From the passage about seven Earth size exoplanets around the star TRAPPIST 1, this comprehension question asks you to identify which factor is described as essential for the possibility of life on a planet, according to the author of the passage.

Introduction / Context:
The passage on the TRAPPIST 1 system discusses why the discovery of seven Earth size exoplanets is exciting for the search for life beyond our solar system. It highlights that all seven planets could possibly have liquid water and that three of them have the greatest chance. In many scientific discussions, liquid water is treated as a key requirement for life as we currently understand it. This question checks whether you can identify precisely which factor the passage emphasises as essential for the possibility of life on a planet, based on the author description rather than general scientific knowledge alone.

Given Data / Assumptions:

• The passage mentions seven Earth size planets orbiting a dwarf star.
• It states that all seven could possibly have liquid water, with three having the greatest chance.
• It specifically calls liquid water a key to life as we know it on Earth.
• The options list several physical conditions that can influence habitability, such as atmosphere, gravity, sunlight, liquid water, and magnetic field.
• The question asks for what is essential to have the possibility of life, according to the passage.

Concept / Approach:
In reading comprehension, it is important to distinguish what the passage explicitly says from what you might know from science or other sources. The safest approach is to locate the phrase where the author directly relates a condition to the possibility of life. The passage states that liquid water is a key to life as we know it on Earth. It also explains that the Goldilocks zone is defined partly by the potential presence of liquid water on a planet surface. Therefore, when the question asks what is essential for life possibility, the answer must be the condition that the author calls a key to life, which is the presence of liquid water.

Step-by-Step Solution:

Verification / Alternative check:
To verify, compare each option with the passage. Presence of atmosphere, gravity, sunlight, and a magnetic field are important for planetary conditions but are not specifically labeled as keys to life in the text. Only liquid water receives that special emphasis. Additionally, the description of the Goldilocks zone highlights that planets in this region are at a distance where liquid water could exist on their surfaces. Because the passage repeatedly links the idea of potential life with liquid water and does not give similar prominence to the other listed conditions, we can confidently conclude that liquid water is the essential factor in the author view.

Why Other Options Are Wrong:
Presence of atmosphere is an important factor for habitability, but the passage does not label it as the key to life, so choosing it would be based on outside knowledge rather than the text. Presence of gravity is a basic property of all massive bodies and again is not singled out in the passage as a special condition for life. Presence of sunlight is important for surface temperature and energy, yet the passage emphasises the balance between too much and too little radiation mainly in relation to liquid water. Presence of a magnetic field can protect a planet from solar radiation, but this detail is not covered at all in the passage, so it cannot be the correct answer. Only the presence of liquid water is explicitly presented as central to the search for life.

Common Pitfalls:
A frequent mistake is to rely on general science knowledge when answering reading comprehension questions, leading candidates to focus on atmosphere or sunlight instead of what the passage stresses. Another pitfall is skimming the text and remembering that several conditions were discussed without remembering which one was highlighted as a key. To avoid such errors, underline or mentally mark phrases like key to life or crucial factor when reading scientific passages. Then, when the question asks for what is essential, you can quickly connect it to the emphasised phrase instead of guessing from general understanding.

Final Answer:
According to the passage, the essential factor for the possibility of life on a planet is the presence of liquid water.