Drake Equation

What is the Drake Equation?[1]

The Drake Equation, a mathematical formula for the probability of finding life or advanced civilizations in the universe. Credit: University of Rochester

Is there life out there in the Universe? That is a question that has plagued humanity long before we knew just how vast the Universe was–i.e. before the advent of modern astronomy. Within the 20th century–thanks to the development of modern telescopes, radio astronomy, and space observatories–multiple efforts have been made in the hopes of finding extra-terrestrial intelligence (ETI).

And yet, humanity is still only aware of one intelligent civilization in the Universe–our own. And until we actually discover an alien civilization, the best we can do is conjecture about the likelihood of their existence. That’s where the famous Drake Equation–named after astronomer Dr. Frank Drake–comes into play. Developed in the 1960s, this equation estimates the number of possible civilizations out there based on a number of factors.

Background:

During the 1950s, the concept of using radio astronomy to search for signals that were extra-terrestrial in origin was becoming widely-accepted within the scientific community. The idea of listening for extra-terrestrial radio communications had been suggested as far back as the late 19th century (by Nikolai Tesla), but these efforts were concerned with looking for signs of life on Mars.

Then, in September of 1959, Giuseppe Cocconi and Philip Morrison (who were both physics professors at Cornell University at the time) published an article in the journal Nature with the title “Searching for Interstellar Communications.” In it, they argued that radio telescopes had become sensitive enough that they could pick up transmissions being broadcast from other star systems. Specifically, they argued that these messages might be transmitted at a wavelength of 21 cm (1420.4 MHz), the same wavelength of radio emissions by neutral hydrogen. As the most common element in the universe, they argued that extra-terrestrial civilizations would see this as a logical frequency at which to make radio broadcasts that could be picked up by other civilizations.

Seven months later, Frank Drake made the first systematic SETI survey at the National Radio Astrono-my Observatory in Green Bank, West Virginia. Known as Project Ozma, this survey relied on the obser-vatory’s 25-meter dish to monitor Epsilon Eridani and Tau Ceti–two nearby Sun-like stars–at frequen-cies close to 21 cm for six hours a day, between April and July of 1960.

Though unsuccessful, the survey piqued the interest of the scientific and SETI communities. It was followed shortly thereafter by a meeting at the Green Bank facility in 1961, where the subjects of SETI and searching for radio signals of extra-terrestrial origin were discussed. In preparation for this meeting, Drake prepared the equation that would come to bear his name. As he said of the equation’s creation:

“As I planned the meeting, I realized a few day[s] ahead of time we needed an agenda. And so I wrote down all the things you needed to know to predict how hard it’s going to be to detect extraterrestrial life. And looking at them it became pretty evident that if you multiplied all these together, you got a number, N, which is the number of detectable civilizations in our galaxy. This was aimed at the radio search, and not to search for primordial or primitive life forms.”

The meeting, which included such luminaries as Carl Sagan, was commemorated with a commemorative plaque that is still in the hall of the Green Bank Observatory today.

The Formula:

The formula for the Drake Equation is as follows:

N = R* x fp x ne x fl x fi x fc x L

Whereas N is the number of civilizations in our galaxy that we might able to communicate with, R* is the average rate of star formation in our galaxy, fp is the fraction of those stars which have planets, ne is the number of planets that can actually support life, fl is the number of planets that will develop life, fi is the number of planets that develop intelligent life, fc is the number civilizations that would develop transmission technologies, and L is the length of time that these civilizations would have to transmit their signals into space.

Limits and Criticism:

Naturally, the Drake Equation has been subject to some criticism over the years, largely because a lot of the values it contains are assumed. Granted, some of the values it takes into account are easy enough to calculate, like the rate of star formation in the Milky Way. There are an estimated 200–400 billion stars within our Milky Way, and modern estimates say that there between 1.65 ± 0.19 and 3 new stars form every year.

Assuming that our galaxy represents the average, and given that that there are as many as 2 trillion galaxies in the observable Universe (current estimates based on Hubble data), that means that there are as many as 1.5 to 6 trillion new stars being added to the Universe with every passing year! However, some of the other values are subject to a great deal of guess work.

For example, estimates on how many stars will have a system of planets has changed over time. Currently, it is estimated that the Milky Way contains 100 billion planets, which works out to about 50% of its stars having a planet of their own. Furthermore, those stars that have multiple planets will likely have one or two that lies within their habitable zone (aka. “Goldilocks Zone”)–where liquid water can exist on their surfaces.

Now let’s assume that 100% of planets located within a habitable zone will be able develop life in some form, that at least 1% of those life-supporting planets will be able to give rise to intelligent species, that 1% of these will be able to communicate, and that they will able to do so for a period of about 10,000 years. If we run those numbers through the Drake Equation, we end up with a value of 10.

In other words, there are possibly 10 civilizations in the Milky Way at any time capable of sending out signals that we could detect. But of course, the values used for four parameters there–fl, fi, fc and L–were entirely assumed. Without any real data to go by, there’s no real way to know how many alien civilizations could really be out there. There could just be 1 in the entire Universe (us), or millions in every galaxy!

The Fermi Paradox:

Beyond the issue of assumed values, the most pointed criticisms of the Drake Equation tend to emphasize the argument put forth by physicist Enrico Fermi, known as the Fermi Paradox. This argument arose in 1950 as a result of conversation between Fermi and some colleagues while he was working at the Los Alamos National Laboratory. When the subject of UFOs and ETI came up, Fermi famously asked, “Where is everybody?”

This simple question summarized the conflict that existed between arguments that emphasized scale and the high probability of life emerging in the Universe with the complete lack of evidence that any such life exists. While Fermi was not the first scientists to ask the question, his name came to be associated with it due to his many writings on the subject.

In short, the Fermi Paradox states that, given the sheer number of stars in the Universe (many of which are billions of years older than our own), the high-probability that even a small fraction would have planets capable of giving rise to intelligent species, the likelihood that some of them would develop interstellar travel, and the time it would take to travel from one side of our galaxy to other (even allowing for sub-luminous speeds), humanity should have found some evidence of intelligent civilizations by now.

Naturally, this has given rise to many hypotheses as to how advanced civilizations could exist within our Universe but remain undetected. They include the possibility that intelligent life is extremely rare, that humanity is an early arrival to the Universe, that they do not exist (aka. the Hart-Tipler Conjecture), that they are in a state of slumber, or that we are simply looking in the wrong places.

The “Great Filter” Hypothesis:

But perhaps the best-known explanation for why no signs of intelligence life have been found yet is the “Great Filter” hypothesis. This states that since that no extraterrestrial civilizations have been so far, despite the vast number of stars, then some step in the process–between life emerging and becomes technologically advanced–must be acting as a filter to reduce the final value.

According to this view, either it is very hard for intelligent life to arise, the lifetime of such civilizations is short, or the time they have to reveal their existence is short. Here too, various explanations have been offered to explain what the form the filter could take, which include Extinction Level Events (ELEs), the inability of life to create a stable environment in time, environmental destruction. and/or technology running amok (some of which we fear might happen to us!)

Alas, the Drake Equation has endured for decades for the very same reason that if often comes under fire. Until such time that humanity can find evidence of intelligent life in the Universe, or has ruled out the possibility based on countless surveys that actually inspect other star systems up close, we won’t be able to answer the question, “Where is everybody?”

As with many other cosmological mysteries, we’ll be forced to guess about what we don’t know based on what we do (or think we do). As astronomers study stars and planets with newer instruments, they might eventually be able to work out just how accurate the Drake Equation really is. And if our recent cosmological and exoplanet-hunting efforts have shown us anything, it is that we are just beginning to scratch the surface of the Universe at large!

In the coming years and decades, our efforts to learn more about extra-solar planets will expand to in clude research of their atmospheres–which will rely on next-generation instruments like the James Webb Space Telescope and the European Extremely-Large Telescope array. These will go a long way towards refining our estimates on how common potentially habitable worlds are.

In the meantime, all we can do is look, listen, wait and see.

There are some great resources out there on the Internet. Check out this Drake Equation calculator.

[1] Matt Williams, “What is the Drake Equation?” Universe Today—Space and astronomy news (June 13, 2017)

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Meaning of Life in the Universe

Meaning of Life in the Universe[1]

What is the meaning of life? It is perhaps the oldest philosophical question; At the end of a hysterical movie, the Monty Python gang told us it’s, “Try and be nice to people, avoid eating fat, read a good book now and then, get some walking in, and try to live together in peace and harmony with people of all creeds and nations.”

Of course, a lot goes into anyone’s personal answer to the question. But in a universe where we know that at least 100 billion or so stars occupy the Milky Way Galaxy alone, then we might say the visible universe contains something like 10,000 billion billion (1022) stars. We know that many of the stars near us host planetary systems. Could we be the only place in the cosmos with life? It doesn’t seem likely. What would an alien sentience consider the meaning of life?

Thus far, Earth is the only place we have evidence for life. Maybe microbes inhabit Europa, Enceladus, Titan, Triton, or even Mars. Perhaps SETI will detect a signal from a civilization elsewhere in the galaxy in the coming years. And yet with all our yearning to find life elsewhere, the cosmic distance scale is unbelievably huge: Contact, if and when it happens, is likely to be a remote exchange rather than shaking hands with aliens when they set down in Central Park.

Still, the question of life, its cosmic prevalence, and its meaning tug at us. From the universe’s point of view, life doesn’t have to have any meaning. The atoms in our bodies, arranged neatly by RNA and DNA, simply reflect their origins in the bellies of massive stars. There is no reason such order couldn’t have arisen in millions of places across the galaxy.

And yet to be a thinking creature, made form stuff in the universe and able to look back out at the stars and reflect on our origins, is the greatest gift of all. Do we–or any species—really need any more meaning than that?

[1] David J. Eicher, Astronomy (44, 10, October, 2016).

Meaning of Life in the Universe

Meaning of Life in the Universe[1]

What is the meaning of life? It is perhaps the oldest philosophical question; At the end of a hysterical movie, the Monty Python gang told us it’s, “Try and be nice to people, avoid eating fat, read a good book now and then, get some walking in, and try to live together in peace and harmony with people of all creeds and nations.”

Of course, a lot goes into anyone’s personal answer to the question. But in a universe where we know that at least 100 billion or so stars occupy the Milky Way Galaxy alone, then we might say the visible universe contains something like 10,000 billion billion (1022) stars. We know that many of the stars near us host planetary systems. Could we be the only place in the cosmos with life? It doesn’t seem likely. What would an alien sentience consider the meaning of life?

Thus far, Earth is the only place we have evidence for life. Maybe microbes inhabit Europa, Enceladus, Titan, Triton, or even Mars. Perhaps SETI will detect a signal from a civilization elsewhere in the galaxy in the coming years. And yet with all our yearning to find life elsewhere, the cosmic distance scale is unbelievably huge: Contact, if and when it happens, is likely to be a remote exchange rather than shaking hands with aliens when they set down in Central Park.

Still, the question of life, its cosmic prevalence, and its meaning tug at us. From the universe’s point of view, life doesn’t have to have any meaning. The atoms in our bodies, arranged neatly by RNA and DNA, simply reflect their origins in the bellies of massive stars. There is no reason such order couldn’t have arisen in millions of places across the galaxy.

And yet to be a thinking creature, made form stuff in the universe and able to look back out at the stars and reflect on our origins, is the greatest gift of all. Do we–or any species—really need any more meaning than that?

[1] David J. Eicher, Astronomy (44, 10, October, 2016, p. ).

Where Is Everybody?

Where Is Everybody?[1]

Perhaps my favorite essay in Aliens: The World’s Leading Scientists on the Search for Extraterrestrial Life is by the astrobiologist Lewis Dartnell, who patiently explains why aliens would not come here to have sex with us or eat us for supper.

I can only assume that he gets these questions a lot.

Here are the answers, should you find these possibilities concerning: The likelihood that we’d be genetically compatible with aliens is terribly remote, which means that they’d almost certainly be immune to our sexual charms. For similar reasons, having to do with biochemistry, we’d be lousy refreshments for them—they would almost certainly lack the proper enzymes to digest us.

As a bonus, Dartnell goes on to reassure us why aliens wouldn’t be especially interested in raiding our planet for raw materials, either (asteroids are a far easier source to mine); and if it were water they were after, they’d be far better off going to Europa, one of Jupiter’s largest moons, which contains more water beneath its icy shell than all the oceans on Earth combined.

If you’re interested in non-Earthly life, don’t look to the movies, is his point.

You could argue that that’s the general point of this modest, eccentric collection. Jim Al-Khalili, a quantum physicist and the editor of Aliens, opens with a question asked by Enrico Fermi in 1950: If the universe is so vast, and its age so old, and its stars so plentiful, where is everybody?

I’m no marketing expert, but “Where Is Everybody?” strikes me as a far catchier title for this book than the one it has, and it’s definitely more accurate. There really is nobody—so far—to write about. (Fighting words, I know. My hands hovered, spaceshiplike, for several minutes over the keyboard before committing that sentence to print.) This doesn’t mean that life elsewhere doesn’t exist. But it probably corresponds very little to what most of us have in mind, and not at all to the ooze-covered beasts of Ridley Scott’s electric dreams[2].

One of the most consistent takeaways from this anthology is just how banal extraterrestrial life might be. Often, when entertaining the possibility of aliens, what we’re really entertaining is the possibility of hardy microbes that can withstand extreme conditions, whether they’re thermophiles (heat lovers), psychrophiles (cold lovers) or halophiles (salt lovers). Read enough of Aliens, and you realize that the search for life is just as much about the most mundane aspects of biology as about the trippier questions of cosmology.

But it is also about philosophy. In this search, it helps to know what life is. Yet there’s no consensus about how to answer this question, strangely. (At the risk of being too Clintonian, it depends on what your definition of “is” is.)

Nor do we know how life began. At some point, the Earth made the transition from chemistry to biology, yes, but we cannot “agree on a definition that separates the nonliving chemistry from life,” as the geneticist Johnjoe McFadden puts it. (He then paraphrases the astronomer Fred Hoyle, who famously said that the odds of assembling something like a bacterium out of the primordial ooze were akin to the odds of a tornado’s assembling a jumbo jet out of a junkyard heap as it sweeps through.)

There are scientists who will go so far as to say that life is a spectacular fluke. Not everyone, mind you: Researchers now estimate that there are one billion Earthlike exoplanets in the Milky Way. “To my mathematical brain, the numbers alone make thinking about aliens perfectly rational,” Stephen Hawking has said.

But a powerful essay by the evolutionary biologist Matthew Cobb will make you wonder whether any form of multicellular life is far less likely than one in a billion. He points out that “there are more single-celled organisms alive on Earth than there are Earthlike planets in the observable universe”; that the number of single-celled organisms that have lived on this planet over the course of 3.8 billion years is beyond calculation; that these organisms have interacted “gazillions” of times (I love it when words of the appropriate magnitude desert even the experts). Yet we’ve never had a second instance of eukaryogenesis—that remarkable moment when one unicellular life form lodged inside another, forming something much more complex—in all this time.

Of course, there are researchers who dispute this theory and Cobb’s reasoning. But you get the idea.

The experience of reading almost any anthology is a bit like traveling across the country in a rental car with only an FM radio for company. Sometimes you get Sinatra; other times you get Nickelback.

This collection has its share of Nickelback. One of its most disappointing essays is about aliens in science fiction, which manages, against stupefying odds, to contain just one interesting insight: that authors tend to be more concerned with physics than with biology. (How did those gigantic sandworms evolve on the desert planet in Dune?)

But the best of these essays are far out in more ways than one. The very first, by the cosmologist Martin Rees, notes that our best hope for interstellar travel isn’t as humans, who don’t live very long and require far too much fuel to get very far, but as post-humans, who will have made the Kurzweilian transition from organic to inorganic, from decaying mortals to silicon-based, eminently portable machines. He adds that alien intelligence, if we ever detect it, will also be in this form.

The final essay, by Seth Shostak, a senior astronomer at the SETI institute (short for Search for Extraterrestrial Intelligence), goes even further, saying that if we really want to be attuned to alien life in the cosmos, it’s so likely to be in the form of machine intelligence that we ought to “be alert to apparent violations of physics.”

These forms of life may well be speaking to us even now. It’s just that our radio telescopes, which listen to the skies for signals from alien beings, can’t understand what they’re hearing. “Even if the search succeeded,” Rees writes, “it would still in my view be unlikely that the ‘signal’ would be a decodable message.”

It’s a whole new twist on George Berkeley’s question. The tree would fall in the forest. We’d hear it. But it would sound nothing like a tree.

[1] See Jennifer Senior, “‘Aliens’ Asks: If the Universe Is So Vast, Where Is Everybody?”, New York Times (May 24, 2017). This article is  review of the book: Al-Khalili, Jim(2007). ALIENS :The World’s Leading Scientists on the Search for Extraterrestrial Life. New York: Picador. Follow Jennifer Senior on Twitter: @jenseniorny. A version of this review appears in print on May 25, 2017, on Page C2 of the New York edition with the headline: “I Think It’s Gonna Be a Long, Long Time.” Downloaded May 26, 2017

[2] Scott is a South African born movie director of sci-fi films, including Alien, and The Martian.

A New Exoplanet and Life

A New Exoplanet May Be Most Promising Yet in Search for Life[1]

At left is an artist’s impression of the newly discovered rocky exoplanet. The exoplanet is close enough that astronomers are hopeful that with the next generation of big telescopes, they will be able to probe its atmosphere for signs of water or other evidence of suitability for life.

A prime planet listing has just appeared on the cosmic real estate market, possibly the most promising place yet to search for signs of life beyond the Solar System, the astronomers who discovered it say.

It is a rocky orb about one and a half times the size of Earth, about 40 light years from here. It circles a dwarf star known as LHS 1140 every 25 days, an orbit that puts it in the “Goldilocks” zone where temperatures are conducive to liquid water and perhaps life as we know it.

It is close enough that astronomers are hopeful that with the next generation of big telescopes, they will be able to probe its atmosphere for signs of water or other evidence of suitability for life.

“This planet is really close to us: If we shrank the Milky Way to the size of the United States, LHS 1140 and the Sun would fit inside Central Park,” David Charbonneau, of the Harvard-Smithsonian Center for Astrophysics, said in an email.

His colleague Jason Dittmann, who led the discovery team and is lead author of a paper published on Wednesday (April 18, 2017) in Nature, said in a statement, “This is the most exciting exoplanet I’ve seen in the last decade.”

The planet was discovered by the MEarth-South survey at the Cerro Tololo Inter-American Observatory in Chile, an array of small telescopes that looks for the dips in starlight when planets pass in front of nearby stars.

The depth of the dip told them how big the new planet is. Then they determined that it was about six times as massive as Earth by using a spectrograph called Harps, for High Accuracy Radial velocity Planet Searcher, at the European Southern Observatory, also in Chile, to measure how much the planet perturbed its home star. The resulting density puts the little world into a rapidly growing class called “superEarths.”

The star LHS 1140 is about one-fifth the size of our Sun. In its close orbit, the planet receives about half as much energy as Earth does from its own Sun, enough for a microbe or something more complicated to make a living.

This discovery continues a recent run of promising new planets circling nearby dwarf stars. Last summer there was the discovery of Proxima b, the nearest star to us, only 4.2 light years from here.

In February astronomers discovered a system of seven Earth-size planets circling a dwarf star known as Trappist-1.

According to Dr. Charbonneau, who originated the MEarth system, red dwarf stars outnumber stars like our Sun by about 10 to 1 in the 30-light-year bubble that constitutes our “block” in the cosmos.

About one in four of them have rocky planets in their habitable zones, according to work by Dr. Charbonneau’s former student Courtney Dressing, now at the California Institute of Technology.

Once upon a time, such planets were not looked upon favorably in the extraterrestrial life sweepstakes, because they were almost undoubtedly tidally locked, keeping one side faced to its star and the other facing out in space. That would result in a burning hell on one side and eternal frostbite on the other, neither side suitable for life.

But recently astronomers have determined that if these planets have thick enough atmospheres, winds can distribute the heat around both hemispheres and make them livable.

“Now we love them,” Sara Seager, a planetary expert at the Massachusetts Institute of Technology, said at a recent meeting on the origins of life sponsored by Harvard at the American Academy of Arts and Sciences in Cambridge. “If they have an atmosphere, they can harbor life.”

Astronomers said that the new planet offered the best hope so far to test that proposition. When the planet crosses in front of LHS 1140 the atmosphere acts like a filter, leaving an imprint on the star’s light that could betray the presence of water and other molecules important for life.

This will be a job for powerful new telescopes like the James Webb Space Telescope, due to be launched next year, or giant ground-based telescopes like the Giant Magellan and European Extremely Large telescopes, now being built in Chile, Dr. Charbonneau said.

Whether such planets actually have atmospheres is still controversial, however. When red dwarf stars are young, Dr. Charbonneau pointed out, they are ferociously luminous and might have blown away the planets’ atmospheres or caused a runaway greenhouse, leaving them barren. But the LHS 1140 planet is heavy enough, he said, that it might have been able to retain its atmosphere or regenerate it by volcanic activity later on.

“But the key point is yes, these are really exciting ideas to test,” he continued. “Do temperate, rocky M-dwarfs planets retain their atmospheres, and do they have life? This world enables those studies.”

[1] Dennis Overbye, “Promising Target in the Search for Extraterrestrial Life”, New York Times (April 20, 2017), p. A19. A version of this article appeared online at the New York Times (April 19, 2017).

Water in Outer Solar System

Water in Outer Solar System[1]

The editors of Astronomy deem the discoveries of water in the outer Solar System the 6th most important astronomy story of 2015.

Saturn’s moon Enceladus continues to show why it’s one of the best in the Solar System to search for life. Astronomers have suspected for years that salty water dredged up from a subsurface sea spews into space out of fissures near the moon’s south pole. But an analysis published in September 2014 in the journal Icarus, of seven years of images from NASA’s Cassini spacecraft indicates that Enceladus has a subsurface global ocean instead of merely a regional sea.

Cornell University planetary scientist Peter Thomas and colleagues measured a slight wobble in the moon’s rotation. If Enceladus were solid, its mass would dampen that motion. The researchers believe, instead, that a liquid water ocean lies between the moon’s icy surface layer and the rocky interior. They say the ocean is deeper and the ice shell thinner at the south polar region where Cassini has spied some 1200 geysers of salt water.

Scientists think that to keep any material in liquid state within Enceladus’ interior requires the push-and-pull tidal energy from Saturn. A global ocean is harder to keep warm than a regional sea, and so this discovery could also indicate that the saturnian satellite has more tidal energy than originally thought. “:If that is correct,” says team member Carolyn Porco, “and its ocean has been around a long, long time, then it may mean that any life within it has had a long time to evolve.”

Some of the material spewing from Enceladus’ underground ocean flows out through the geysers, flows toward Saturn because of the planet’s gravitational pull, and then orbits the planet as its E ring. In the March 12, 2015 issue of Nature, Frank Postberg at the universities of Heidelberg and Stuttgart in Germany and colleagues described how they used the Cassini spacecraft to study some of the material from the E ring. They saw silicon-rich molecules (called silicates) just a few nanometers wide. When this type of material is found in space, it almost always originates from rock being dissolved in water. But to learn the precise characteristics of that water-rock interaction, Postberg’s team collaborated with researchers from Japan to mimic the conditions needed at Enceladus to produce the sizes and composition of silicate particles they observed. They found the water needs to be at least 194 °F and have a pH between 8.5 and 10.56. These characteristics imply hot-spring heated water; the only other place where such hydrothermal vents have ever been see is on Earth, and these sites host extreme organisms.

The chemical reaction that produces the silicates also creates molecular hydrogen, and a different instrument on board Cassini looked for this gas during a late 2015 flight through Enceladus’ plumes. If more molecular hydrogen is detected (to be analyzed in the future) than expected, it will confirm hydrothermal activity, says Postberg.

In 2016 astronomers also found the best evidence so far of water at yet another location in the Solar System: Jupiter’s large moon Ganymede. NASA’s Galileo spacecraft, which studies the jovian system in the late 1990s and early 2000s, studied Ganymede’s magnetic field to learn whether the moon holds a global ocean under its surface. But the analysis from only 20 minutes of flyby observations was inconclusive. Fast forward to 2015, when Joachim Saur of the University of Cologne and his colleagues studied data from two 7-hour Hubble Space Telescope observations.

Ganymede has an auroral belt in each hemisphere just like Earth does. Jupiter’s magnetic field also influences these aurorae and causes them to rock during Jupiter’s 10-hour rotation period. Saur’s team knew that if Ganymede did not have an ocean, the aurora belts would change their positions slightly, tilting about 6°. “However, when a salty and thus electrically conductive ocean is present, this ocean counterbalances Jupiter’s magnetic influence and thus reduces the rocking of the auroras to only 2°,” says Saur. “We observed Ganymede with the Hubble Space Telescope for more than 5 hours and saw that the aurora barely moved and rocked by only 2°. This thus confirms the existence of an ocean.” The researchers think the ocean lies about 90 miles below the moon’s rock-ice crust and is about 60 miles thick. This strong evidence of Ganymede’s ocean continues to increase the number of worlds in our Solar System known to host water.

[1] See Astronomy (44, 1, 2016, p.)