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Why are some moons spherical while others are shaped like potatoes?

Why are some moons spherical while others are shaped like potatoes?

This blog post was written by Physicist Dr. Matt Caplan, who used to run the QuarksAndCoffee blog. That blog no longer exists, but I’m showing this archived copy of one of his posts for my students.


Why are some moons spherical while others are shaped like potatoes?

Short answer: Gravity likes to pull things together, which makes spheres. If the body is small enough gravity isn’t strong enough to deform it, which makes potatoes.

Long answer: Put a ball on top of a hill. What happens? It rolls down to the bottom. Why? Because gravity said so. This isn’t just how it works on the earth, but everywhere in the universe. Clearly, gravity is trying to make spheres. If you tried to dig a super deep hole stuff would fall in from the edges to fill it up. And what happen if we start to pile up rocks? Eventually, the pile of rocks reaches the point where it will all crumble down under its own weight. A sphere is the only shape that has no holes to fill or hills to crush. This is why every planet and star in the universe is round.

Of course, the earth and moon and planets aren’t perfect spheres. They’re lumpy. They’ve got hills and valleys and although none of them are that big compared to the planet, they’re still there. This is because gravity is strong enough to destroy (or prevent the formation) of a really big mountain, but not a small mountain. A small mountain’s own rigidness is enough to support its weight against gravity [1].

Mountain failure How High

This image shows two failure modes for mountains. The mountain on the left experiences shear failure, with the stress from the weight above the diagonal line exceeding the breaking point of the material. The mountain on the right fails due to compression of the base material.

Because materials have some intrinsic rigidity there must be bodies whose gravity isn’t strong enough to pull them into a sphere. Rather, the material is stiff enough to keep an oblong shape. After all, satellites and astronauts and cows don’t collapse into spheres in space.

The limit where gravity is strong enough to overcome the material properties of a body and pull it into a sphere is called the Potato Radius, and it effectively marks the transition from asteroid to dwarf planet [2]. It’s about 200-300 km, with rocky bodies having a slightly larger Potato Radius than icy bodies.

You can use some complicated math with material elasticity, density, and gravity to calculate the Potato Radius from scratch, or you could just look at Mt Everest. It turns out that the same physics determining the maximum height of mountains can be used to determine the Potato Radius – after all, they’re both just the behavior of rocks under gravity.

Check this out. The heights of the tallest mountains on Earth and Mars obey an interesting relation:

height gravity mountains equation

If you know the height of Everest and that Mars surface gravity is 2/5ths of Earth, then you know that Olympus Mons (tallest mountain on Mars) is about 5/2× taller than Everest! This relation also works with Maxwell Montes, the tallest mountain on Venus, but not for Mercury. Planetary science is a lot like medicine in this sense- there are always exceptions because everything is completely dependent on the body you’re looking at.

This is more than a curiosity. It tells us something important. The height of the tallest mountain a planet can support, multiplied by that planet’s surface gravity, is a constant.

For this sake of this piece I’ll call it the Rock Constant because that sounds cool. So why am I spending so long on a tangent about mountains in a piece about potato moons? It’s because the Potato Radius and Rock Constant are determined by the same things – gravity and the elasticity of rock! We can use the Rock Constant to estimate the Potato Radius!

Consider an oblong asteroid. Let’s pretend this asteroid is actually a sphere with a large mountain whose height is equal to the radius of that sphere.

potato shaped asteroid or moon

As the radius of a body increases the maximum height of a mountain decreases. If the radius was any bigger the mountain would have to be shorter and our asteroid would be entering ‘sphere’ territory. Let’s check if the radius of this imaginary asteroid is close to the Potato Radius using our relation for the Rock Constant:

height asteroid gravity equation

Height asteroid 3

And now we have everything we need:


height asteroid 4

This works out to about 240 km [1], right in the middle of the 200-300 km range of the more rigorous calculation!

(1) How High Can A Mountain Be? P. A. G. Scheuer, Journal of Astrophysics and Astronomy, vol. 2, June 1981, p. 165-169.

Web.archive.org copy

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Saturn is a gas giant planet, the sixth planet from the Sun, and the second largest planet in our solar system.

A gas giant is a giant planet composed mainly of hydrogen and helium.

Jupiter and Saturn are the Solar System’s gas giants.

Gas giants consist mostly of hydrogen and helium. Heavier elements make up between 3 and 13 percent of the mass.

They have an outer layer of hydrogen gas, surrounding a layer of liquid metallic hydrogen. They likely have a molten, rocky core.

The outermost portion of the atmosphere has many layers of visible clouds, composed of water and ammonia.

The layer of metallic hydrogen makes up the bulk of each planet. This is referred to as “metallic” because the very high pressure turns hydrogen into an electrical conductor.

In Roman mythology, Saturn is the god of agriculture and wealth.

Its radius is about nine times that of Earth. It’s volume is about 95 times larger than Earth.

It has one-eighth the average density of Earth.

Its astronomical symbol (♄) represents Saturn’s sickle.

Saturn's satellites and ring structures

Saturn’s rings

What are they made of?


How were they formed?


What does it look like as we travel trough them?

Go to this NASA press release for an amazing short movie.

Cassini’s ‘Inside-Out’ Rings Movie: NASA JPL

This movie sequence of images from NASA’s Cassini spacecraft offers a unique perspective on Saturn’s ring system. Cassini captured the images from within the gap between the planet and its rings, looking outward as the spacecraft made one of its final dives through the gap as part of the mission’s Grand Finale.

Using its wide-angle camera, Cassini took the 21 images in the sequence over a span of about four minutes during its dive through the gap on Aug. 20, 2017. The images have an original size of 512 x 512 pixels; the smaller image size allowed for more images to be taken over the short span of time.

The entirety of the main rings can be seen here, but due to the low viewing angle, the rings appear extremely foreshortened. The perspective shifts from the sunlit side of the rings to the unlit side, where sunlight filters through. On the sunlit side, the grayish C ring looks larger in the foreground because it is closer; beyond it is the bright B ring and slightly less-bright A ring, with the Cassini Division between them. The F ring is also fairly easy to make out.

More resources

Planets: Gas giants and Ice giants

A 3d model of Saturn and It’s Major Moons, from 3dwarehouse

Possible habitat for life on Enceladus, a moon of Saturn

How Saturn and the other planets got their names


Viewing space from Earth

How to get into backyard astronomy

EWS A quick and dirty guide to backyard astronomy. PCWorld

How to get started in amateur astronomy. Instructables

Astronomy for beginners

The best places to place a telescope


Light pollution

How light pollution makes it difficult for us to see objects in space nowadays.

what do the planets look like from Earth?

These are the relative sizes of the planets in our solar system, compared to the size of the moon. Note that this only shows their relative size, not their location! They are never lined up like this.

milwaukee public museum Size of planets compared to moon

Relative size of planets compared to the moon. From Milwaukee Public Museum, Soref Dome Theater & Planetarium.

What do comets and asteroids look like from Earth?

What are asteroids? What do they look like in space, and from here on Earth?

What are comets? What do they look like up close, and from here on Earth?

what do stars look like from Earth?

What are stars?


the largest objects in the night sky

Look up at the night sky – Are there immensely huge things that are just a bit too faint for the human eye to see? You betcha! Check out this amazing composite photo. This shows the actual apparent size of deep space objects, in our night sky, if they were brighter.


Actual size of deep space objects if they were brighter labeled

The images are in scale with one another, including the Moon, but not to the Milky Way background.
1. The Moon.
2. Andromeda Galaxy.
3. Triangulum Galaxy.
4. Orion Nebula.
5. Lagoon Nebula.
6. Pinwheel Galaxy.
7. Sculptor Galaxy.
8. Supernova remnant 1006.
9. Veil Nebula.
10. Helix Nebula.
11. Sombrero Galaxy.
12. Crab Nebula.
13. Comet Hale-Bopp (c. 1997)
14. Venus.
15. Jupiter.
16. International Space Station.

Learning Standards

Learning standards for astronomy



What is a “moon”?

Lunar libration with phase 2

Lunar libration with phase

Earth has a moon, Luna, but most of the other planets in our solar system also have moons.


(adapted from Wikipedia)

Interestingly, the Moon was called a “planet” until Copernicus’ publication of De revolutionibus orbium coelestium in 1543.

Until the discovery of the Galilean satellites around Jupiter, in 1610, there was no opportunity for referring to such objects as a class. Galileo chose to refer to his discoveries as Planetæ (“planets”.) It was only later discoverers who chose other terms to distinguish them from the objects they orbited.

The first to use of the term satellite to describe orbiting bodies was the German astronomer Johannes Kepler in his pamphlet Narratio de Observatis a se quatuor Iouis satellitibus erronibus (“Narration About Four Satellites of Jupiter Observed”) in 1610. Kepler derived the term from the Latin word satelles, meaning “guard”, or “companion”, because the satellites accompanied their primary planet in their journey through the heavens.

The term satellite became the normal one for referring to an object orbiting a planet, as it avoided the ambiguity of the word “moon”. In 1957, however, the launching of the artificial object Sputnik created a need for new terminology. The terms man-made satellite and artificial moon were very quickly abandoned in favor of the simpler satellite, and as a consequence, the term has become linked primarily with artificial objects flown in space – including, sometimes, even those not in orbit around a planet.

Because of this shift in meaning, the term “moon,” which had continued to be used in a generic sense in works of popular science and in fiction, has regained respectability and is now used interchangeably with natural satellite, even in scientific articles.

To avoid ambiguity, the convention is to capitalize the word Moon when referring to Earth’s natural satellite, but not when referring to other natural satellites.

Many authors define “satellite” or “natural satellite” as orbiting some planet or minor planet, synonymous with “moon” – by such a definition all natural satellites are moons, but Earth and other planets are not satellites.


Mars and its moons, 3D visualization



Pull out from Jupiter Showing Moon Orbits, showing 63 moons


Jupiter And The Galilean moons



Saturn and its major moons



Uranus and its major moons



Neptune and its major moon Triton


Neptune and its rings


Pluto and its moon Charon



Astronomical engineering: a strategy for modifying planetary orbits

How’s this for an idea for a science-fiction story?

The sun has unexpectedly started to swell into a red giant – which would engulf and destroy the Earth. So, “to save humanity, the world’s governments have banded together and constructed thousands of rocket engines across the Earth’s surface. Once installed, they propel the planet out of its solar system and onto a 2,500 year journey to resettle in Alpha Centauri.” (Grant Watson.)

The Wandering Earth (Chinese: 流浪地球) is a 2019 Chinese science fiction film directed by Frant Gwo, loosely based on the novella of the same name by author Liu Cixin. Here’s an image of one of the many “Earth Engines.”

The Wandering Earth Movie (Engine)

Our question – Could this be done in real life?

What science in the film did they get wrong?

Helium flash

Thrusting the Earth out of orbit with rockets: consider, how much reaction mass would we need to do this?

Even if you could build engines large enough, mining the Earth (as these engines do in the film) causes a problem. There would barely be any Earth left by the point you mined enough dirt to thrust the planet to Proxima Centauri, 4.2 light-years away. “It would take about 95 percent of the mass of Earth to do this,” Elliott estimates.

Stopping the rotation of the Earth?

Gravitational slingshot around Jupiter

Surviving the radiation around Jupiter

Could ‘The Wandering Earth’ Actually Happen? Here’s What a NASA Engineer Says

Wandering Earth: Rocket scientist explains how we could move our planet. ARS Technia

Other options

We could eventually move human civilization to Mars, which become habitable.


What science could actually work to change Earth’s orbit?

Astronomical engineering: a strategy for modifying planetary orbits
D. G. Korycansky, Gregory Laughlin, Fred C. Adams (7 Feb 2001)

The Sun’s gradual brightening will seriously compromise the Earth’s biosphere within ~ 1E9 years. If Earth’s orbit migrates outward, however, the biosphere could remain intact over the entire main-sequence lifetime of the Sun.

In this paper, we explore the feasibility of engineering such a migration over a long time period. The basic mechanism uses gravitational assists to (in effect) transfer orbital energy from Jupiter to the Earth, and thereby enlarges the orbital radius of Earth.

This transfer is accomplished by a suitable intermediate body, either a Kuiper Belt object or a main belt asteroid. The object first encounters Earth during an inward pass on its initial highly elliptical orbit of large (~ 300 AU) semimajor axis.

The encounter transfers energy from the object to the Earth in standard gravity-assist fashion by passing close to the leading limb of the planet. The resulting outbound trajectory of the object must cross the orbit of Jupiter; with proper timing, the outbound object encounters Jupiter and picks up the energy it lost to Earth.

With small corrections to the trajectory, or additional planetary encounters (e.g., with Saturn), the object can repeat this process over many encounters. To maintain its present flux of solar energy, the Earth must experience roughly one encounter every 6000 years (for an object mass of 1E22 g). We develop the details of this scheme and discuss its ramifications.


As for the Moon, reasoning by analogy with cases of stellar binaries and third-body encounters suggests that the Moon will tend to become unbound by encounters in which O passes inside the Moon’s orbit. (As well, there is the non-zero probability of collisions between O and the Moon, which must be avoided.) Again, detailed quantitative work needs to be done, but it seems that the Moon will be lost from Earth orbit during this process. On the other hand, a subset of encounters could be targeted to “herd” the Moon along with the Earth should that prove necessary.

It has been suggested (cf. Ward and Brownlee, 2000) that the presence of the Moon maintains the Earth’s obliquity in a relatively narrow band about its present value and is thus necessary to preserve the Earth’s habitability. Given that the Moon’s mass is 1/81 that of the Earth, a similarly small increment of the number of encounters should be sufficient to keep it in the Earth’s environment.

The fate of Mars in this scenario remains unresolved. By the time this migration question becomes urgent, Mars (and perhaps other bodies in the solar system) may have been altered for habitability, or at least become valuable as natural resources. Certainly, the dynamical consequences of significantly re-arranging the Solar System must be evaluated. For example, recent work by Innanen et al. (1998) has shown that if the Earth were removed from the Solar System, then Venus and Mercury would be destabilized within a relatively short time. In addition, the Earth will traverse various secular and mean-motion resonances with the other planets as it moves gradually outward. A larger flux of encounters might be needed to escort the Earth rapidly

Journal reference: Astrophys.Space Sci.275:349-366, 2001. Astronomical Engineering: A Strategy For Modifying Planetary Orbits, Springer Link
Cite as: arXiv:astro-ph/0102126 (or arXiv:astro-ph/0102126v1 for this version)


Moving our sun and entire solar system

Caplan thruster

from Scientist figures out how to move our sun to avoid space collisions, BigThink

Stellar engines: Design considerations for maximizing acceleration, Acta Astronautica, 12/2019

Caplan envisions two stellar engine designs, with one of them based on the idea of encapsulating the sun in a megastructure that would take advantage of its energy. Another engine would make use of a giant sail to move the solar system by about 50 light years during the course of a million years….

One big reason would be to move the solar system if we’re anticipating running into a mega-explosion from a supernova or some such cataclysmic scenario. Of course, we’d need to be way more ahead technologically for any such endeavor.

If you were to be moving the solar system, the convenient thing is that theoretically everything inside it would move along at the same time. Being pulled by the sun’s gravity would keep the contents of the system in consistent orbit.

One of the stellar engine designs involves a thin mirror-like solar sail, like the “Shkladov thruster”. The reflective material would be thinner than a red blood cell. The sail would be positioned over the poles of the sun and would not be orbiting. It would be important to install it in such a way that it won’t interfere with the Earth’s temperature. This would also affect the direction in which we’d be steering the solar system.

Thrust for the sail design would be created by solar radiation reflecting onto the mega-mirror. This is definitely not the fastest way to travel, with the sun being pushed along at the rate of 100 light-year in 230 million years. That’s actually not fast enough to get out of the way of a supernova explosion, admits Caplan.

What would work better is a speedier “active” thruster, called the “Caplan thruster” by Kurzgesagt, which initially approached Caplan to design such engines. It would be propelled by thermonuclear blasts of photon particles. This thruster is a modified version of the “Bussard ramjet,” conceptualized in the 1960s, which works on fusion energy. The engine would need millions of tons of fuel per second to function, creating fusion from matter it collects in the solar wind by utilizing a giant electromagnetic field. More energy would also be gathered by a Dyson sphere megastructure, built around the sun.

Caplan imagines the engine having two jets, with one using hydrogen pointed at the sun, to prevent colliding with it, and another, employing helium, directed away from the star. This would cause net momentum, like from a tug boat, and move the thruster forward.

The astrophysicist calculates this type of thruster would be fast enough to escape a supernova. It could also redirect the galactic orbit of our solar system in as little as 10 million years.

Related articles

Gregory Benford and Larry Niven solved the problems with Shkavdov thrusters for a propulsion system for moving stars

On the Possibility of Detecting Class A Stellar Engines Using Exoplanet Transit Curves,”  Journal of the British Interplanetary Society

See this video

Learning Standards

2016 Massachusetts Science and Technology/Engineering Curriculum Framework

8.MS-ESS1-2. Explain the role of gravity in ocean tides, the orbital motions of planets, their moons, and asteroids in the solar system.

HS-PS2-4. Use mathematical representations of Newton’s law of gravitation and Coulomb’s law to both qualitatively and quantitatively describe and predict the effects of gravitational and electrostatic forces between objects.

Next Generation Science Standards

HS-PS2.B.1 ( High School Physical Sciences ): Newton’s law of universal gravitation and Coulomb’s law provide the mathematical models to describe and predict the effects of gravitational and electrostatic forces between distant objects.

Next Generation Science Standards Appendix F: Science and Engineering Practices

Ask questions that arise from careful observation of phenomena, or unexpected results, to clarify and/or seek additional information.
Ask questions that arise from examining models or a theory, to clarify and/or seek additional information and relationships.
Ask and/or evaluate questions that challenge the premise(s) of an argument, the interpretation of a data set, or the suitability of a design.

A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas (2012)

PS2.B: TYPES OF INTERACTIONS: Gravitational, electric, and magnetic forces between a pair of objects do not require that they be in contact. These forces are explained by force fields that contain energy and can transfer energy through space. These fields can be mapped by their effect on a test object (mass, charge, or magnet, respectively). Objects with mass are sources of gravitational fields and are affected by the gravitational fields of all other objects with mass. Gravitational forces are always attractive. For two human-scale objects, these forces are too small to observe without sensitive instrumentation. Gravitational interactions are non-negligible, however, when very massive objects are involved. Thus the gravitational force due to Earth, acting on an object near Earth’s surface, pulls that object toward the planet’s center. Newton’s law of universal gravitation provides the mathematical model to describe and predict the effects of gravitational forces between distant objects.

Black holes: Videos

A CURIOUS WORLD: BLACK HOLES: What are black holes made of and how do they work?


NOVA search


Rebuilding the Interstellar Black Hole


Black holes are not as black as we once thought. They are theorized to die a slow death by evaporation, emitting energy known as Hawking radiation.


What’s inside a black hole?


Four types of black holes


PBS NOVA: Black Hole Apocalypse. Season 45, Episode 1

“Black holes are the most enigmatic and exotic objects in the universe. They’re also the most powerful, with gravity so strong it can actually trap light. And they’re destructive. Anything that falls into them vanishes…gone forever. But now, astrophysicists are realizing that black holes may be essential to understanding how our universe unfolded.”




Black hole eating a star GIF


The Drake Equation

“The Drake Equation is a way to estimate the number of communicating advanced civilizations (N) inhabiting the Galaxy. It is named after Frank Drake who first summarized the things we need to know to answer the question, “how many of them are out there?” The equation breaks this big unknown, complex question into several smaller (hopefully manageable) parts. Once you know how to deal with each of the pieces, you can put them together to come up with a decent guess.”

– Nick Strobel, Astronomy Notes

drake equation SETI

Image from universetoday.com, drake-equation


Luciano Ingenito Drake Equation SETI

Luciano Ingenito | CC by-nc-nd 4.0


The Drake Equation interactive PBS NOVA



SETI notes

The search for extraterrestrial intelligence (SETI) is a collective term for any scientific searches for intelligent extraterrestrial life. It is primarily done by monitoring radio signals for signs of transmissions from civilizations on other planets.

What are radio waves?

Radio waves are just a part of the EM (electromagnetic) spectrum.

Gamma rays Spectrum Properties NASA

That sounds dandy, except, what exactly is the “electromagnetic spectrum”?

All parts of the EM spectrum – radio, visible light, etc. – are oscillating electric and magnetic fields. To learn the details, see Light is an electromagnetic field.


How are radio waves different from other parts of the EM spectrum?

They’re made of the same thing, behaving in exactly the same way. Only difference is that radio waves are hundreds of meters to thousands of meters long. Other parts of the spectrum have shorter waves.

What creates radio waves?

With a radio receiver we can hear radio waves coming from all around us. They are naturally produced, and comes from all over the Earth, and outer space. Radio waves are naturally created by:

* Wind whipping over a surface, creating static electricity

* Lightning

* atoms trapped in the magnetic fields around the Earth, and around all other planets as well.

* The Sun (puts out all frequencies of EM radiation!)

* All stars

* Ionized interstellar gas surrounding bright, hot stars

* Supernovas

* There are even more complex radio waves naturally generated, you can read this paper: Natural and man-made terrestrial electromagnetic noise

By the late 1800’s humans had learned not only how to receive radio waves, but how to generate them. Today we artificially create radio waves for all sorts of purposes, including

  • Traditional, over-the-air, radio stations (AM and FM radio)

  • Traditional, old-fashioned, TV (television)

  • Wi-Fi

  • Bluetooth

  • Cellphone communication (cell towers and the phones)

Cornell.edu: Observational-astronomy. SETI and extraterrestrial life

Kaiserscience – All about the electromagnetic spectrum

What is a radio telescope?

The technology of how we detect radio waves.


radio telescope

Image from website of James Schombert, Dept of Physics, Univ. Oregon

How does an antenna pick up radio waves?

“If we place a conducting material on the path of such a wave, the passing wave will create an oscillating electric field inside the material; and that field will accelerate charges back and forth through the conductor.”

radio antenna dipole oscillating electric field



History of SETI

This section has been adapted from “Search for extraterrestrial intelligence.” Wikipedia, The Free Encyclopedia. 4 Mar. 2019

There have been many earlier searches for extraterrestrial intelligence within the Solar System. In 1896, Nikola Tesla suggested that an extreme version of his wireless electrical transmission system could be used to contact beings on Mars. He conducted an experiment at his Colorado Springs experimental station.

In the early 1900s, Guglielmo Marconi, Lord Kelvin and David Peck Todd also stated their belief that radio could be used to contact Martians, with Marconi stating that his stations had also picked up potential Martian signals.

On August 21–23, 1924, Mars entered an opposition closer to Earth than at any time in the century before or the next 80 years. In the United States, a “National Radio Silence Day” was promoted during a 36-hour period from August 21–23, with all radios quiet for five minutes on the hour, every hour.

At the United States Naval Observatory, scientists used a radio receiver, miles above the ground in a dirigible, to listen for any potential radio messages from Mars.

A 1959 paper by Philip Morrison and Giuseppe Cocconi first pointed out the possibility of searching the microwave spectrum, and proposed frequencies and a set of initial targets.

In 1960, Cornell University astronomer Frank Drake performed the first modern SETI experiment, named “Project Ozma”, after the Queen of Oz in L. Frank Baum’s fantasy books. Drake used a radio telescope at Green Bank, West Virginia, to examine the stars Tau Ceti and Epsilon Eridani.

Soviet scientists took a strong interest in SETI during the 1960s and performed a number of searches. Soviet astronomer Iosif Shklovsky wrote the pioneering book in the field, Universe, Life, Intelligence (1962), which was expanded upon by American astronomer Carl Sagan as the best-selling book Intelligent Life in the Universe (1966).

In the March 1955 issue of Scientific American, John D. Kraus described an idea to scan the cosmos for natural radio signals using a radio telescope. Ohio State University soon created a SETI program.

In 1971, NASA funded a SETI study that involved Drake, Bernard M. Oliver of Hewlett-Packard Corporation, and others. The resulting report proposed the construction of an Earth-based radio telescope array with 1,500 dishes known as “Project Cyclops”. It was not built, but the report formed the basis of much SETI work that followed.

Why don’t any organisms detect radio waves?

(Honors Biology topic)

Whyevolutionistrue.wordpress.com : Why-don’t-any-organisms-detect-radio-waves?

How difficult will this be?

It is very difficult to pick up Earth’s radio waves from another solar system. As such, we can imagine that it would  very difficult to pick up radio waves here, from some other solar system. That’s why we aren’t really looking for random radio waves that happen to escape out into space. Rather, the current projects are looking for much more powerful signals, that we hope would be sent out on purpose.


“That’s a rather extraordinary claim, so I spoke to SETI expert and scifi novelist David Brin about it — and he’s not convinced detection is this easy. He told me that, even if an ETI had a one square kilometer array, they would have to point it a at Earth for the duration of an entire year. “Because it would take that long,” he told io9. “But why stare if you don’t already have a reason to suspect?”

Like SETI Institute’s Seth Shostak, Brin believes that Earth is not detectable beyond five light years. “With one exception: Narrow-focused, coherent (laser-like) planetary radars that are aimed to briefly scan the surfaces of asteroids and moons,” he says, “And not to be confused with military radars that disperse.””

How can we differentiate between natural or artificial (intelligent) signals?

Consider listening to the sound of radio static. Compare that to the sound of a song, or a person giving a speech. Both are sounds – how are they similar? How are they different?

Come up with ideas on how we could differentiate between natural or artificial (intelligent) signals.

“Humanity has received some odd signals in the past. We’ve also sent out some signals ourselves. How could we determine that a signal we’d received was artificial in origin? Or of course inversely, how could an extraterrestrial civilization determine a signal we had sent out was was artificial?”

Space.stackexchange.com – What-evidence-would-be-needed-to-determine-a-signal-was-artificial-in-origin?

Listening for Extraterrestrial Blah Blah: At the cosmic dinner party, intelligence is the loudest thing in the room. By Laurance R. Doyle, Illustrations by Tianhua Mao

Listening for Extraterrestrial Blah Blah

The Water hole: What radio frequencies should we listen to?


Hailing Frequencies Open, Captain! What is the “water hole”?


Misconceptions about listening with radio telescopes

Radio signals diminish in strength very rapidly with distance – they decrease according to the inverse square law. What does that mean Consider cooking spray. Oil is sprayed through an opening. In this image, the cooking spray hits a piece of toast and deposits an even layer of butter, 1 mm thick.

Hewitt Inverse Square Law Butter gun

Hewitt textbook

When the butter gets twice as far, it becomes only 1/4 as this.
If it travels 3 times as far, it will spread out to cover 3 x 3,
or 9, pieces of toast.
So now the butter will only be 1/9th as thick.
(1/9 is the inverse square of 3)
This pattern is called an inverse-square law.
The same is true for a can of spray paint: as the paint travels further, it covers a wider area, so the paint per area is inversely less thick.

Hewit inverse square law spray paint

Hewitt Conceptual Physics worksheets

The same pattern of spreading out and weakening, the inverse square laws, is true for radio waves.

Inverse Square Law GIF

Animations of the inverse-square law – animated clip: Inverse-square law for light

Okay – so by the time that radio signals reach even the next solar system they would be unbelievably weak. The radio signals would be even millions of times weaker by the time they travelled across even 1% of the galaxy. Our radio telescopes could never pick up such radio signals.

So if that’s the case, what then are SETI researchers listening for?We are looking for a civilization that wants to be known, one that has deliberately built a high power radio beacon, aimed in one direction at a time. A tightly beamed signal would be millions of times stronger – if by chance we happen to be in its path.

Do SETI researchers believe that someone out there is deliberately sending a signal to us here on Earth specifically? No. However, we know that there are billions of stars, and tens of billions of planets. Many of these planets might support life. Therefore, at any given time there could be many thousands of other worlds with intelligent life

The hope is that some of them would want to communicate, sending a tight, beamed radio signal out into space. If so, then one day we might intercept such a communication.

Article: Is there anybody out there? Jason Davis, October 25, 2017, The Planetary Society

Planetary.org – Is there anybody out there?

Goldilocks Zone/Circumstellar habitable zone

This section from evolution.berkeley.edu, A Place for Life: A special astronomy exhibit of Understanding Evolution

From the known properties of stars and of the chemistry of water, astronomers can define “habitable zones” around stars where liquid water (and hence life) could exist on the surface of planets.

Too close to the star, and water will boil; too far, and it will freeze. This so-called Goldilocks zone, where the temperature is just right, depends on both the distance from the star and the characteristics of the star itself.

The habitable zone around luminous giant stars is further from the star than the habitable zone around faint dwarfs.

Of course, as noted previously, life may also exist outside these zones, for example in subsurface oceans on icy moons heated from the moon’s interior.

We know that there are around 200 billion stars in our Galaxy. Recent research has revealed that most of them have planets, and that tens of billions of these planets are likely similar in size to Earth, made of rock, and orbit in their stars’ habitable zones.

The question that remains to be answered is what fraction of those potentially habitable worlds host life.


Image converted using ifftoany

from the NASA Kepler Mission



Habitable Zones of Different Stars. NASA/Kepler Mission/Dana Berry.


Habitable zones for binary star systems

What about planets in a solar system with two stars?

Most stars in the Galaxy have at least one stellar companion—binary or multiple star systems. Stars like our Sun with no stellar companion are in the minority.

It would probably be difficult for there to be stable, only slightly elliptical planet orbits in a binary or multiple star system.

Complex life (multi-cellular) will need to have a stable temperature regime to form so the planet orbit cannot be too eccentric. Simple life like bacteria might be able to withstand large temperature changes on a planet with a significantly elliptical orbit but complex life is the much more interesting case.

Suitable binary stars would be those systems where either:

(a) the binary stars orbit very close to each other with the planet(s) orbiting both of them at a large distance (called a “circumbinary planet”)

or (b) the binary stars orbit very far from each other so the planet(s) could reside in stable orbits near each of the stars—the one star’s gravity acting on a planet would be much stronger than that of the other star.

Goldilocks habitable zone binary star

Image by Nick Strobel

image from https://www.astronomynotes.com/lifezone/s2.htm

A cool article on this subject:  I Built a Stable Planetary System with 416 Planets in the Habitable Zone


Strong magnetic fields may be necessary

Earth has a strong magnetic field. Turns out that this might be necessary on a planet if complex life is to evolve.

magnetic_field Earth


Why does Earth have such a strong magnetic field? Earth’s core is still hot and molten. Metal still moves inside it, and moving metal has moving free electrons. Electrons moving around – by definition – are an electrical current. And it turns out that electrical currents create their own magnetic field!

Electric current creates magnetic field

Image from S-cool revision. GCSE » Physics » Magnetism and Electromagnetism

Inside the earth

This field protects the Earth’s atmosphere from some of the Sun’s radiation.

Without such a field most of a planet’s atmosphere is likely to be blown away into space, as happened to Mars.

Mars now has very little atmosphere, and its surface is constantly irradiated by solar radiation.

Mars Earth biosphere due to magnetic field

The Drake Equation

An equation named after Frank Drake, who first summarized the things we need to know to answer the question, “how many possible extraterrestrial civilizations are out there?” The equation breaks this complex question into several smaller (hopefully manageable) parts.

The Drake Equation

Learning Standards

(for the entire SETI unit)

Common Core, English Language Arts Standards » Science & Technical Subjects

CCSS.ELA-LITERACY.RST.9-10.1 – Cite specific textual evidence to support analysis of science and technical texts, attending to the precise details of explanations or descriptions.

CCSS.ELA-LITERACY.RST.9-10.2 – Determine the central ideas or conclusions of a text; trace the text’s explanation or depiction of a complex process, phenomenon, or concept; provide an accurate summary of the text.

CCSS.ELA-LITERACY.RST.9-10.4 – Determine the meaning of symbols, key terms, and other domain-specific words and phrases as they are used in a specific scientific or technical context.

CCSS.ELA-LITERACY.RST.9-10.5 –  Analyze the structure of the relationships among concepts in a text, including relationships among key terms (e.g., force, friction, reaction force, energy).

CCSS.ELA-LITERACY.RST.9-10.6 – Analyze the author’s purpose in providing an explanation, describing a procedure, or discussing an experiment in a text, defining the question the author seeks to address.

2016 Massachusetts Science and Technology/Engineering Curriculum Framework

HS-PS2-5. Provide evidence that an electric current can produce a magnetic field and that a changing magnetic field can produce an electric current.

6.MS-PS4-1. Use diagrams of a simple wave to explain that (a) a wave has a repeating pattern with a specific amplitude, frequency, and wavelength, and (b) the amplitude of a wave is related to the energy of the wave.

HS-PS4-1. Use mathematical representations to support a claim regarding relationships among the frequency, wavelength, and speed of waves traveling within various media. Recognize that electromagnetic waves can travel through empty space (without a medium) as compared to mechanical waves that require a medium.

HS-PS4-5. Communicate technical information about how some technological devices use the principles of wave behavior and wave interactions with matter to transmit and capture information and energy.

Next Generation Science Standards: Science & Engineering Practices
● Ask questions that arise from careful observation of phenomena, or unexpected results, to clarify and/or seek additional information.
● Ask questions that arise from examining models or a theory, to clarify and/or seek additional information and relationships.
● Ask questions to determine relationships, including quantitative relationships, between independent and dependent variables.
● Evaluate a question to determine if it is testable and relevant.
● Ask and/or evaluate questions that challenge the premise(s) of an argument, the interpretation of a data set, or the suitability of the design

Common Core Math Standards (Inverse-square law)

CCSS.Math.Content.7.RP.A.2a ( Grade 7 ): Decide whether two quantities are in a proportional relationship, e.g., by testing for equivalent ratios in a table or graphing on a coordinate plane and observing whether the graph is a straight line through the origin.
CCSS.Math.Content.7.RP.A.2c ( Grade 7 ): Represent proportional relationships by equations.

Stars – Resources

Star formation by collapse of molecular clouds: Computer simulation

“ collapse and fragmentation of a molecular cloud presented in “The Formation of Stars and Brown Dwarfs and the Truncation of Protoplanetary Discs in a Star Cluster” by Matthew R. Bate,”


How Stars Are Formed and Born, National Geographic


Birth of a Star | Out There | The New York Times


NASA/JWST | Planetary formation (HD)


The Life of a star, Dillon Gu


Boardgame – “Stellar Journey : The Game”, by Other Worlds Educational Enterprises

Formation of Black Holes

Black Holes Explained – From Birth to Death


Stephen Hawking explains black holes in 90 seconds – BBC News


Sound of Two Black Holes Colliding


Neil deGrasse Tyson Explains Wormholes and Black holes



Stars are powered by nuclear fusion

Stars are powered by nuclear fusion

Before reading this section, you will first need to know

  • Atoms are the smallest stable building blocks of matter in the universe

  • Atoms are not solid. They are made of protons, neutrons, and electrons.

  • All solids, liquids, gases and plasma in our universe are made of these particles.

  • All matter is attracted to other matter through gravity. If you have enough mass floating around in space, over time, large amounts of matter will be attracted together to form giant gas clouds – nebulas.

  • Nebulas themselves can contract, due to gravity. This leads to the development of stars

  • Generally speaking, all matter in our universe is conserved (conservation of matter)

  • Generally speaking, all energy in our universe is conserved (conservation of energy)

  • Here’s the wacky bit: scientists discovered fascinating and unexpected violations of those supposedly inviolable laws, but instead of that being magic, it pointed towards an even greater discovery: the law of conservation of matter and  energy.  This was discovered by Albert Einstein, and is known as mass–energy equivalence.

As such, first read our lesson on the discovery of nuclear physics and radioactivity.

At this point you now have the background for what comes next.

Inside a star, gravity pulls billions of tons of matter towards the center. Atoms are pushed very close together. So close that sometimes two atoms will fuse into one, heavier atom. The mass of this new atom is slightly less than the mass of the pieces that it was made of in the first place? Where the did missing go? It effectively becomes energy – which we see as photons, or as the heat/motion energy of other particles.

As an example, here we see deuterium fusing with tritium. The resulting product has less mass than the parts going in to the collision. That missing mass we see becomes 3.5 mega electron-volts of energy,

Here we some typical nuclear fusion reactions that go on inside yellow dwarf stars like our sun.


Here is a step-by-step cascade showing how hydrogen atoms can fuse to create Helium, giving off gamma rays and neutrinos in the process.

Solar nucleosynthesis

In a different form we see the same process here.issue3_fusion1_large

(More text TBA)