Planet P is the least dense of the planets of the solar system. It was first visited by NASA's Pioneer II in 1979.
Which of the following could be planet P ?

Options

A. Earth
B. Jupiter
C. Uranus
D. Saturn

Explanation

To solve the question about which planet P is, we can follow these steps:

### Step 1: Understand the properties of the planets
- We need to identify the characteristics of the planets listed in the options: Earth, Jupiter, Uranus, and Saturn. Specifically, we are looking for the least dense planet.

### Step 2: Recall the densities of the planets
- The densities of the planets in the solar system vary. Generally, gas giants (like Jupiter, Saturn, Uranus) have lower densities compared to terrestrial planets (like Earth).

### Step 3: Identify the least dense planet
- Among the gas giants:
- Jupiter has a density of about 1.33 g/cm³.
- Saturn has a density of about 0.69 g/cm³.
- Uranus has a density of about 1.27 g/cm³.
- Earth, being a terrestrial planet, has a density of about 5.52 g/cm³.
- From this, we can see that Saturn has the lowest density among the listed planets.

### Step 4: Check historical data
- The question mentions that Planet P was first visited by NASA's Pioneer II in 1979. We need to confirm which planet was visited by Pioneer II.
- NASA's Pioneer 11 (not Pioneer II) was the spacecraft that flew by Saturn in 1979.

### Step 5: Conclusion
- Based on the information gathered, we can conclude that Planet P, being the least dense planet and visited by NASA's Pioneer spacecraft in 1979, is Saturn.

### Final Answer
- Planet P is Saturn.

---