Celebrating Our Rare and Special Planet on Earth Day

Universe-Sandbox-²-Earth Day

I hear a lot of people say that looking at our universe from a solar system, galactic, or cosmological scale makes them feel small and insignificant. Insignificant to who and what? I look at our universe and I see the incredible complexity that has arisen from a few forces and a few fundamental laws. You see it in the structure of the cosmic background, you see it in the shapes of galaxies, you see it in the heavy elements formed in our stars. Nowhere do you see that complexity more than when you look at life. It was a glorious accident that allowed us to exist, and we exist in an incredibly delicate balance.

99.99999999…% of the universe is empty space. 99.9999999…% of that which is not empty space is lifeless. Why does it matter if somewhere light years away something doesn’t care about your individual existence?  We have this amazing planet and it might not be unique in the universe in its complexity, but it is incredibly rare and special, and I feel very significant — overwhelmed with a sense of purpose and duty, in fact — because it is up to us to try as hard as we can to preserve this rare and special place and all the life on it.

The structure of the universe doesn’t make me feel insignificant. It makes me feel incredibly lucky. By some chance I exist in this rare place. Thus it is my duty to try to keep life existing here, and to try to make that life as pleasant as possible for the other rare and lucky creatures who share it with me.

– Jenn Seiler, astrophysicist and Universe Sandbox ² developer.


Learn how we simulate Earth’s climate and how you can explore it in Universe Sandbox ²:


Gravitational Waves & Universe Sandbox ²

A black hole in Universe Sandbox ². Researchers have concluded the gravitational waves they detected were the result of two black holes colliding.

A black hole in Universe Sandbox ². Researchers concluded the detected gravitational waves resulted from two black holes colliding.

What’s the significance of discovering gravitational waves?

This announcement is a huge deal.  It is on par with the discovery of the Higgs Boson particle which provided the missing evidence for a prediction of the Standard Model of particle physics. Gravitational waves are a century-old (almost exactly) prediction now confirmed by a huge number of relentless, and brilliant people after many years of hard work. It is the first direct confirmation of the prediction from Einstein’s General Relativity that matter and energy determine the motion of bodies by warping the fabric of spacetime itself, and in so doing, emanate ripples when massive bodies are accelerated through that space.

It is not only confirmation of general relativity, though. It is also the first of many future observations that will look at the universe in a completely new way. Up until now we’ve used only photons (telescopes all along the electromagnetic spectrum) and sometimes neutrinos. Now we can add listening to the fabric of space to our list of tools. This will allow us to see the dark and the obscured parts of the universe: the early universe, centers of galaxies, things blocked by dust clouds, and so on, by listening for changes in space itself. It is the start of a new age in astronomy.

In addition to this detection being the first direct proof that the predictions of general relativity that matter and energy warp space time are true, and some of the strongest evidence for the reality of black holes, this is also a new kind of astronomy.  Though gravity is the weakest force and gravitational waves are very hard to detect, they do have a few advantages over observations of photons.

  • First, gravitational waves are practically impervious to matter in their path. This means we can see into regions of space that are blocked to optical observatories, such as inside dense clouds of dust, the centers of galaxies, behind large or close bodies.
  • Second, this is an observation of the warping of space itself, meaning we can detect things that have mass but might not produce observable light, such as black holes, dense sources of dark matter (if such were to exist), cosmic string breaks, etc. 
  • Third, gravitational waves fall off in amplitude much more slowly than light. This means that we can receive signals from very far away that we might not notice optically.
  •  And fourth, because gravitational waves also travel at the speed of light and don’t have to bounce off intervening matter, and begin to be potentially detectable from bodies getting close rather than just after the moment of collision, this means that we can work with other telescopes and tell them “Look over there! You’re probably going to see something exciting!”

This all of means that this detection means the beginning of a new kind of observational astronomy, as well as a better understanding of of of the fundamental forces of the universe, gravity.


What role did Jenn, astrophysicist and Universe Sandbox ² developer, play in the discovery?

While I was in the field I ran super-computer simulations to make predictions about the gravitational wave signals that would be produced by binary black hole mergers. Those waveforms are used as templates in the detector pipeline. The detector matches the template banks against the incoming data to find real signals amidst the noise of the detector, while also doing searches for large burst signals (how this one was found). Those waveforms are then used again to determine where the signal came from, what it was (two black holes, a neutron star and a black hole, two neutron stars, etc), and the properties of the bodies that created the signal (spins, masses, separation, etc.). I also worked on developing the analytical formulas to determine those spins and masses from those signals.

Here’s one of the scientific papers on the process of determining the properties of the source of the signal, with three papers cited on which Jenn Seiler was an author:

The Einstein equations for general relativity are ten highly non-linear partial differential equations. This means that it is only possible to obtain exact solutions for astrophysical situations for some very idealized conditions (such as spherical symmetry and a single body). In order to predict the gravitational waveforms produced by compact multi-body systems, or stellar collapse, it is necessary to solve the equations numerically (computationally). This means formulating initial data for spacetimes of interest (such as two in-spiralling black holes of various spins and mass ratios) and evolving them by integrating the solutions of the Einstein equations stepping forward in time by discrete steps. To prove that these computer simulations approximate reality more than just by equations on paper we would run these simulations at multiple resolutions for our discrete spacetimes and show that our solutions converged to a single solution as we approach infinite resolution (that would represent real continuous space) at the rate we expect for the method we were using. 

There were many obstacles in creating these simulations: vast amounts of computational power required for accuracy; the fact that we needed to run tons of these large, slow, computationally intensive simulations in order to cover the parameter space (spins, masses, orientations, etc) of potential sources of gravitational waves; and so on. For black holes, one major challenge was the fact that they contain a singularity. A singularity means an infinity, and computers don’t like to simulate infinities. Numerical relativity researchers had to find a way to simulate black holes without having the singularity point in the slicing of the spacetime integrated in the simulation. The first successful simulation of this kind didn’t happen until 2005 (

Once we had working simulations, groups around the world set to work on simulating the gamut of major potential gravitational wave signal sources. These simulation results were not just useful to the detectors to help identify signals, but also to the theorists to help formulate predictions about the results of such astrophysical events. Predictions such as: the resulting velocity of merged black holes from binaries of various spins, the amount of energy released by black hole mergers, the effect that black hole spins have on the spins and orbits of other bodies, etc.

When will you add gravitational waves into Universe Sandbox ²?

We really can’t do gravitational waves in an n-body simulation, which is the method Universe Sandbox ² uses to simulate gravity. N-body simulations look at the effect that each body has on each other body in a system at small discrete time steps.

General relativity requires simulating the spacetime itself. That is, taking your simulation space, discretizing it to a hi-res 3-D grid and checking the effect that each and every point in that grid has on all neighboring points at every timestep. Instead of simulating N number of bodies, you are simulating a huge number of points. You start with some initial data of the shape of your spacetime and then see how it evolves according to the Einstein equations, which are 10 highly non-linear partial differential equations. Accurate general relativity simulations require supercomputers.

There are some effects and features related to relativity that would be possible to add to Universe Sandbox ², however. Here are a few we are discussing:

  • Gravity travelling at the speed of light.

    • Currently if you delete a body in a simulation, the paths of all other bodies instantly respond to the change. The reality is that it would not be instantaneous; it would take time for that information about the altered gravitational landscape to reach a distant object.

  • Spinning black holes.

    • Most black holes are very highly spinning. If you imagine a spinning star collapsing it is easy to understand why. This is the same effect as when a spinning figure skater pulls in their arms; because of conservation of angular momentum, they spin faster. A consequence of this spin is that, while the event horizon would remain spherical, there would be an oblate spheroid (squished ball) around the black hole called an ergosphere. This ergosphere twists up the spacetime contained within it and accelerates bodies that enter this region (as well as affecting their spins). Because it is outside of the event horizon, this means one can slingshot away from this region and even steal energy from the rotation of that black hole.

  • Corrections to the motions of bodies to approximate general relativity.

    • Loss of momentum due to the emission of gravitational waves causes close massive bodies to inspiral. With this you could recreate the decaying orbits of binary pulsars.

    • Spins of close bodies affect each other’s motion and spins (see above). This would give you things like spun up accretion disks around black holes.

    • These corrections would be made by adding post-newtonian corrections to body velocities.


Learn more

Bad Astronomy article: LIGO Sees First Ever Gravitational Waves as Two Black Holes Eat Each Other

Video and Comic Explaining Gravitational Waves

Reddit AMA (Ask Me Anything) by LIGO Scientists

Paper by LIGO Researchers: Observation of Gravitational Waves from a Binary Black Hole Merger

Alpha 18.2 | Planet Nine | Now Available

Planet Nine

The discovery of a hypothetical ninth planet in our solar system was announced on January 20th, 2016 by researchers at the California Institute of Technology.

Universe Sandbox ² Alpha 18.2 features two simulations of Planet Nine. Run Steam to update, then check them out in Home -> Open -> Possible Planet Nine [and] Evidence of a Ninth Planet. 

Or buy now for instant access to Universe Sandbox ² on Steam Early Access:

The announcement comes after years of research into explaining the peculiar, but very similar, orbits of six small bodies orbiting beyond Neptune. Many theories have been proposed, but none has been as compelling as a very distant ninth planet pulling these bodies into their highly elliptical orbits. Using mathematical modeling, the two researchers, Konstantin Batygin and Mike Brown, have shown that a ninth planet fits very well into the data we have about objects in the Kuiper Belt and beyond.

There's only a 1 in 15,000 chance that the clustering of the orbits on the left is coincidental. Another explanation is the gravitational influence of a ninth planet, whose orbit is represented by the yellow line on the right. (from Universe Sandbox ²)

There’s only a 1 in 15,000 chance that the clustering of the orbits on the left is coincidental. Another explanation is the gravitational influence of a ninth planet, whose orbit is represented by the yellow line on the right. (from Universe Sandbox ²)

Planet Nine has not been directly observed yet by telescope, which is why it is hypothetical. But the researchers say there is a very good chance of spotting it in the next five years. It is suspected to be about 10 times the mass of Earth, similar in size to Neptune, with an orbit that’ll take it around the Sun every 10,000 – 20,000 years. 

Of course, we don’t know how Planet Nine got there. Brown and Batygin propose that this planet was formed in the early days of the solar system, along with Jupiter, Saturn, Uranus, and Neptune. Then it could have been shot outward by one of the gas giants, and instead of leaving the solar system entirely, it may have been slowed down by gas in the Sun’s protoplanetary disk, enough to keep it in orbit.

Universe Sandbox ² - Planet Nine Alt Angle

Alternative angle of Planet Nine (yellow orbital line) and the six objects used in the analysis. (from Universe Sandbox ²)

If the ninth planet does exist, then it will be the second time our solar system will have claim to nine planets… After, of course, Pluto was demoted in 2006. But Brown says there’s no question that the hypothetical ninth planet is indeed a planet. It’s likely much bigger than Earth, and has a large influence on other bodies in the solar system. And besides, Brown would know — his discovery of Eris was the reason Pluto was voted out.

Here’s a great discussion of Planet Nine by Mike Merrifield, an astronomer and professor at the University of Nottingham:

We’re hope you’re as excited about this possible discovery as we are! Make sure you check out the new simulations in Universe Sandbox ²: Home -> Open -> Possible Planet Nine [and] Evidence of a Ninth Planet.

See the complete list of What’s New in Alpha 18.2:  What’s New


Additional links about Planet Nine:

Astronomers say a Neptune-sized planet lurks beyond Pluto 

Evidence grows for giant planet on fringes of Solar System

Evidence for a Distant Giant Planet in the Solar System (research paper)

Simulating the World of Game of Thrones in Universe Sandbox ²

Universe Sandbox ² - 20150707-140519 UI

An Unpredictably Long Winter is Coming

Fans of George R. R. Martin’s fantasy series A Song of Ice and Fire or the television series, Game of Thrones, know well the often repeated warning, “Winter is coming.” 

For those living on the continent of Westeros in this fantasy world, summers can be long, and so can the winters. But some winters are especially cold and last for several years, while others are relatively mild and short.

What causes this variance in seasons?  Martin doesn’t offer an explanation, so we’re free to speculate.

One research paper (“Winter is coming”) proposes that perhaps there’s a natural explanation: Westeros is on a circumbinary planet, meaning its orbit extends around a binary star system.

Simulating  Westeros in Universe Sandbox ²

The paper may be tongue-in-cheek, but that doesn’t mean we can’t use its parameters to try simulating it in Universe Sandbox ². Like the paper, we were unable to find stable orbital parameters that would create the level of unpredictability discussed in the books or the show.

We could, however, create a system that has variable winter and summer intensities on regular predictable intervals with a large northern polar ice region. Though our results didn’t exactly match those in the paper, we managed to recreate similar seasonal patterns to what the authors describe in their paper.

If you own Universe Sandbox ², you can see this simulation for yourself in Alpha 15: Home -> Open -> Fiction -> Lands of Ice & Fire | Game of Thrones.

To open the temperature graph, open the Westeros planet’s Properties, select the Climate tab, hover over the Surface Temperature icon and click the Graph button.

If you don’t own Universe Sandbox ², you can buy it now to get instant access to the alpha via Steam code:

Universe Sandbox ² Alpha 15.2 | The Pluto Encounter Update

Universe Sandbox ² - New Horizons Pluto 2

If you already own Universe Sandbox ², just run Steam to update to the latest version.

Or you can buy Universe Sandbox ² here:

New Horizons

We’ve just released Alpha 15.2, which features a simulation of NASA’s New Horizons trip past Pluto and its moons. The spacecraft will be closest to the icy dwarf planet next Tuesday, July 14th. You can find the simulation in Home -> Open.

We will be updating Pluto’s and its moons’ textures as data is received from the New Horizons spacecraft.

If you keep the simulation running to 2019, you will see New Horizons approach its second target, 2014 MU69 (or PT1), an object with a diameter of 30-45 km orbiting in the Kuiper belt. New Horizons will likely be closer to PT1 than our simulation reflects, though, as NASA will be using a portion of its remaining fuel to get closer to its target.

You can also check out NASA’s own New Horizons simulation.

Recent Updates & Changes

In this update we’ve also made it possible to draw trails relative to a body and made additional tweaks and fixes.

In Alpha 15.1, released on June 26th, we updated the look of Ceres based on the latest photos from NASA, added a random asteroid feature, new moons of Pluto, pulsar jets, and improved the look of brown dwarfs. We also re-introduced the ability to customize launch bodies: Hover over bodies in the Add panel then press a number key to assign the body to that launch slot.

Full list of What’s New




Watch a Demo of Exoplanet Detection in Universe Sandbox ²

The video below was created by Eric, an astronomer working on Universe Sandbox ².

He explains how we calculate both radial velocity and normalized light curves, two key components in methods of detecting exoplanets.

You will be able to try out these new features for yourself in an upcoming alpha of Universe Sandbox ².

If you do not yet own Universe Sandbox ², you can buy it now to get instant access to the alpha, as well as free updates up to and including the final release:



Scott Manley Simulating the Universe for Fun

Watch “Astronogamer” Scott Manley run through a series of simulations in Universe Sandbox ² as he discusses a bit of the science behind it all:

Thanks, Scott!

Remember, Universe Sandbox ² is still in alpha, so there are many fixes and improvements that are on their way.

Buy Universe Sandbox ² and get instant access to the alpha on Steam for Windows, Mac, and Linux and pre-order the finished game:

Orbital Resonance in Universe Sandbox ²

Universe Sandbox ² is as much about breaking the rules as it is following them.  This is why you’re given the ability to add Saturn-like rings to any body, whether it’s Earth, Mars, or even the Sun. And it’s also why we allow you to put in all kinds of interesting but unrealistic shapes for the ‘rings,’ like spirals and cubes.  But, of course, we are also committed to incorporating and simulating as much science as we can. So we’ve included resonance gaps.

Adding realistic rings to the Sun produces several resonance gaps.

Adding realistic rings to the Sun produces several resonance gaps.

The basic idea here is that a larger body, like a moon (in the case of a ring around a planet), or a planet (in the case of rings around a star) creates gaps, or resonances, as seen in the picture above.

Here’s how it works (and for simplicity’s sake, just imagine circular orbits): According to Kepler’s third law, the distance an object is away from the planet determines how long it takes to orbit. That means that for an object at a given distance (and orbital period),  there must be a distance where the orbit takes only half the time, and another distance that takes three times as long. Whenever the two orbits take integer ratios of each other, we say the orbits are in resonance. For example, if the moon takes 28 days to orbit, there is a distance that only takes 14 days to orbit. This would be the 2:1 resonance (two orbits for every one orbit of the moon). If there is a disk present, and there is material at that distance, then every other time the material in the disk orbits, the moon has gone the whole way around, and the disk material comes as close to the moon as it can get — basically they are in the same part of their respective orbits. Each time that happens, the moon’s gravity pulls on the material just a little bit. For certain ratios of orbit periods, this little extra pull will clear the material out of that particular orbit. It’s really an amazing process that takes a fair bit of time to occur. But we see it in Saturn’s rings and we see it in the asteroid belt, where resonances with Jupiter have cleared out gaps (the Kirkwood gaps). And here is a great little animation showing resonance in Jupiter’s moons.

We calculate a handful of unstable resonances (3:1, 5:2, 7:3, 2:1, 7:6, and 1:1). So when you’re playing around with Universe Sandbox 2, and you include resonance gaps, you’ll see these rifts in the rings.  If you select the Sun and try to add rings, we find the planets that orbit the Sun, compute the resonances, and put gaps there, as pictured above. It’s pretty slick, if I do say so myself.

This feature includes the gaps when a ring is placed around an object. But Universe Sandbox is a gravity simulator, and this is a gravitational process. In principle, the gaps would develop on their own if you let the simulation run long enough. In practice, though, the simulation would need very, very high precision and to run for a very, very long time, and until we are running Universe Sandbox on a supercomputer, ring gaps won’t develop spontaneously.

You can also place your own rings by specifying the inner and outer boundaries of a single ring. So you could build up a single ring system with gaps anywhere you’d like them to be. We hope you have fun playing around with this feature when we make Universe Sandbox ² available for purchase. I sure had fun coding it up.

In other ring-related news, it was announced last week, to everyone’s surprise, that an asteroid in our Solar System called Chariklo has 2 rings. Here it is in Universe Sandbox ²:

Chariklo, Pluto, Mars, Venus, & Earth

Chariklo, Pluto, Mars, Venus, & Earth

Why do small stars not age past 12.6 billion years in Universe Sandbox ²?

Universe Sandbox ², currently in development, is a powerful gravity simulator that invites you to learn about our amazing universe and fragile planet via an expanding realm of realistic, interconnected astronomy and climate physics systems. Read more about this upcoming version here: The New Universe Sandbox.  Or find out how to purchase the currently available version at

Q:  In Universe Sandbox 2, small stars, such as red dwarfs, stop aging when they reach 12.6 billion years old.  Why is this?

A:  Stellar evolution is incredibly complicated. We understand the basics of it quite well, and for a lot of stars, our models do a really good job of matching up to measurements of real stars. It is, however, very hard to do this right, for several reasons:

1. We don’t have perfect data.

We can’t directly measure the mass or radius of a real star. And even something like the temperature of a star isn’t always easy to measure. This means that a lot of the data are rough estimations.
One really big issue is what astronomers call ‘metallicity’. Basically, what is the relative fraction of elements in a star? How much iron is there, relative to hydrogen, etc?

2. We can’t actually observe the evolution of a single star.

We can watch it, but so far, we’ve only been watching for maybe a few decades or so, depending on when you consider our technology to have been good enough to do any of this.  In star years, that’s not even a blink of the eye.  So we have to make assumptions about the way a star will age by looking at other, older stars. It’s like looking at a whole bunch of people and guessing how you will age by seeing what older people look like right now. You can make an estimation, but it won’t be exact, because how you age depends a bit on what you eat, what happens to you, your genes, etc., and these factors are inevitably different than those of the older folks. Similarly, we can guess what the sun will do, but it has slightly different properties of the stars we think it will look like.

3.  We can’t see inside a star.

And this is where most of the action happens. So this is all based on physics calculations. We’re good at that, but not perfect.

Hertzsprung–Russell diagram in Universe Sandbox. The HR Diagram (star's temperature versus luminosity) was introduced in the early 20th century and helped pave the way for a better understanding of stellar evolution.

Hertzsprung–Russell diagram in Universe Sandbox. The HR Diagram (star’s temperature versus luminosity) was introduced in the early 20th century and helped pave the way for a better understanding of stellar evolution.

Okay, now on to how this directly relates to Universe Sandbox ²:

Since stellar evolution is so hard, we let the full-time, professional astronomers compute the models. We’ve adopted a whole suite of these stellar evolution models, or “isochrones.”  These isochrones tell us the temperature, radius, and luminosity for a range of stellar masses and ages, and we turn that data into what you see in Universe Sandbox.

Unfortunately, although these isochrones are quite good and pretty accurate, they don’t give us predictions for very low-mass stars (less than 10% of the Sun’s mass) or very high-mass stars. And they don’t give us predictions for longer timeframes, such as 12.6 billion years (recall the age of the universe is a bit less than 14 billion years).

So this is what we’re stuck with for the time being. In the future, we’re planning on incorporating some other isochrones that are focused on lower-mass stars (which call for different physics, since the stellar atmospheres are very different; at low temperatures, molecules and even clouds can form in the stellar atmosphere, requiring different models), higher mass stars, different metallicities, and longer timeframes.

This is a work in progress, but we’ll be making it better and better as we go.

Follow the links below to read more about stellar evolution and some of the terms used in this post.

Impacting a Comet

More than 5 years ago on July 4, 2005, NASA crashed a 370 kg (815 lb) copper mass into the comet Tempel 1.

This impact kicked up more dust than expected and prevented the host spacecraft, Deep Impact, from getting a good photograph of the resulting crater.

Now more than 5 years later, another spacecraft, Stardust , has taken a photo of the impact site.
Before impact is on the left. After impact is on the right.

I have to say I’m disappointed by the result. The right photo appears blurry because it’s taken from much further away than the composite on the left and the crater isn’t very obvious even with the yellow arrows pointing it out.

What is amazing is that humans impacted a comet, then flew by it again with another spacecraft years later to take a follow up photo. Even thought the image isn’t as visually impressive as what one might expect from a collision, there’s lots for scientists to learn from it and what happens when you slam something into a comet at 10 km/s (about 1/3 the speed the Earth travels around the Sun).

You can learn more about the impact from Bad Astronomy’s analysis of this event and their follow up post.

In Universe Sandbox you can slam moons into the Earth, Earths into Jupiters, or Jupiters into Sun to your hearts content.

Launch Earth at Jupiter

  1. Download & Install Universe Sandbox (it’s free and includes a 60 minute trial of all the premium features, including the add and launch tools)
    You may want to run through the short tutorial to get a feel for how to navigate in the simulator.
  2. Open the Jupiter & Moons simulation.
  3. Select the Add Tool (the Saturn icon with the +) and then select the Earth icon.
  4. Select the Launch tool (looks like a crosshair) and click on Jupiter to launch Earth at it.
  5. Keep clicking to launch more than one.