Directions for teachers:

Ask your students to read the Science News article “Heavy metal may rain from the sky of an exoplanet,” then collaborate to answer the partner questions about phase changes below. Finally, prompt students to imagine their own exoplanet and its atmosphere and develop related phase diagrams before sharing responses with classmates.

Want to make it a virtual lesson? Post the Science News article “Heavy metal may rain from the skies of planet WASP 76b” to your online classroom and ask students to read the article. Pair up students to discuss and answer the partner questions below using a video conferencing platform, or talking by phone. They can collaborate in a shared document during the conversation. Post the class discussion prompt to an online discussion board. Have students respond individually and give feedback or ask a question about a classmate’s response.

Partner questions

1. What substance was identified as a gas in the atmosphere of exoplanet WASP 76b? When was the gas seen and when was it not seen in the atmosphere? What do scientists think happens to the substance over the course of a day and night on the planet? What does that mean in terms of the substance’s phase of matter? (In explaining your answer, it might be helpful to sketch what happens to the substance during the transition from daytime to nighttime and back again.)

Iron gas was identified in the atmosphere of exoplanet WASP 76b where the planet transitions from daytime to nighttime. As nighttime approaches, scientists think that gaseous iron in the atmosphere condenses, meaning it turns from a gas to a liquid, and during the night that liquid iron falls toward the center of the planet.   

2. Name the phases of matter of a substance, and describe the general differences in their shape and volume. Explain your answers in terms of the phase’s relative strength of intermolecular attraction forces, or the forces of attraction between the atoms or molecules within a substance.

Atoms and molecules can exist in the solid, liquid or gaseous phase. Solids have a fixed shape, and liquids and gases take the shape of their container. Solids and liquids have a fixed volume, and gases take the volume of their container. The intermolecular attraction between atoms or molecules of a substance in a solid is stronger than the attraction between atoms or molecules in a liquid, which is stronger than the attraction within a gas. That’s why gaseous molecules are free to move within a space, and atoms or molecules within a solid are essentially stuck.

3. Based on the article, what factor affects the phase of matter of a substance? Explain.

The article mentions that the temperature of a substance can affect its phase of matter. Where the atmosphere was transitioning from the dayside to the nightside on WASP 76b, an extreme decrease in temperature, almost a 1,000-degree Celsius decrease, appears to have caused the iron to condense and change phase from a gas to a liquid.   

4. A phase diagram shows how temperature and external pressure on a substance affect its phase of matter. Draw a general phase diagram including regions for solid, liquid and gas. Use this Purdue University page as a reference, if needed. Label your axes with appropriate titles and units, and include regions for each phase of matter.

See the example on the website provided. The y-axis should be titled “Pressure (in atmospheres)” and the x-axis “Temperature (in degrees Celsius).” Starting near the origin and moving clockwise by region, the phases of matter are solid, liquid and then gas.    

5. A normal boiling point is the temperature at which a substance in the liquid phase becomes a gas at an external pressure of 1 atmosphere. The term “normal” refers to an external, or atmospheric, pressure equal to 1 atmosphere. Assume that you drew the phase diagram for H₂O in the question above. Mark the normal melting, boiling and condensing points for water on the phase diagram, and appropriately label the points with pressure and temperature values. Then, explain how condensation differs from boiling in terms of energy. (Please note: The phase diagram for H₂O would actually have a negatively sloping line between the solid and liquid phase.)

Students should identify a pressure of 1 atmosphere on the graph and mark where the phase boundaries intersect that pressure. Moving from left to right, students should label the first point of intersection “freezing point” and the second point of intersection as both “boiling” and “condensing” points. The first point should be labeled: 1 atmosphere, 0° Celsius. The second point should be labeled: 1 atmosphere, 100° Celsius. In order for a substance to condense, energy must leave the system. The opposite is true for boiling.

6. How can a change in pressure affect the phase of matter of a substance at a constant temperature (assume the temperature is one where all three phases of matter can exist)? Explain your answer using the diagram and your knowledge about the strength of attraction between molecules.

At a specific temperature, as the pressure increases, the atoms or molecules are forced more closely together, strengthening their intermolecular attraction. At a temperature where all three phases of matter can exist, increasing the pressure of a gas would make it a liquid. With further increases in pressure, the liquid would become a solid. You can see these phase transitions on the graph if you follow a vertical line that passes through all three phase regions.      

7. Based on the info in the article and your knowledge of iron, how do you think the phase diagram for iron would compare with the phase diagram of H₂O? Explain. Then draw a quick sketch of iron’s phase diagram. Draw a line to represent the phase transition described in the article. Mark any data points that you can on the diagram.

The phase diagram of iron would likely have the same general shape as that of H₂O, but the boundaries between phase regions would occur at much more extreme temperatures and pressures relative to H₂O’s phase diagram. From the article, the condensing and boiling points would be over 1,000° Celsius, for example, where H₂O’s is 100° Celsius. On the phase diagram for iron, students should draw a horizontal line from the gas to the liquid region and indicate a change in temperature of nearly 1,000 degrees Celsius.  

8. How do you think Earth’s atmosphere compares with that of WASP 76b’s? Explain your answer in terms of temperature and pressure and potential composition.

WASP 76b’s atmosphere is considerably warmer than Earth’s atmosphere during the day and night, so it may contain more elements with higher boiling points as gases. Iron may be one of many heavy metals in WASP 76’s atmosphere. Also, smaller molecules made up of lighter elements that exist in Earth’s atmosphere, such as water or carbon dioxide, might not be able to exist in the liquid phase in the extreme environment of WASP 76. We don’t have much information on the relative atmospheric pressure of WASP 76b compared with Earth. If gases exist in the same abundance in both atmospheres, the higher temperature on WASP 76b might make its atmospheric pressure higher, too. Gases at higher temperatures move faster and exert a higher pressure.

9. What are some factors of the atmosphere that affect weather patterns? Given these factors, what might contribute to differences in wind weather patterns on WASP 76b and Earth? How does the “iron weather” on WASP 76b compare with “water weather” on Earth?

Air temperature, air pressure, humidity, wind speed and direction are all aspects of the atmosphere that affect weather patterns. WASP 76b has extreme temperature differences between daytime and nighttime that seem to cause at least one heavy metal to condense and potentially rain nightly. The stark temperature changes likely cause other weather events, such as high winds. On Earth, there are not temperature differences that are so extreme, and the phase of water molecules depends on air pressure and temperature. Water vapor in Earth’s atmosphere condenses into clouds, which under the right conditions will cause rain, releasing the water out of the atmosphere to fall toward Earth.

Class discussion prompt

Imagine your own exoplanet with an atmosphere different from Earth’s. Explain the general conditions on the planet, as well as the atmospheric composition and general weather patterns that make the planet special. Research an additional phase diagram to inform your answer. Include at least two data points from a substance’s phase diagram in your response. For additional inspiration, check out NASA’s Exoplanet Exploration, which describes known exoplanets.