What if Earth were twice as close to the Sun?
Half the distance. Four times the sunlight.
Inner Orbit
A passing star nudged Earth inwards. Now the Sun is four times stronger.
1. The Nudge
It took astronomers months to accept what they were seeing. A dim wandering star had passed close to the Solar System, and its gravity had tugged Earth onto a new orbit, closer to the Sun. In the end Earth settled at half its old distance.
Ira noticed the Sun first. It looked twice as wide. Halve the distance and the sunlight falling on every square metre of ground becomes four times as strong. Her school in Nagpur moved lessons to the night.
2. Steam
That first summer, the oceans began to give up their water. The air grew so humid that sweat wouldn't evaporate. On the news, scientists talked about the wet-bulb temperature, the limit beyond which a human body can't cool itself, and the number kept going up.
People lived in cooled shelters and came out only after dark. The fields browned. Water vapour is a greenhouse gas, the scientists warned, so every bit of ocean that evaporated trapped more heat and made the next bit evaporate faster.
3. Fast Years
The year shrank to about 129 days. Ira had a birthday almost every four months, and the seasons rushed past so quickly that the monsoon barely had time to start before it ended. Calendars were reprinted every few weeks.
Her little brother asked why the years were so short. Ira drew it for him: a smaller orbit is a shorter path, and the Sun's pull is stronger close in, so Earth moves faster too. 'Kepler worked it out four hundred years ago,' she said.
4. The Venus Road
At the astronomy club, Ira looked through a telescope at Venus, the bright planet that sits at about 0.72 times Earth's old distance from the Sun. Its surface was about 465 °C under air ninety times thicker than Earth's. Long ago, scientists thought, Venus might have had oceans too.
'That's where we're heading,' the club leader said quietly, 'unless we leave.' On the wall behind her, a poster showed a red planet much further out, colder and safer. Underneath someone had written: NEXT STOP, MARS.
The end. Now see the science that the story is built on.
See it happen
What would happen, step by step
Sunlight spreads out as it travels, so its strength falls with the square of the distance. Halve the distance and every square metre of Earth gets four times as much energy.
The science behind it
The inverse-square law
Sunlight ∝ 1 ÷ d² ⟹ at 0.5 AU: 1 ÷ 0.5² = 4 times as much
Light spreads out as it travels. At twice the distance it is spread over four times the area, so each square metre gets a quarter as much. Halve the distance and each square metre gets four times as much. Today about 1,361 watts of sunlight arrive on each square metre facing the Sun above our air; at half the distance it would be about 5,400.
Kepler's third law
T² ∝ a³ ⟹ T = 0.5^1.5 years ≈ 0.354 years ≈ 129 days
Johannes Kepler found in 1619 that the square of a planet's year is proportional to the cube of its orbit's size. Closer planets have shorter paths and move faster, so their years are much shorter. Mercury, at about 0.39 AU, goes round in 88 days.
The runaway greenhouse
More heat evaporates more ocean, water vapour traps more heat, and the cycle feeds itself. Scientists think something like this happened on Venus, which now has a thick carbon dioxide atmosphere, a surface pressure about 92 times Earth's and temperatures around 465 °C: hotter than Mercury, even though Mercury is closer to the Sun.
In one line: The inverse-square law: intensity ∝ 1/d². Kepler's third law: T² ∝ a³, so T = (0.5)^1.5 ≈ 0.354 of a year.
Planets on the Space path →Think about it
If Earth moved twice as far from the Sun instead, how much sunlight would each square metre get, and how long would a year last?