubble focussed on Haumea, and captured 10 consecutive orbits to try to understand its family of satellites better.
The second thing at the heart of the study is called a “non-linear shift and stack method.” This is a novel technique which allows the detection of extremely faint and distant objects. When used in this study, it specifically ruled out the existence of small moons like the ones that orbit Pluto. This method may allow for future detection of other moons and Kuiper Belt Objects.
The study itself outlines some of Haumea’s properties that make it such an object of fascination for astronomers. It’s the fastest-rotating large body in the Solar System. In fact it rotates so quickly, that it’s near the rate at which the dwarf planet would break up. Haumea also has an unexpectedly high density, and a high albedo resulting from a surface of water ice. It’s two moons are in dynamically excited orbits, and its family of icy fragments is not near as dispersed as it should be. As the paper says, “There is no simple high-probability formation scenario that naturally explains all of these observational constraints.”
In the paper, the authors emphasize the puzzling nature of Haumea’s formation. To quote the paper, “Though multiple explanations and variations have been proposed, none have adequately and self-consistently explained all of the unique features of this interesting system and its family.”
Some of the explanations proposed in other studies include a collision between objects in the scattered disk, which overlaps the Kuiper Belt and extends much further, rather than objects in the Kuiper Belt itself. Another proposes that Haumea’s two largest moons–Hi’iaka and Namaka–are themselves second generation moons formed from the breakup of a progenitor moon.
Though the study shows that the Pluto system and the Haumea system, erstwhile cousins in the Solar System, have followed different pathways to formation, it also concludes that a collision was indeed the main event for both dwarf planets. But what happened after that collision, and where exactly those collisions took place, are still intriguing questions.
Planetary scientists using the Hubble Space Telescope have spotted a dark mini-moon orbiting the distant dwarf planet Makemake. The moon, nicknamed MK 2, is roughly 160 km (100 miles) wide and orbits about 20,000 km (13,000 miles) from Makemake. Makemake is 1,300 times brighter than its moon and is also much larger, at 1,400 km (870 miles) across, about 2/3rd the size of Pluto.
“Our discovery of the Makemakean moon means that every formally-designated Kuiper Belt dwarf planet has at least one moon!” said Alex Parker on Twitter. Parker, along with Mark Buie, both from the Southwest Research Institute, led the same team that found the small moons of Pluto in 2005, 2011, and 2012, and they used the same Hubble technique to find MK 2. NASA says Hubble’s Wide Field Camera 3 has the unique ability to see faint objects near bright ones, and together with its sharp resolution, allowed the scientists to pull the moon out from bright Makemake’s glare.
Previous searches for moons around Makemake came up empty, but Parker said their analysis shows the moon has a very dark surface and it is also in a nearly edge-on orbit, which made it very hard to find.
This moon might be able to provide more details about Makemake, such as its mass and density. For example, when Pluto’s moon Charon was discovered in 1978, astronomers were able to measure Charon’s orbit and then calculate the mass of Pluto, which showed Pluto’s mass was hundreds of times smaller than originally estimated.
“Makemake is in the class of rare Pluto-like objects, so finding a companion is important,” Parker said. “The discovery of this moon has given us an opportunity to study Makemake in far greater detail than we ever would have been able to without the companion.”
Parker also said the discovery of a moon for Makemake might solve a long-standing mystery about the dwarf planet. Thermal observations of Makemake by the Spitzer and Herschel space observatories seemed to show the bright world had some darker, warmer material on its surface, but other observations couldn’t confirm this.
Parker said perhaps the dark material isn’t on Makemake’s surface, but instead is in orbit. “I modeled the emission we expect from Makemake’s moon, and if the moon is very dark, it accounts for most previous thermal measurements,” he said on Twitter.
The researchers will need more Hubble observations to make accurate measurements to determine if the moon’s orbit is elliptical or circular, and this could help determine its origin. A tight circular orbit means that MK 2 probably formed from a collision between Makemake and another Kuiper Belt Object. If the moon is in a wide, elongated orbit, it is more likely to be a captured object from the Kuiper Belt. Many KBOs are covered with very dark material, so that might explain the dark surface of MK 2.
Every year, the NASA Innovative Advanced Concepts (NIAC) program puts out the call to the general public, hoping to find better or entirely new aerospace architectures, systems, or mission ideas. As part of the Space Technology Mission Directorate, this program has been in operation since 1998, serving as a high-level entry point to entrepreneurs, innovators and researchers who want to contribute to human space exploration.
This year, thirteen concepts were chosen for Phase I of the NIAC program, ranging from reprogrammed microorganisms for Mars, a two-dimensional spacecraft that could de-orbit space debris, an analog rover for extreme environments, a robot that turn asteroids into spacecraft, and a next-generation exoplanet hunter. These proposals were awarded $100,000 each for a nine month period to assess the feasibility of their concept.