Habitable Worlds Lab Help

Your first experiment

An exoplanet is a planet outside our solar system. A world is habitable for a particular kind of life if its environment can support that life. In this lab, you’ll look for a rocky world with surface liquid water and conditions roughly like the ones humans experience on Earth.

1. Open the lab with Close-in flare world selected. Read Incoming starlight and High-energy exposure index. The first is the energy that can warm the planet. The second is a separate estimate of the stress that energetic radiation can put on the upper atmosphere.
2. Click Load 3D planet view and wait for the progress bar to finish. Then click Play 5 billion years and watch the globe as the age advances. The atmospheric rim thins and the surface can change as the modeled conditions change. Follow the current world type, remaining atmosphere, and temperature below the play button. The starting flare world finishes as a barren desert with about 8% of its atmosphere left.
3. Move the Atmosphere over time slider back to zero, then change only Stellar activity from Flare-active to Quiet. Replay the timeline. The incoming starlight stays the same in the lab, while the high-energy index and rate of atmospheric loss change.
4. Read the World after 5 billion years card. It names the final world and says Challenge won or Challenge lost. Compare what happened to the atmosphere and oxygen over the run. Change one setting, then replay and compare. Then visit Observe your planet from Earth to see which signals a simulated observer could measure.

Two play buttons, two clocks. Play motion animates the orbit, spin and flares. The planet’s age holds steady. Play 5 billion years advances the climate and atmosphere. You can run the time-lapse with the orbital animation paused.

What each control changes

Camera and scene

Orbit view and Follow planet

Orbit view frames the whole system. Follow planet keeps the camera near your world as it travels. Drag the scene or use Turn left, Turn right, Tilt up and Tilt down. Zoom and Full screen change how much of the scene you see.

Orbit line, star, rim and gas tail

The line traces the modeled orbit. The planet can pass behind the star from some camera angles. The stellar surface and flares are illustrations. The colored rim represents an atmosphere. A blue tail illustrates escape stress and can fade when seen end-on. Use the readouts to judge loss. The scene compresses sizes, distances and speeds so you can see the system.

One orbit takes

The orbital period is the time it takes to complete one trip around the star. It is the length of that planet’s year, given in Earth days or Earth years. A closer orbit around the same star generally takes less time. The scene animation uses its own playback speed.

Planet settings

Starting world

Choose a preset to load a fresh experiment at year zero. Earth reference gives you a comparison. Close-in flare world is the opening challenge. Ice world, Global ocean world, Mars-like desert, Gas giant, Neptune-like world, and Rocky super-Earth let you explore other outcomes. Changing a setting marks your design Custom.

Planet type

A rocky planet has a solid surface where the lab can estimate temperature, pressure, and water conditions. Jupiter-like and Neptune-like worlds have deep gaseous envelopes and use the observing tools. The rocky surface and atmospheric-loss timeline do not apply to them. A super-Earth is a size or mass category. The lab illustrates a rocky example at twice Earth’s mass or more when the surface is temperate with land and water.

Water supply

Choose exposed land with oceans, a global ocean, or a dry surface. This tells the model whether water is available. Temperature and pressure then determine whether it passes the liquid-water screen. A world can fail the land requirement and still be interesting to astrobiologists.

Host star

Sun-like stars, orange dwarfs and red dwarfs differ in mass, size and luminosity. Luminosity is the total power a star gives off. The lab uses representative stars with 100%, 25%, and 2% of the Sun’s luminosity, respectively. A dim red dwarf can deliver Earth-like starlight at a much closer distance.

Stellar activity

Quiet and Flare-active set the level of high-energy radiation used by the model. Flares are bursts of energy from a star. Ultraviolet and X-ray radiation can heat upper atmospheres, and stellar particles can drive other kinds of escape. The lab multiplies its exposure index by 12 in the active setting. Real stars vary widely over time.

Orbital distance

Distance from the star in astronomical units (AU). One AU is about the Earth–Sun distance. Move twice as far from the same star and each square meter receives one quarter as much starlight. Distance also affects the modeled high-energy exposure and the orbital period.

Rocky planet mass

Mass tells you how much matter the planet contains. One Earth mass is written as 1 × Earth. The lab estimates a larger radius for a heavier rocky planet and uses both values to calculate escape speed. Changing the mass can also change the transit depth because the planet’s disk gets bigger or smaller.

Dominant gas

Choose the gas that makes up most of the atmosphere. The model uses nitrogen (N₂), carbon dioxide (CO₂), or hydrogen (H₂) as the representative molecule for its escape calculation. At the same temperature, lighter molecules move faster on average. Choose No atmosphere to explore a bare world. The greenhouse setting is a separate control.

Greenhouse warming

Some gases absorb infrared energy emitted by a planet and help warm its surface. This control adds a starting temperature increment, such as +25 °C / +45 °F. For a temperature change, multiply by 1.8 to get the Fahrenheit change. The 32-degree offset is used when converting a temperature reading. In the lab, the increment shrinks as the modeled atmosphere thins. In reality, warming depends on the gases, their concentrations, clouds and other feedbacks.

Light reflected

This percentage is the planet’s albedo. A value of 30% means the planet reflects 30% of the incoming starlight and absorbs 70% in the model. All else equal, a higher value leaves less energy to warm the world.

Starting surface pressure

Pressure is force per unit area. At the surface, the weight of the atmosphere above you contributes to that pressure. One bar is 100,000 pascals, close to standard sea-level pressure on Earth. The slider sets the starting pressure. The readout shows its value at the current age.

Starting oxygen share

The starting percentage of the air made up of oxygen (O₂). Earth’s dry air is about 21% oxygen. The percentage tells you the mix. To compare the amount of oxygen available at different total pressures, read Oxygen partial pressure.

Oxygen replenishment

Steady source keeps the oxygen share constant in the model as the total atmosphere changes. No ongoing source lets the oxygen share decline on a prescribed schedule. On Earth, photosynthesis supplies oxygen and processes such as respiration and reactions with rock consume it. This switch illustrates a balance between sources and sinks.

How to read the indicators

The Earth baseline column is a fixed comparison. The Your planet column responds to your settings and the selected age.

Incoming starlight

Power received per square meter relative to Earth. 1.00 × Earth is the reference, and 1.25 × Earth is 25% more. This value stays fixed through a run while the star and distance are fixed.

Estimated surface temperature

A single global estimate in °C and °F based on absorbed starlight plus the selected greenhouse warming, scaled by the remaining atmosphere. Real worlds have nights, seasons, local climates and weather.

Escape speed

The speed needed to escape the planet’s gravity from its surface without further propulsion, shown in kilometers per second. Earth’s reference is about 11.2 km/s. The lab compares this speed with the typical speed of the selected gas molecules in the heated upper atmosphere.

High-energy exposure index

An illustrative comparison to the quiet Earth reference. The lab flags values greater than 6 × Earth. This index is a teaching tool for atmospheric escape, with no direct conversion to a radiation dose at the surface.

Atmosphere remaining

The fraction of your own starting inventory left. 40% means 60% of that inventory has been lost. The starting inventory may correspond to a thicker or thinner atmosphere than Earth’s, depending on the pressure setting.

Surface pressure and oxygen partial pressure

Total pressure is the pressure of the whole mixture. Partial pressure is one gas’s share of that pressure. Multiply total pressure by the oxygen fraction. At 1.00 bar and 21% oxygen the result is 0.210 bar. At 0.50 bar and the same 21% it’s 0.105 bar.

Surface water

The lab checks water supply, its 0.10-bar pressure cutoff, and a broad temperature window around 0–100 °C (32–212 °F). Actual freezing and boiling depend on pressure and composition. A frozen surface may still hide an ocean.

Atmosphere

Reports whether the selected gas passes the thermal retention screen and whether high-energy exposure adds stress. A gas can pass the thermal screen while its atmosphere still thins over time. Real worlds can lose gas through multiple mechanisms.

Air for humans

Compares the dominant gas, total pressure, and oxygen partial pressure with the challenge targets at the current age. Passing this check means those values fit the lab’s ranges. A real assessment also needs data about harmful gases, radiation, temperature extremes, and other conditions.

Follow five billion years of change

Click Play 5 billion years or move the Atmosphere over time slider to a specific age. The progress bar and current world readout follow the planet as it changes. The final card compares the starting and ending temperature, reports the remaining atmosphere and final pressure, and gives one clue if the world loses. Under How your planet evolved, compare its starting conditions, its state at 2.5 billion years, and its final state. The explanation connects these changes to your settings. Use Resume to continue and Replay to start from year zero. View the world’s history records the starting world and the billion-year milestones reached under the current settings. The table shows age, world appearance, estimated temperature, atmosphere remaining, and oxygen partial pressure.

The selected world loses a fraction of what remains over equal spans of time. That produces exponential decay. For example, losing 20% each interval would leave 80% after one interval and 64% after two. The lab makes loss faster under high-energy exposure or weak thermal retention. As the atmosphere thins, pressure and the greenhouse contribution can fall even though the star’s brightness stays fixed.

Understand the challenge result

Complete a full playback to see the verdict. Use the age slider to inspect a moment in the simulation. If you start playback after scrubbing, the challenge begins at year zero.

The challenge looks for a rocky planet around a red dwarf that sustains Earth-like air and climate, with oceans and exposed land. If you lose, use the clue to decide what to investigate. Try changing one setting per run so you can tell what made the difference.

Habitability depends on the kind of life you’re looking for. Early Earth supported life long before its air resembled today’s. Some life can thrive beneath an icy surface or near deep-sea vents. Explore Oceans Beyond Earth.

Observe your world from Earth

Transit light curve

A transit happens when the planet passes between us and its star and covers some of the star’s disk. A light curve plots the change over time. In this chart, deeper downward dips mean more starlight was blocked. The vertical axis uses parts per million (ppm). Ten thousand ppm equals 1%. The horizontal axis shows hours before and after the midpoint. The dot marks the current phase when it’s inside the plotted window. The readout reports the mid-transit and current amounts blocked.

Transit alignment is the closest projected distance between the centers of the star and planet, measured in stellar radii. Zero crosses the center. Near one stellar radius, the planet may graze the edge. Beyond the edge plus the planet’s radius, the curve stays flat. Time around transit lets you scrub the marker through the dip. At a full crossing in this uniform-star model, the blocked fraction is about (planet radius ÷ star radius)². An Earth-size planet crossing a Sun-size star blocks about 84 ppm. The same planet can produce a deeper dip around a smaller star.

Analyze a real Kepler light curve in the Exoplanet Transit Lab.

Transmission spectrum

During a transit, some starlight filters through the planet’s atmosphere. Molecules absorb particular wavelengths, or colors of light, and the planet can appear to block slightly more of the star at those wavelengths. The horizontal axis uses micrometers (µm), where 1 µm is one millionth of a meter. The vertical axis shows the extra light blocked by gas in ppm. Peaks mark stronger modeled absorption. The lab can illustrate oxygen, water vapor, carbon dioxide and methane under selected conditions.

If the planet misses the star, there is no modeled transmission spectrum. A depleted atmosphere or a combination with no supported bands can also leave the plot flat. Actual spectra are affected by cloud and haze, stellar activity, instrument noise, and other gases with overlapping features. A gas detection alone cannot establish whether a world has life, breathable air, or oceans.

Explore how light reveals chemistry in the Spectroscopy Lab.

The science behind the model

Sources and further reading

• NASA Science · The Habitable Zone
• NASA Webb · Observing a world around a red dwarf
• NASA Exoplanet Archive · Equilibrium temperature and incident flux
• NASA Technical Reports Server · Atmospheric escape
• NASA Science · Earth’s atmosphere
• NASA Science · What’s a transit?
• NASA Webb · Transmission spectrum of an Earth-like atmosphere
• NASA Astrobiology · Detecting life’s influence on atmospheres

Return to the Habitable Worlds Lab. If you opened this guide from the lab, switch back to the original tab to keep working on your current world.

Teaching with the lab

Open the classroom worksheet and educator notes.