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What's real and what's simulated

Gravity Lab uses real data wherever it exists, and labels the rest clearly.

A model of the universe has to fill in gaps. Nobody has photographed a planet in the Andromeda Galaxy or mapped the far side of every exoplanet. Gravity Lab's rule is simple: use real measurements wherever they exist, base everything else on what science says is likely, and never present a guess as fact.

Real

The Solar System

• Planets, moons, dwarf planets, asteroids and comets move along their published orbits, worked out for the exact date on Gravity Lab's clock.
• Spacecraft such as Voyager 1 and 2, the Pioneers, New Horizons, Parker Solar Probe, JUICE, Europa Clipper, Psyche, Lucy, BepiColombo and Solar Orbiter follow trajectories from NASA/JPL's Horizons system.
• Earth satellites, including the International Space Station, use live orbit data from CelesTrak.
• Rovers and landers sit at their real landing sites on Mars and the Moon.

The surfaces

• Earth uses satellite imagery and real elevation data, detailed enough to fly down to individual mountains.
• Mars uses the Viking colour mosaic, sharp images from the CTX camera and the MOLA laser altimeter's elevation map.
• The Moon uses NASA's mosaics and elevation from the LOLA laser altimeter on the Lunar Reconnaissance Orbiter.
• Venus is mapped from the Magellan spacecraft's radar. Its atmosphere's temperature, pressure and winds follow measurements from the Pioneer Venus and Venera probes.

Beyond the Solar System

• Stars sit at their real positions and distances, with real sizes and colours.
• Exoplanets come from NASA's Exoplanet Archive: about 6,300 confirmed planets around about 4,700 stars, on their measured orbits. Where a planet's size hasn't been measured, it's estimated from its mass using a published relation (Chen & Kipping, 2017).
• Nebulae, star clusters, black holes and galaxies are placed at their real sky positions and distances.
• Inside other galaxies, the named objects astronomers have actually seen are included: giant star-forming nebulae, star clusters, supernovae and record-breaking stars. That includes the only two planet candidates ever reported beyond the Milky Way, PA-99-N2 b in Andromeda and M51-ULS-1b in the Whirlpool Galaxy.

Modelled from real measurements

Some things can't be photographed in full, but their structure is well measured. Gravity Lab builds these from the measurements:

• Galaxy shapes. Each detailed galaxy is built from its real structure: the size of its central bulge, the number and winding of its arms, any bar, rings, dust lanes or jets, and the tilt it shows from Earth. It's drawn with tens of thousands of stars. The result matches what telescopes see from Earth, and lets you fly around to views no telescope can reach. The individual stars, though, are illustrative.
• Black holes. The appearance comes from ray-tracing light through the black hole's gravity. See How black holes look.

Simulated, and always labelled

No planet has been confirmed in any other galaxy. They're far too distant for today's telescopes. Rather than leave other galaxies empty, Gravity Lab gives each one five simulated star systems of the kind it most likely holds. They're built from what we do know:

• the mix of stars each type of galaxy contains (old elliptical galaxies have no young blue stars, while starburst galaxies are full of them);
• how often planets appear around stars like those in our own galaxy, from the Kepler and TESS surveys (small planets are common, giant planets rarer, and rarer still where heavy elements are scarce);
• orbits spaced the way stable planetary systems space them;
• each world's temperature, worked out from its star's light.

Every one of these systems and planets is marked SIMULATED in its information card. In Andromeda they're named M31 Sim-1 to M31 Sim-5. In search results they are tagged "simulated system" or "simulated world".

✨ Explore a Star is simulated too. It generates a new star system on the spot, following the same rules, so you can visit somewhere no one has charted.

Sandbox Mode

Sandbox Mode starts from the real Solar System at the current moment. After that, what happens is the result of your changes and the physics, a simulation of what would happen. Reports explain what happened and why, and impact estimates are clearly given as estimates.

Spotted a mistake?

If something is in the wrong place, out of date, or labelled wrongly, please email [email protected]. Corrections are fixed quickly.