If Earth Were an Exoplanet, it Would Still be Tricky to Figure Out if There’s Life Here

Earth as seen by the JUNO spacecraft in 2013. Credit: NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill.

How would Earth appear to alien astronomers? What would their observations tell them about Earth if they searched the heavens for signs of habitability like we are? It’s a fun thought experiment.

But the experiment is more than just fun: it’s scientifically instructive. In many ways, it’s easier to study our planet and how it appears and then extrapolate those results as far as they go.

A new study shows that finding evidence of life on Earth may depend on the season alien astronomers are observing.

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Hubble Shows Saturn in the Middle of its Summer

Hubble Space Telescope view of Saturn on July 4, 2020. Credit: NASA, ESA, A. Simon (Goddard Space Flight Center), M.H. Wong (University of California, Berkeley), and the OPAL Team.

If you want an iconic picture of the planet Saturn, it doesn’t get any better than this. The latest picture from the Hubble Space Telescope shows a spectacular view of the ringed giant, taken on July 4, 2020. This shows a “summertime” view of Saturn’s northern hemisphere.

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Astronomy Cast Ep. 353: Seasons on Saturn

You think we’re the only place that experiences seasons? Well, think again. Anything with a tilt enjoys the changing seasons, and that includes one of the most dramatic places in the Solar System: Saturn, with its rings and collection of moons.

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Why Are There Seasons?

Why Are There Seasons?

We’re in the middle of Summer here on Vancouver Island, the Sun is out, the air is warm, and the river is great for swimming.

Three months from now, it’s going to be raining and miserable.

Six months from now, it’s still going to be raining, and maybe even snowing.

No matter where you live on Earth, you experience seasons, as we pass from Spring to Summer to Fall to Winter, and then back to Spring again.

Why do we have variations in temperature at all? What causes the seasons?

If you ask people this question, they’ll often answer that it’s because the Earth is closer to the Sun in the summer, and further in the winter.

But this isn’t why we have seasons. In fact, during Winter in the Northern Hemisphere, the Earth is actually at the closest point to the Sun in its orbit, and then farthest during the Summer. It’s the opposite situation for the Southern hemisphere, and explains why their seasons are more severe.

So if it’s not the distance from the Sun, why do we experience seasons?

We have seasons because the Earth’s axis is tilted.

Consider any globe you’ve ever used, and you’ll see that instead of being straight up and down, the Earth is at a tilt of 23.5-degrees.

The Earth’s North Pole is actually pointed towards Polaris, the North Star, and the south pole towards the constellation of Octans. At any point during its orbit, the Earth is always pointed the same direction.

For six months of the year, the Northern hemisphere is tilted towards the Sun, while the Southern hemisphere is tilted away. For the next six months, the situation is reversed.

Whichever hemisphere is tilted towards the Sun experiences more energy, and warms up, while the hemisphere tilted away receives less energy and cools down.

Consider the amount of solar radiation falling on part of the Earth.

When the Sun is directly overhead, each square meter of Earth receives about 1000 watts of energy.

But when the Sun is at a severe angle, like from the Arctic circle, that same 1000 watts of energy is spread out over a much larger area.

This tilt also explains why the days are longer in the Summer, and then shorter in the Winter.

The longest day of Summer, when the Northern Hemisphere is tilted towards the Sun is known as the Summer Solstice.

And then when it’s tilted away from the Sun, that’s the Winter Solstice.

When both hemispheres receive equal amounts of energy, it’s called the Equinox. We have a Spring Equinox, and then an Autumn Equinox, when our days and night are equal in length.

So how does distance from the Sun affect us?

The distance between the Earth and has an effect on the intensity of the seasons.

The Southern Hemisphere’s Summer happens when the Earth is closest to the Sun, and their winter when the Earth is furthest. This makes their seasons even more severe.

You might be interested to know that the orientation of the Earth axis is actually changing.

full-526px-earth_precessionsvgOver the course of a 26,000 year cycle, the Earth’s axis traces out a great circle in the sky. This is known as the precession of the equinoxes.

At the halfway point, 13,000 years, the seasons are reversed for the two hemispheres, and then they return to original starting point 13,000 years later.

You might not notice it, but the time of the Summer Solstice comes earlier by about 20 minutes every year; a full day every 70 years or so.

I hope this helps you understand why the Earth – and any planet with a tilted axis – experiences seasons.

Astronomy Cast Ep. 282: Seasons

Spring, Summer, Autumn and Winter. These are the seasons we experience here on Earth as our planet completes an orbit around the Sun. But what’s going on? Why do we experience such different temperatures and weather over the course of 365 days? Do other planets experience the seasons like we do?

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“Seasons” on the Astronomy Cast website, with shownotes and transcript.

And the podcast is also available as a video, as Fraser and Pamela now record Astronomy Cast as part of a Google+ Hangout:

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