# Planetary atmospheres and habitability claims

> A simple equilibrium-temperature model balances absorbed stellar power and emitted thermal power. Albedo, redistribution and greenhouse effects require separate treatment.

Course: [Orbits, missions and exoplanets](https://studybrix.com/en/courses/orbits-missions-and-exoplanets/index.md)
Level: university
Prerequisites: Radiation, chemistry and scientific inference.

## Notes

### Energy balance

A simple equilibrium-temperature model balances absorbed stellar power and emitted thermal power. Albedo, redistribution and greenhouse effects require separate treatment.

### Spectroscopy

Atmospheric inference compares measured spectral features with models. Clouds, chemistry and instrument effects can produce degeneracies.

### Habitability

A habitable-zone label concerns a model of conditions compatible with surface liquid water; it is not evidence that water or life actually exists there.

### Claims

Separate observed features, model-dependent estimates and speculation. Strong claims need independent checks and quantified uncertainty.

## Practice questions

### 1. Does being in a model habitable zone prove a planet has life?

A. Yes, by definition
B. No
C. Yes, if it has any atmosphere
D. Only if its name is short

Answer: No

Explanation: The label is a model-dependent potential-condition statement.

### 2. A surface reflects fraction 0.3 of incident power. Find the absorbed fraction in a simple no-transmission model.

Answer: 0.7

Explanation: Absorbed fraction=1−albedo.

### 3. What can create an atmospheric retrieval degeneracy?

A. A perfectly known temperature always
B. Different models fitting similar spectra
C. Eliminating all uncertainty by renaming parameters
D. Only one possible atmosphere in every case

Answer: Different models fitting similar spectra

Explanation: Similar observational signals need not imply a unique atmosphere.

### 4. Distinguish equilibrium temperature from an actual measured surface temperature.

Sample answer / marking guidance: Equilibrium temperature follows a specified radiation-balance idealization. Actual surface conditions also depend on atmosphere, circulation and internal energy, and may not be directly measured.

Explanation: Credit modelling assumptions and additional processes.

### 5. Absorbed power is 80 W in a simple steady balance with no storage. Find emitted power.

Answer: 80 W

Explanation: Steady energy balance equates absorbed and emitted power in the stated model.

### 6. Why should a single tentative spectral feature not be called a confirmed biological signal?

Sample answer / marking guidance: Noise, instrumental effects and nonbiological chemistry may offer alternatives. Robust detection, model comparison and independent evidence are required.

Explanation: Credit competing explanations and confirmation.

### 7. What is albedo in a simple planetary energy model?

A. Planet mass in kilograms
B. The fraction of incident radiation reflected
C. The fraction of stars with planets
D. The speed of gravity

Answer: The fraction of incident radiation reflected

Explanation: Albedo describes reflectivity under the specified spectral/geometry convention.

### 8. If reflected fraction rises from 0.2 to 0.4 at fixed incident power, find new absorbed power divided by old.

Answer: 0.75

Explanation: The absorbed fractions change from 0.8 to 0.6, so the ratio is 0.6/0.8.

## Further reading

- https://science.nasa.gov/exoplanets/

Original AI-assisted educational draft; not independently teacher-reviewed. References support further study, not copied exercises, affiliation or curriculum accreditation.