6  Unit 1: Closing — Which Exoplanet Is Most Earth-Like?

Constructing your argument from evidence

HS-ESS1-1 HS-ESS1-3 HS-ESS1-4 Time: 1–5 Days

🔭 Which Exoplanet Is Most Earth-Like? 🔭

7 The Performance Task

7.1 🎯 Your Task

Which exoplanet is most likely to support life?

Using your explanatory models for what has made Earth the only habitable planet in our solar system, argue from evidence about which exoplanet in the data set is most likely to be habitable.

Your argument must:

• ✅ Make a clear claim about which exoplanet is most habitable
• ✅ Support your claim with evidence from all three lesson sequences (Sun, Stars, Orbits)
• ✅ Include scientific reasoning connecting your evidence to your claim
• ✅ Address at least one counterargument (why someone might pick a different planet)
• ✅ Reference your revised model of habitability

8 The CER Framework

8.0.1 📌 CLAIM

“Based on the evidence, [exoplanet name] is the most likely to support life because [brief reason connecting star properties, stability, and orbital features].”

8.0.2 📊 EVIDENCE

You need at least one piece of evidence from each chapter:

• ☀️ Sun chapter: Star composition, fusion process, energy output stability
• ⭐ Stars chapter: Star mass, lifespan, H-R position, spectral type
• 🪐 Orbits chapter: Habitable zone position, eccentricity, Kepler’s Law calculations

8.0.3 🧠 REASONING

Connect your evidence using the mechanisms you’ve learned: Why does star mass determine lifespan? Why does distance determine temperature? Why does eccentricity matter for water?

9 Evidence Review

9.0.1 ☀️ From How the Sun Works

Evidence	What It Shows
Spectroscopy reveals 73% H, 25% He	Sun’s composition enables sustained fusion
Nuclear fusion: 4H → He + energy	Mechanism of energy production
Sun outputs \(3.8 \times 10^{26}\) W steadily	Stable energy for 4.6 billion years
Only fusion explains duration + scale	Chemical/gravitational sources insufficient

9.0.2 ⭐ From Star Life Cycles

Evidence	What It Shows
Mass-lifespan relationship: \(L \propto M^{-2.5}\)	Higher mass → shorter life
H-R diagram position	Main sequence stars are stable
Stars > 8 M☉ → supernova	Massive stars destroy planets
G/K type stars: 10–17 Gyr lifespans	Long enough for complex life

9.0.3 🪐 From Planets & Orbits

Evidence	What It Shows
Kepler’s Third Law: \(T^2 = a^3\)	Period reveals distance
Habitable zone depends on luminosity	HZ = \(\sqrt{L/1.1}\) to \(\sqrt{L/0.53}\) AU
Low eccentricity needed	Planet must stay in HZ all orbit
Earth: \(e = 0.017\), always in HZ	Model for ideal orbit

Plot = require("@observablehq/plot")

ptData = [
  {name: "Kepler-442b", starMass: 0.61, starType: "K", lifespan: 28, distance: 0.409, ecc: 0.04, inHZ: "Yes", score: 9},
  {name: "Kepler-452b", starMass: 1.04, starType: "G", lifespan: 9.2, distance: 1.046, ecc: 0.035, inHZ: "Yes", score: 10},
  {name: "TRAPPIST-1e", starMass: 0.089, starType: "M", lifespan: 100, distance: 0.029, ecc: 0.005, inHZ: "Yes", score: 7},
  {name: "Kepler-22b", starMass: 0.97, starType: "G", lifespan: 10.8, distance: 0.849, ecc: 0.0, inHZ: "Yes", score: 8},
  {name: "55 Cancri e", starMass: 0.91, starType: "K", lifespan: 12.5, distance: 0.015, ecc: 0.17, inHZ: "No", score: 2}
]

Plot.plot({
  title: "Exoplanet Habitability Scorecard",
  subtitle: "Scored on star type, lifespan, HZ position, and eccentricity (10 = most Earth-like)",
  width: 700,
  height: 300,
  marginLeft: 120,
  x: {label: "Habitability Score (0–10)", domain: [0, 10]},
  y: {label: null},
  color: {range: ["#2ecc71", "#f39c12", "#e74c3c"]},
  marks: [
    Plot.barX(ptData, {
      y: "name",
      x: "score",
      fill: d => d.score >= 8 ? "#2ecc71" : d.score >= 5 ? "#f39c12" : "#e74c3c",
      sort: {y: "-x"}
    ,
        tip: true}),
    Plot.text(ptData, {
      y: "name",
      x: "score",
      text: d => `${d.score}/10 — ${d.starType}-type, ${d.lifespan} Gyr, e=${d.ecc}`,
      dx: 5,
      textAnchor: "start",
      fontSize: 10
    })
  ]
})

9.0.4 📝 Write Your Argument

Construct a written argument (300–500 words) or prepare a 2–3 minute oral presentation that:

1. States your claim for which exoplanet is most habitable
2. Provides evidence from star properties, stability, and orbital data
3. Explains the scientific reasoning connecting evidence to claim
4. Addresses why someone might argue for a different exoplanet
5. Discusses: Is traveling to an exoplanet really a viable option for humans?

10 Reflection

10.0.1 🔄 How Has Your Thinking Changed?

1. Compare your initial model from the Opening to your final model. What did you add?
2. What was the most surprising thing you learned about what makes Earth habitable?
3. Do you think humans will ever travel to an exoplanet? Why or why not?
4. What questions do you still have about habitability?

10.1 🎓 What You Figured Out in Unit 1

Question	Answer
How does the Sun produce energy?	Nuclear fusion: 4H → He + energy (\(E = mc^2\))
Why has the Sun been stable?	Balance of gravity and fusion pressure; 10 Gyr fuel supply
What makes a star good for life?	G or K type, 0.5–1.5 M☉, main sequence, > 4 Gyr lifespan
How do we know star composition?	Spectroscopy — absorption line patterns
What determines a habitable orbit?	Kepler’s Laws + habitable zone + low eccentricity
Which exoplanet is most Earth-like?	Your argument from evidence!