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An artist's impression of a gravitational microlensing event by a free-floating planet.
Credit: J. Skowron/Warsaw University Observatory
There probably aren't nearly as many giant planets zooming alone through the Milky Way galaxy as scientists had thought, a new study reports.
Previous research had suggested that huge "rogue" or "unbound" worlds, which have no discernible host star, are extremely common in the Milky Way, perhaps outnumbering stars by a factor of 2 to 1. But that's probably not the case, according to the new study.
"We found that Jupiter-mass [rogue] planets are at least 10 times less frequent than previously thought," study lead author Przemek Mróz, a researcher at the Warsaw University Observatory in Poland, told Space.com via email. [Gallery: The Strangest Alien Planets]
Astronomers think most rogue planets were likely booted out of their native solar systems by interactions with neighboring planets. Scientists generally hunt for these lonely worlds using a technique called gravitational microlensing, which involves watching for a foreground object to pass in front of a distant star. When this happens, the closer body's gravity bends and magnifies the star's light, in ways that can reveal clues about the foreground object's mass and other characteristics.
A 2011 study, based on 474 microlensing events detected by a telescope in New Zealand, estimated that gas-giant rogue worlds are nearly twice as common as main-sequence ("normal") stars in the Milky Way. (The number of stars in our galaxy is a matter of debate, with estimates ranging from 100 billion to 1 trillion.)
In the new study, Mróz and his team analyzed a much bigger data set — more than 2,600 microlensing events that were detected between 2010 and 2015 by the Optical Gravitational Lensing Experiment (OGLE). This survey, which is run by researchers at the University of Warsaw, depends primarily on observations made at the Las Campanas Observatory in Chile.
The researchers determined that the Milky Way likely hosts a maximum of one Jupiter-like rogue for every four main-sequence stars — still a lot, but to be sure, but not nearly as many as the previous study had suggested.
The gravity of a free-floating “rogue” planet may deflect and focus light from a distant star when passing closely in front of it. Owing to the distorted image, the star temporarily seems much brighter.
Credit: J. Skowron/Warsaw University Observatory
The new results make sense on a number of levels, Mróz said.
"Our new microlensing observations are in agreement with theoretical expectations on the frequency of free-floating Jupiters and with infrared surveys for planetary-mass objects in star-forming regions," he said.
Intriguingly, the OGLE survey also spotted a few extremely brief microlensing events, which Mróz said were likely caused by much smaller worlds — ones about the size of Earth, or just a bit bigger.
"Because our sensitivity to such short events was very low, free-floating Earths should be very common, perhaps more frequent than stars, but we are unable to provide a precise number owing to [the] small number of detections," he told Space.com.
Increasing the number of ground-based microlensing detections would give astronomers a somewhat better understanding of the population of small rogue planets, Mróz said. But big gains may have to wait for future space observatories, such as Europe's Euclid mission and NASA's Wide-Field Infrared Survey Telescope (WFIRST).
"Thanks to the superb quality of photometry from space-based observatories and the possibility of continuous observations during approximately 100-day-long windows, future space-based missions, such as WFIRST and Euclid, will have the potential to explore the population of free-floating Earth-mass planets in more detail," Mróz and his colleagues wrote in the new study, which was published online today (July 24) in the journal Nature.
Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at Orion’s Nebula’s “little brother”, the De Marian’s Nebula!
During the 18th century, famed French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky. Having originally mistaken them for comets, he began compiling a list of them so that others would not make the same mistake he did. In time, this list (known as the Messier Catalog) would come to include 100 of the most fabulous objects in the night sky.
One of these is the spiral galaxy located in the constellation Canes Venatici known as the Whirlpool Galaxy (aka. Messier 51). Located between 19 and 27 million light-years from the Milky Way, this deep sky object was the very first to be classified as a spiral galaxy. It is also one of the best known galaxies among amateur astronomers, and is easily observable using binoculars and small telescopes.
Description:
Located some 37 million light years away, M51 is the largest member of a small group of galaxies, which also houses M63 and a number of fainter galaxies. To this time, the exact distance of this group isn’t properly known… Even when a 2005 supernova event should have helped astronomers to correctly calculate! As K. Takats stated in a study:
“The distance to the Whirlpool galaxy (M51, NGC 5194) is estimated using published photometry and spectroscopy of the Type II-P supernova SN 2005cs. Both the expanding photosphere method (EPM) and the standard candle method (SCM), suitable for SNe II-P, were applied. The average distance (7.1 +/- 1.2 Mpc) is in good agreement with earlier surface brightness fluctuation and planetary nebulae luminosity function based distances, but slightly longer than the distance obtained by Baron et al. for SN 1994I via the spectral fitting expanding atmosphere method. Since SN 2005cs exhibited low expansion velocity during the plateau phase, similarly to SN 1999br, the constants of SCM were recalibrated including the data of SN 2005cs as well. The new relation is better constrained in the low-velocity regime, that may result in better distance estimates for such SNe.”
Visible light (left) and infrared image (right) of M51, taken by the Kitt Peak National Observatory and NASA’s Spitzer Space Telescope, respectively. Credit: NASA/JPL-Caltech/R. Kennicutt (Univ. of Arizona)/DSS
Of course, one of the most outstanding features of the Whirlpool Galaxy is its beautiful spiral structure – perhaps result of the close interaction between it and its companion galaxy NGC 5195? As S. Beckwith,
“This sharpest-ever image of the Whirlpool Galaxy, taken in January 2005 with the Advanced Camera for Surveys aboard NASA’s Hubble Space Telescope, illustrates a spiral galaxy’s grand design, from its curving spiral arms, where young stars reside, to its yellowish central core, a home of older stars. At first glance, the compact galaxy appears to be tugging on the arm. Hubble’s clear view, however, shows that NGC 5195 is passing behind the Whirlpool. The small galaxy has been gliding past the Whirlpool for hundreds of millions of years. As NGC 5195 drifts by, its gravitational muscle pumps up waves within the Whirlpool’s pancake-shaped disk. The waves are like ripples in a pond generated when a rock is thrown in the water. When the waves pass through orbiting gas clouds within the disk, they squeeze the gaseous material along each arm’s inner edge. The dark dusty material looks like gathering storm clouds. These dense clouds collapse, creating a wake of star birth, as seen in the bright pink star-forming regions. The largest stars eventually sweep away the dusty cocoons with a torrent of radiation, hurricane-like stellar winds, and shock waves from supernova blasts. Bright blue star clusters emerge from the mayhem, illuminating the Whirlpool’s arms like city streetlights.”
But there were more surprises just waiting to be found – like a black hole, surrounded by a ring of dust. What makes it even more odd is a secondary ring crosses the primary ring on a different axis, a phenomenon that is contrary to expectations and a pair of ionization cones extend from the axis of the main dust ring. As H. Ford,
“This image of the core of the nearby spiral galaxy M51, taken with the Wide Field Planetary camera (in PC mode) on NASA’s Hubble Space Telescope, shows a striking , dark “X” silhouetted across the galaxy’s nucleus. The “X” is due to absorption by dust and marks the exact position of a black hole which may have a mass equivalent to one-million stars like the sun. The darkest bar may be an edge-on dust ring which is 100 light-years in diameter. The edge-on torus not only hides the black hole and accretion disk from being viewed directly from earth, but also determines the axis of a jet of high-speed plasma and confines radiation from the accretion disk to a pair of oppositely directed cones of light, which ionize gas caught in their beam. The second bar of the “X” could be a second disk seen edge on, or possibly rotating gas and dust in MS1 intersecting with the jets and ionization cones.”
History of Observation:
The Whirlpool Galaxy was first discovered by Charles Messier on October 13th, 1773 and re-observed again for his records on January 11th, 1774. As he wrote of his discovery in his notes:
“Very faint nebula, without stars, near the eye of the Northern Greyhound [hunting dog], below the star Eta of 2nd magnitude of the tail of Ursa Major: M. Messier discovered this nebula on October 13, 1773, while he was watching the comet visible at that time. One cannot see this nebula without difficulties with an ordinary telescope of 3.5 foot: Near it is a star of 8th magnitude. M. Messier reported its position on the Chart of the Comet observed in 1773 & 1774. It is double, each has a bright center, which are separated 4’35”. The two “atmospheres” touch each other, the one is even fainter than the other.”
It would be his faithful friend and assistant, Pierre Mechain who would discover NGC 5195 on March 21st, 1781. Even though it would be many, many years before it was proven that galaxies were indeed independent systems, historic astronomers were much, much sharper than we gave them credit for. Sir William Herschel would observe M51 many times, but it would be his son John who would be the very first to comment on M51’s scheme:
“This very singular object is thus described by Messier: – “Nebuleuse sans etoiles.” “On ne peut la voir que difficilement avec une lunette ordinaire de 3 1/2 pieds.” “Elle est double, ayant chacune un centre brillant eloigne l’un de l’autre de 4′ 35″. Les deux atmospheres se touchent.” By this description it is evident that the peculiar phenomena of the nebulous ring which encircles the central nucleus had escaped his observation, as might have been expected from the inferior light of his telescopes. My Father describes it in his observations of Messier’s nebulae as a bright round nebula, surrounded by a halo or glory at a distance from it, and accompanied by a companion; but I do not find that the partial subdivision of the ring into two branches throughout its south following limb was noticed by him. This is, however, one of its most remarkable and interesting features. Supposing it to consist of stars, the appearance it would present to a spectator placed on a planet attendant on one of them eccentrically situated towards the north preceding quarter of the central mass, would be exactly similar to that of our Milky Way, traversing in a manner precisely analogous the firmament of large stars, into which the central cluster would be seen projected, and (owing to its distance) appearing, like it, to consist of stars much smaller than those in other parts of the heavens. Can it, then, be that we have here a brother-system bearing a real physical resemblance and strong analogy of structure to our own? Were it not for the subdivision of the ring, the most obvious analogy would be that of the system of Saturn, and the idea of Laplace respecting the formation of that system would be powerfully recalled by this object. But it is evident that all idea of symmetry caused by rotation on an axis must be relinquished, when we consider that the elliptic form of the inner subdivided portion indicates with extreme probability an elevation of that portion above the plane of the rest, so that the real form must be that of a ring split through half its circumference, and having the split portions set asunder at an angle of about 45 deg each to the plane of the other.”
Sketch of M51 by William Parsons, 3rd Earl of Rosse (Lord Rosse) in 1845. Credit: Public Domain
As with other Messier Objects, Admiral Smyth also had some insightful and poetic observations to add. As he wrote of this galaxy in September of 1836:
“We have then an object presenting an amazing display of the uncontrollable energies of the Omnipotence, the contemplation of which compels reason and admiration to yield to awe. On the outermost verge of telescopic reach we perceive a stellar universe similar to that to which we belong, whose vast amplitudes no doubt are peopled with countless numbers of percipient beings; for those beautiful orbs cannot be considered as mere masses of inert matter.
And it is interesting to know that, if there be intelligent existence, an astronomer gazing at our distant universe, will see it, with a good telescope, precisely under the lateral aspect which theirs presents to us. But after all what do we see? Both that wonderful universe, our own, and all which optical assistance has revealed to us, may be only the outliers of a cluster immensely more numerous.
The millions of suns we perceive cannot comprise the Creator’s Universe. There are no bounds to infinitude; and the boldest views of the elder Herschel only placed us as commanding a ken whose radius is some 35,000 times longer than the distance of Sirius from us. Well might the dying Laplace explain: “That which we know is little; that which we know not is immense.”
Lord Rosse would continue on in 1844 with his 6-feet (72-inch) aperture, 53-ft FL “Leviathan” telescope, but he was a man of fewer words.
“The greater part of the observations were made when the eye was affected by lamp-light, which made it difficult to estimate correctly the centre of the nucleus; it was of importance that no time should be unnecessarily spent, and after the lamp had been used a new measure was taken, as it was judged that the object was sufficiently seen. With the brighter stars this would frequently happen before the nucleus was well defined, as all impediments to vision seem to affect nebulae much more than stars the light of which would be estimated as of the same intensity. In the foregoing list the greatest discrepancies are in the measures of bright objects, and this is probably the proper account of it. No stars have been inserted in the sketch which are not in the table of the measurements. The general appearance of the object would have been better given if the minute stars had been put in from the eye-sketch, but it would have created confusion.”
May the stars from this distant island universe fill your eyes!
The Whirlpool Galaxy (Spiral Galaxy M51, NGC 5194), a classic spiral galaxy located in the Canes Venatici constellation, and its companion NGC 5195. Credit: NASA/ESA
Locating Messier 51:
Locating M51 isn’t too hard if you have dark skies, but this particular galaxy is very difficult where light pollution of moonlight is present. To find it, start with Eta UM, the star at the handle of the Big Dipper. In the finderscope or binoculars, you’ll clearly see 24 UM to the southwest. Now, center your optics there and move slowly southwest towards Cor Caroli (Alpha CVn) and you’ll find it!
In locations where skies are clear and dark, it is easy to see spiral structure in even small telescopes, or to make out the galaxy in binoculars – but even a change in sky conditions can hide it from a good location. Rich field telescopes with fast focal lengths to an outstanding job on this galaxy and companion and you may be able to make out the nucleus of both galaxies on a good night from even a bad location.
Object Name: Messier 51 Alternative Designations: M51, NGC 5194, The Whirlpool Galaxy Object Type: Type Sc Galaxy Constellation: Canes Venatici Right Ascension: 13 : 29.9 (h:m) Declination: +47 : 12 (deg:m) Distance: 37000 (kly) Visual Brightness: 8.4 (mag) Apparent Dimension: 11×7 (arc min)
With the dawning of the Space Age in the 1950s, human beings were no longer confined to studying the Solar planets and other astronomical bodies with Earth-based instruments alone. Instead crewed missions have gone into orbit and to the Moon while robotic missions have traveled to every corner of the Solar System. And in the process, we have learned some interesting things about the planets, planetoids, and asteroids in our Solar neighborhood.
For example, we have learned that all the Solar planets have their own particular patterns and cycles. For instance, even though Mercury is an airless body, it does have a tenuous exosphere and experiences seasons of a sort. And while it is known for being extremely hot, it also experiences extremes of cold, to the point that ice can exist on its surface. While it is by no means what we are used to here on Earth, Mercury still experiences a kind of “weather”.
Mercury’s Atmosphere:
As noted, Mercury has no atmosphere to speak of, owing to its small size and extremes in temperature. However, it does have a tenuous and variable exosphere that is made up of hydrogen, helium, oxygen, sodium, calcium, potassium and water vapor, with a combined pressure level of about 10-14 bar (one-quadrillionth of Earth’s atmospheric pressure).
The Fast Imaging Plasma Spectrometer on board MESSENGER has found that the solar wind is able to bear down on Mercury enough to blast particles from its surface into its wispy atmosphere. Shannon Kohlitz, Media Academica, LLC
It is believed this exosphere was formed from particles captured from the Sun (i.e solar wind) as well as volcanic outgassing and debris kicked into orbit by micrometeorite impacts. In any case, Mercury’s lack of a viable atmosphere is the reason why it is unable to retain heat from the Sun, which leads to extreme variations between night and day for the rocky planet.
Orbital Resonance:
Mercury’s temperature variations are also attributed to its orbital eccentricity of 0.2056, which is the most extreme of any planet in the Solar System. Essentially, its distance from the Sun ranges from 46 million km (29 million mi) at its closest (perihelion) to 70 million km (43 million mi) at its farthest (aphelion). As a result, the side facing the Sun reaches temperatures of up to 700 K (427° C), the side in shadow dips down to 100 K (-173° C).
With an average rotational speed of 10.892 km/h (6.768 mph), Mercury also takes 58.646 days to complete a single rotation. This means that Mercury has a spin-orbit resonance of 3:2, where it completes three rotations on its axis for every two rotations completed around the Sun. This does not, however, mean that three days last the same as two years on Mercury.
The Orbit of Mercury during the year 2006. Credit: Wikipedia Commons/Eurocommuter
In fact, its high eccentricity and slow rotation mean that it takes 176 Earth days for the Sun to return to the same place in the sky (aka. a solar day). In short, a single day on Mercury is twice as long as a single year! Mercury also has the lowest axial tilt of any planet in the Solar System – approximately 0.027 degrees compared to Jupiter’s 3.1 degrees (the second smallest).
Polar Ice:
This low tilt means that the polar regions are constantly in shadow, which leads to another interesting feature about Mercury. Yes, despite how hot its Sun-facing side can become, the existence of water ice and even organic molecules have been confirmed on Mercury’s surface. But this only true at the poles, where the floors of deep craters are never exposed to direct sunlight, and temperatures within them therefore remain below the planetary average.
These icy regions are believed to contain about 1014–1015 kg (1 to 10 billion metric tons, 1.1 to 11 billion US tons) of frozen water, and may be covered by a layer of regolith that inhibits sublimation. The origin of the ice on Mercury is not yet known, but the two most likely sources are from outgassing of water from the planet’s interior or deposition by the impacts of comets.
The Big Bear Solar Observatory Captures a high-res image of this week’s transit of Mercury across the face of the Sun. Image credit: NJIT/BBSO
When one talks about the “weather” on Mercury, they are generally confined to talking about variations between the Sun-facing side and the night side. Over the course of two years, that weather will remain scorching hot or freezing cold. In that respect, we could say that a single season on Mercury lasts a full four years, and includes a “Midnight Sun” that lasts two years, and a “Polar Night” that lasts the same.
Between its rapid and very eccentric orbit, its slow rotation, and its strange diurnal and annual patterns, Mercury is a very extreme planet with a very extreme environment. It only makes sense that its weather would be similarly extreme. Hey, there’s a reason nobody lives there, at least not yet…
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
Explanation: Mercury had never been seen like this before. In 2008, the robotic MESSENGER spacecraft buzzed past Mercury for the second time and imaged terrain mapped previously only by comparatively crude radar. The featured image was recorded as MESSENGER looked back 90 minutes after passing, from an altitude of about 27,000 kilometers. Visible in the image, among many other newly imaged features, are unusually long rays that appear to run like meridians of longitude out from a young crater near the northern limb. MESSENGER entered orbit around Mercury in 2011 and finished its primary mission in 2012, but took detailed measurements until 2015, at which time it ran out of fuel and so was instructed to impact Mercury's surface.
Planets and other objects in our Solar System. Credit: NASA.
Remembering the order of the planets can be a tricky task. With eight celestial bodies, and all the names taken from classical nomenclature, getting them mixed up is a common mistake. First the quick facts: Our Solar System has eight “official” planets which orbit the Sun. Here are the planets listed in order of their distance from the Sun:
If you add in the dwarf planets, Ceres is located in the asteroid belt between Mars and Jupiter, while the remaining dwarf planets are in the outer Solar System and in order from the Sun are Pluto, Haumea, Makemake, and Eris. There is, as yet, a bit of indecision about the Trans-Neptunian Objects known as Orcus, Quaoar, 2007 O10, and Sedna and their inclusion in the dwarf planet category.
A mnemonic for this list would be “My Very Educated Mother Could Just Serve Us Noodles, Pie, Ham, Muffins, and Eggs” (and Steak, if Sedna is included.) Now, let’s look at a few details including the definition of a planet and a dwarf planet, as well as details about each of the planets in our Solar System.
Artistic impression of the Solar System, with all known terrestrial planets, as giants, and dwarf planets. Credit: NASA
What is a planet? In 2006, the International Astronomical Union (IAU) decided on the definition of a planet. The definition states that in our Solar System, a planet is a celestial body which:
is in orbit around the Sun,
has sufficient mass to assume hydrostatic equilibrium (a nearly round shape),
has “cleared the neighborhood” around its orbit.
is not a moon.
This means that Pluto, which was considered to be the farthest planet since its discovery in 1930, now is classified as a dwarf planet. The change in the definition came after the discovery three bodies that were all similar to Pluto in terms of size and orbit, (Quaoar in 2002, Sedna in 2003, and Eris in 2005).
With advances in equipment and techniques, astronomers knew that more objects like Pluto would very likely be discovered, and so the number of planets in our Solar System would start growing quickly. It soon became clear that either they all had to be called planets or Pluto and bodies like it would have to be reclassified.
With much controversy then and since, Pluto was reclassified as a dwarf planet in 2006. This also reclassified the asteroid Ceres as a dwarf planet, too, and so the first five recognized dwarf planets are Ceres, Pluto, Eris, Makemake and Haumea. Scientists believe there may be dozens more dwarf planets awaiting discovery.
Later, in 2008, the IAU announced the subcategory of dwarf planets with trans-Neptunian orbits would be known as “plutoids.” Said the IAU, “Plutoids are celestial bodies in orbit around the Sun at a distance greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighborhood around their orbit.”
This subcategory includes Ceres, Pluto, Haumea, Makemake, and Eris.
The Planets in our Solar System: Having covered the basics of definition and classification, let’s get talking about those celestial bodies in our Solar System that are still classified as planets (sorry Pluto!). Here is a brief look at the eight planets in our Solar System. Included are quick facts and links so you can find out more about each planet.
Mercury:Mercury is the closest planet to our Sun, at just 58 million km (36 million miles) or 0.39 Astronomical Unit (AU) out. But despite its reputation for being sun-baked and molten, it is not the hottest planet in our Solar System (scroll down to find out who that dubious honor goes go!)
Mercury, as imaged by the MESSENGER spacecraft, revealing parts of the never seen by human eyes. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Mercury is also the smallest planet in our Solar System, and is also smaller than its largest moon (Ganymede, which orbits Jupiter). And being equivalent in size to 0.38 Earths, it is just slightly larger than the Earth’s own Moon. But this may have something to do with its incredible density, being composed primarily of rock and iron ore. Here are the planetary facts:
Diameter: 4,879 km (3,032 miles)
Mass: 3.3011 x 1023 kg (0.055 Earths)
Length of Year (Orbit): 87.97 Earth days
Length of Day: 59 Earth days.
Mercury is a rocky planet, one of the four “terrestrial planets” in our Solar System. Mercury has a solid, cratered surface, and looks much like Earth’s moon.
If you weigh 45 kg (100 pounds) on Earth, you would weigh 17 kg (38 pounds) on Mercury.
Mercury does not have any moons.
Temperatures on Mercury range between -173 to 427 degrees Celcius (-279 to 801 degrees Fahrenheit)
Just two spacecraft have visited Mercury: Mariner 10 in 1974-75 and MESSENGER, which flew past Mercury three times before going into orbit around Mercury in 2011 and ended its mission by impacting the surface of Mercury on April 30, 2015. MESSENGER has changed our understanding of this planet, and scientists are still studying the data.
Venus: Venus is the second closest planet to our Sun, orbiting at an average distance of 108 million km (67 million miles) or 0.72 AU. Venus is often called Earth’s “sister planet,” as it is just a little smaller than Earth. Venus is 81.5% as massive as Earth, and has 90% of its surface area and 86.6% of its volume. The surface gravity, which is 8.87 m/s², is equivalent to 0.904 g – roughly 90% of the Earth standard.
A radar view of Venus taken by the Magellan spacecraft, with some gaps filled in by the Pioneer Venus orbiter. Credit: NASA/JPL
And due to its thick atmosphere and proximity to the Sun, it is the Solar Systems hottest planet, with temperatures reaching up to a scorching 735 K (462 °C). To put that in perspective, that’s over four and a half times the amount of heat needed to evaporate water, and about twice as much needed to turn tin into molten metal (231.9 °C)!
Diameter: 7,521 miles (12,104 km)
Mass: 4.867 x 1024 kg (0.815 Earth mass)
Length of Year (Orbit): 225 days
Length of day: 243 Earth days
Surface temperature: 462 degrees C (864 degrees F)
Venus’ thick and toxic atmosphere is made up mostly of carbon dioxide (CO2) and nitrogen (N2), with clouds of sulfuric acid (H2SO4) droplets.
Venus has no moons.
Venus spins backwards (retrograde rotation), compared to the other planets. This means that the sun rises in the west and sets in the east on Venus.
If you weigh 45 kg (100 pounds) on Earth, you would weigh 41 kg (91 pounds) on Venus.
Venus is also known and the “morning star” or “evening star” because it is often brighter than any other object in the sky and is usually seen either at dawn or at dusk. Since it is so bright, it has often been mistaken for a UFO!
More than 40 spacecraft have explored Venus. The Magellan mission in the early 1990s mapped 98 percent of the planet’s surface. Find out more about all the missions here.
Earth:Our home, and the only planet in our Solar System (that we know of) that actively supports life. Our planet is the third from the our Sun, orbiting it at an average distance of 150 million km (93 million miles) from the Sun, or one AU. Given the fact that Earth is where we originated, and has all the necessary prerequisites for supporting life, it should come as no surprise that it is the metric on which all others planets are judged.
Earth, pictured by the crew of the Apollo 17 mission. Credit: NASA
Whether it is gravity (g), distance (measured in AUs), diameter, mass, density or volume, the units are either expressed in terms of Earth’s own values (with Earth having a value of 1) or in terms of equivalencies – i.e. 0.89 times the size of Earth. Here’s a rundown of the kinds of
Diameter: 12,760 km (7,926 miles)
Mass: 5.97 x 1024 kg
Length of Year (Orbit): 365 days
Length of day: 24 hours (more precisely, 23 hours, 56 minutes and 4 seconds.)
Surface temperature: Average is about 14 C, (57 F), with ranges from -88 to 58 (min/max) C (-126 to 136 F).
Earth is another terrestrial planet with an ever-changing surface, and 70 percent of the Earth’s surface is covered in oceans.
Earth has one moon.
Earth’s atmosphere is 78% nitrogen, 21% oxygen, and 1% various other gases.
Mars:Mars is the fourth planet from the sun at a distance of about 228 million km (142 million miles) or 1.52 AU. It is also known as “the Red Planet” because of its reddish hue, which is due to the prevalence of iron oxide on its surface. In many ways, Mars is similar to Earth, which can be seen from its similar rotational period and tilt, which in turn produce seasonal cycles that are comparable to our own.
Global image of the planet Mars. Credit: NASA
The same holds true for surface features. Like Earth, Mars has many familiar surface features, which include volcanoes, valleys, deserts, and polar ice caps. But beyond these, Mars and Earth have little in common. The Martian atmosphere is too thin and the planet too far from our Sun to sustain warm temperatures, which average 210 K (-63 ºC) and fluctuate considerably.
Diameter: 6,787 km, (4,217 miles)
Mass: 6.4171 x 1023 kg (0.107 Earths)
Length of Year (Orbit): 687 Earth days.
Length of day: 24 hours 37 minutes.
Surface temperature: Average is about -55 C (-67 F), with ranges of -153 to +20 °C (-225 to +70 °F)
Mars is the fourth terrestrial planet in our Solar System. Its rocky surface has been altered by volcanoes, impacts, and atmospheric effects such as dust storms.
Mars has a thin atmosphere made up mostly of carbon dioxide (CO2), nitrogen (N2) and argon (Ar).If you weigh 45 kg (100 pounds) on Earth, you would weigh 17 kg (38 pounds) on Mars.
Mars has two small moons, Phobos and Deimos.
Mars is known as the Red Planet because iron minerals in the Martian soil oxidize, or rust, causing the soil to look red.
Jupiter:Jupiter is the fifth planet from the Sun, at a distance of about 778 million km (484 million miles) or 5.2 AU. Jupiter is also the most massive planet in our Solar System, being 317 times the mass of Earth, and two and half times larger than all the other planets combined. It is a gas giant, meaning that it is primarily composed of hydrogen and helium, with swirling clouds and other trace gases.
Io and Jupiter as seen by New Horizons during its 2008 flyby. (Credit: NASA/Johns Hopkins University APL/SWRI).
Jupiter’s atmosphere is the most intense in the Solar System. In fact, the combination of incredibly high pressure and coriolis forces produces the most violent storms ever witnessed. Wind speeds of 100 m/s (360 km/h) are common and can reach as high as 620 km/h (385 mph). In addition, Jupiter experiences auroras that are both more intense than Earth’s, and which never stop.
Diameter: 428,400 km (88,730 miles)
Mass: 1.8986 × 1027 kg (317.8 Earths)
Length of Year (Orbit): 11.9 Earth years
Length of day: 9.8 Earth hours
Temperature: -148 C, (-234 F)
Jupiter has 67 known moons, with an additional 17 moons awaiting confirmation of their discovery – for a total of 67 moons. Jupiter is almost like a mini solar system!
Jupiter has a faint ring system, discovered in 1979 by the Voyager 1 mission.
If you weigh 45 kg (100 pounds) on Earth, you would weigh 115 kg (253) pounds on Jupiter.
Jupiter’s Great Red Spot is a gigantic storm (bigger than Earth) that has been raging for hundreds of years. However, it appears to be shrinking in recent years.
Many missions have visited Jupiter and its system of moons, with the latest being the Juno mission will arrive at Jupiter in 2016. You can find out more about missions to Jupiter here.
Saturn’s relatively thin main rings are about 250,000 km (156,000 miles) in diameter. (Image: NASA/JPL-Caltech/SSI/J. Major)
Saturn:Saturn is the sixth planet from the Sun at a distance of about 1.4 billion km (886 million miles) or 9.5 AU. Like Jupiter, it is a gas giant, with layers of gaseous material surrounding a solid core. Saturn is most famous and most easily recognized for its spectacular ring system, which is made of seven rings with several gaps and divisions between them.
Diameter: 120,500 km (74,900 miles)
Mass: 5.6836 x 1026 kg (95.159 Earths)
Length of Year (Orbit): 29.5 Earth years
Length of day: 10.7 Earth hours
Temperature: -178 C (-288 F)
Saturn’s atmosphere is made up mostly of hydrogen (H2) and helium (He).
If you weigh 45 kg (100 pounds) on Earth, you would weigh about 48 kg (107 pounds) on Saturn
Saturn has 53 known moons with an additional 9 moons awaiting confirmation.
Five missions have gone to Saturn. Since 2004, Cassini has been exploring Saturn, its moons and rings. You can out more about missions to Saturn here.
Uranus:Uranus is the seventh planet from the sun at a distance of about 2.9 billion km (1.8 billion miles) or 19.19 AU. Though it is classified as a “gas giant”, it is often referred to as an “ice giant” as well, owing to the presence of ammonia, methane, water and hydrocarbons in ice form. The presence of methane ice is also what gives it its bluish appearance.
Uranus as seen by NASA’s Voyager 2 space probe. Credit: NASA/JPL
Uranus is also the coldest planet in our Solar System, making the term “ice” seem very appropriate! What’s more, its system of moons experience a very odd seasonal cycle, owing to the fact that they orbit Neptune’s equator, and Neptune orbits with its north pole facing directly towards the Sun. This causes all of its moons to experience 42 year periods of day and night.
Diameter: 51,120 km (31,763 miles)
Mass:
Length of Year (Orbit): 84 Earth years
Length of day: 18 Earth hours
Temperature: -216 C (-357 F)
Most of the planet’s mass is made up of a hot dense fluid of “icy” materials – water (H2O), methane (CH4). and ammonia (NH3) – above a small rocky core.
Uranus has an atmosphere which is mostly made up of hydrogen (H2) and helium (He), with a small amount of methane (CH4). The methane gives Uranus a blue-green tint.
If you weigh 45 kg (100 pounds) on Earth, you would weigh 41 kg (91 pounds) on Uranus.
Uranus has 27 moons.
Uranus has faint rings; the inner rings are narrow and dark and the outer rings are brightly colored.
Voyager 2 is the only spacecraft to have visited Uranus. Find out more about this mission here.
Neptune: Neptune is the eighth and farthest planet from the Sun, at a distance of about 4.5 billion km (2.8 billion miles) or 30.07 AU. Like Jupiter, Saturn and Uranus, it is technically a gas giant, though it is more properly classified as an “ice giant” with Uranus.
Neptune photographed by the Voyager 2 space probe. Credit: NASA/JPL
Due to its extreme distance from our Sun, Neptune cannot be seen with the naked eye, and only one mission has ever flown close enough to get detailed images of it. Nevertheless, what we know about it indicates that it is similar in many respects to Uranus, consisting of gases, ices, methane ice (which gives its color), and has a series of moons and faint rings.
Diameter: 49,530 km (30,775 miles)
Mass: 1.0243 x 1026 kg (17 Earths)
Length of Year (Orbit): 165 Earth years
Length of day: 16 Earth hours
Temperature: -214 C (-353 F)
Neptune is mostly made of a very thick, very hot combination of water (H2O), ammonia (NH3), and methane (CH4) over a possible heavier, approximately Earth-sized, solid core.
Neptune’s atmosphere is made up mostly of hydrogen (H2), helium (He) and methane (CH4).
Neptune has 13 confirmed moons and 1 more awaiting official confirmation.
Neptune has six rings.
If you weigh 45 kg (100 pounds) on Earth, you would weigh 52 kg (114 pounds) on Neptune. Neptune was the first planet to be predicted to exist by using math.
Voyager 2 is the only spacecraft to have visited Neptune. You can find out more about this mission here.
Find out more about Neptune at this series of articles on Universe Today and this NASA webpage. We have written many articles about the planets for Universe Today. Here are some facts about planets, and here’s an article about the names of the planets.If you’d like more info on the Solar System planets, dwarf planets, asteroids and more, check out NASA’s Solar System exploration page, and here’s a link to NASA’s Solar System Simulator.We’ve also recorded a series of episodes of Astronomy Cast about every planet in the Solar System. Start here, Episode 49: Mercury.Venus is the second planet from the Sun, and it is the hottest planet in the Solar System due to its thick, toxic atmosphere which has been described as having a “runaway greenhouse effect” on the planet.
Now you know! And if you find yourself unable to remember all the planets in their proper order, just repeat the words, “My Very Educated Mother Just Served Us Noodles.” Of course, the Pie, Ham, Muffins and Eggs are optional, as are any additional courses that might be added in the coming years!
Matt Williams is the Curator of the Guide to Space for Universe Today, a a regular contributor to HeroX, a science fiction author, and a Taekwon-Do instructor. He lives with his family on Vancouver Island in beautiful BC.
A March 2013 picture of the Shapley Supercluster from the European Space Agency’s Planck observatory. ESA describes it as “the largest cosmic structure in the local Universe.” Credit: ESA & Planck Collaboration / Rosat/ Digitised Sky Survey
Comet 2P/Encke as imaged by Damian Peach on October 12th. Taken with a 20″ CDK telescope and a FLI Proline PL11002 Colr CCD camera; LRGB: L: 6×2 minutes, RGB exposure: 1×2 minutes. (Credit: D. Peach)
Galaxy z8_GND_5296 (seen in the inset) is the earliest galaxy that astronomers have measured the distance to accurately. It formed approximately 700 million years after the Big Bang, and is forming stars at an incredibly rapid rate. [Credit: V. Tilvi (Texas A&M), S. Finkelstein (UT Austin), the CANDELS team, and HST/NASA]
![Galaxy z8_GND_5296 (seen in the inset) is the earliest galaxy that astronomers have measured the distance to accurately. It formed approximately 700 million years after the Big Bang, and is forming stars at an incredibly rapid rate. [Credit: V. Tilvi (Texas A&M), S. Finkelstein (UT Austin), the CANDELS team, and HST/NASA]](https://lh3.googleusercontent.com/blogger_img_proxy/AEn0k_toI4iY5jLcxbFrVg3iTwPBwbTwm5pD-CACt3OpA59NPxOH_1eFHRUjPPlHR3QIlnkn9zMBfcy3dO_CBASssjYynlycfgO7fEWq7xVfua97F6JIP5IETw-jRg1Xg7c0ZAEnzZ816Pd8QaqxwYVT5DcGyy5l=s0-d "Taking Measure: A ‘New’ Most Distant Galaxy")
