Walking on Sunshine

I’ve always felt it’s worth exploring closer to home before venturing too far out. It’s not a hard rule or anything, just a belief that local gems deserve a look before you start chasing treasures abroad. With that in mind, let’s turn our eyes to the Sun and the planets, moons, and other cosmic neighbours that surround her.

Our view of the Solar System has shifted a lot over time. At one point, most people believed the Earth sat at the center of the Universe, with everything else spinning around it. The idea that distant stars might be suns like ours was unimaginable. We simply assumed our place was special, unique, and central. Who could have guessed our planet was racing through space at roughly 107,200 km/h?¹ It doesn’t feel like we’re rocketing through the cosmos.

Five centuries ago, Copernicus² and other pioneering astronomers began opening our eyes to a deeper truth. The Sun is a star, and we orbit her, not the other way around.

That shift in perspective made us wonder if we were important at all. We are just one of many planets circling one of countless stars. And the conditions that gave rise to life here might not be so rare after all. Maybe Mars had life. And if not Mars, then surely one of the endless stars out there had a planet with at least simple, single-celled life.

That’s a bit too deep for this post, but let’s examine what our corner of the cosmos has to offer. And if life does exist in our solar system outside of Earth, where would it be?

The Sun holds 99.86% of all the mass in the Solar System, leaving just 0.14% behind for everything else: planets, moons, asteroids, comets.³ She really puts the “solar” in solar system. 🥁 *ta dum tsk* Lucky for us, that 0.14% is still about ~2.8 × 10²⁷ kg and more than enough for some really cool stuff to happen.⁴

It’s easy to laugh at our ancestors for thinking the Sun was a god,⁵ but really, how else would you explain the giant ball of fire that rises every day and chases away darkness? People today believe in gods that do far less. If I were going to worship something, the flaming sky orb that helps me spot predators sounds like a decent pick.

The sun is currently in its main sequence stage,²³ the longest and most stable period in a star’s life. Inside its core, hydrogen atoms are being fused into helium, producing energy and light. This process creates hydrostatic equilibrium, where the inward force of gravity is matched by the outward pressure from fusion.

Here’s how the proton-proton chain reaction works:

1. Two protons (hydrogen nuclei) fuse together, forming a deuterium nucleus (one proton, one neutron), releasing a positron and a neutrino.
2. The deuterium nucleus fuses with another proton, forming helium-3 (two protons, one neutron) and releasing a gamma-ray photon.
3. Two helium-3 nuclei fuse to form helium-4 (two protons, two neutrons), ejecting two protons in the process.

Every second, the sun fuses 600 million tons of hydrogen into 596 millions tons of helium. The remaining 4 millions tons (0.7%) is converted to energy (via E = mc²),²⁴ which powers the Sun’s light and holds off gravitational collapse.

Our Sun isn’t the biggest star out there. That title goes to UY Scuti,²⁵ a red supergiant²⁶ with a diameter around 1,700 times wider than the Sun. On the flip side, red dwarfs²⁷ are the cosmic runts, tiny compared to our star. The Sun’s somewhere in the middle, like Goldilocks’ porridge, just right. It’s a G-type star,²⁸ also called a yellow dwarf. We’ll get into what that means in a later post, but for now, just know that its size gives it a yellowish glow and about 5.5 billion more years of fusion.

Astronomers use the Sun’s mass (1.9885 × 10³⁰ kilograms) as a unit of measurement. By definition, the Sun has a mass of one solar mass. (1 M☉).²⁹ Because of this, it will have a total lifespan of about 10 billion years.

About 58 million km³⁴ from the Sun lies the first of the four rocky planets. Named after the messenger god of the Romans, Mercury zooms across the night sky faster than any other planet. Due to its close proximity to the Sun, Mercury has an orbital period of ~88 Earth days, significantly faster than Venus (~225 days), Earth (~365 days), and Mars (~687 days).³⁵

Mercury has hardly any axial tilt. Earth’s seasons are caused by her tilt of ~23.5°, but Mercury has a measly 0.034°. Not that it had any real chance at a seasonal, Earth-like climate anyway.

Due to its proximity to the Sun, the planet is in a rare 3:2 spin-orbit resonance. This means for every 3 times the planet rotates, it revolves around the sun twice (on Mercury 3 days = 2 years = ~196 Earth days). Another side effect of the proximity means the surface facing the Sun can reach ~430 °C, but Mercury’s weak atmosphere cannot distribute this heat (no winds) or retain warmth (no greenhouse effect), so, at night or in shadowed areas, temperatures plummet to ~-180 °C.

Mercury has the highest metal-to-silicate ratio of the four rocky planets. Its iron core makes up over 60% of its mass,³⁷ possibly due to a previous collision that stripped away most of its original mantle.³⁸

Unlike Venus and Mars, Mercury does have a global magnetic field but it’s very weak (only ~1% of Earth’s)³⁹ and oddly shaped. Most magnetic fields tend to align with the planet’s rotation axis, which is why Earth’s magnetic north is close to the planet’s north pole. Mercury’s is offset about 20% of the way toward its north pole, which is unusual (and still being studied).⁴⁰

With only ~38% of Earth’s diameter and ~5.5% of Earth’s mass, Mercury is really small compared to its siblings. Some of the outer planets have moons that are bigger than it.⁴¹

Being so small, Mercury’s gravity is too weak to hold onto gases long-term. Its proximity to the Sun, extreme temperature swings (approx. –180 to +430 °C), relentless solar wind, and constant micrometeorite impacts ensure that gases are quickly blasted into space. ⁴²

Mercury does have a tenuous exosphere though, formed by:⁴³

• sputtering from solar wind ions 
• vaporisation caused by micrometeorite impacts 
• radioactive decay within the crust releasing noble gases 

But this exosphere is around 10^-14 bar, billions of times thinner than Earth’s atmosphere . With hostile conditions, no atmosphere, extreme heat, and solar radiation, Mercury offers no window for life as we know it .

Named for the Roman goddess of love, Venus is a powerful reminder that being in a star’s habitable zone does not guarantee a planet will be suitable for life. Often called Earth’s angry twin, the two planets share a similar size, composition, and distance from the Sun, but Venus is smothered by a thick atmosphere made up of about 96.5% carbon dioxide (CO²), a potent greenhouse gas. This fuels a runaway greenhouse effect where heat is trapped and re-radiated so efficiently that surface temperatures soar to around 465 °C, making it hotter than Mercury despite orbiting nearly twice as far from the Sun. The planet is cloaked in thick, unbroken layers of acidic clouds that block nearly all sunlight from reaching the surface. There is no clear day or night, only a dim orange twilight and unrelenting heat.

Apart from a few trace gases, nitrogen (N₂) accounts for the remaining 3.5% of Venus’s atmosphere. That may seem small next to the overwhelming carbon dioxide, but because Venus’s atmosphere is about 92 times denser than Earth’s, it actually contains more nitrogen in total, even though Earth’s atmosphere is roughly 78% nitrogen.⁴⁷

Venus is one of only two planets in the Solar System that rotate in retrograde, the other being Uranus.⁴⁹ This means it spins in the opposite direction to most planets, including Earth. Since planets form from the same rotating protoplanetary disc as their star, they are expected to spin in the same direction.⁵⁰ Something must have altered Venus’ original rotation, possibly a massive collision, prolonged tidal effects, or internal dynamics that reversed its spin over time.⁵¹ And Venus spins slower than any other planet in the Solar System. A day on Venus (~243 Earth days) lasts longer than a year (~225 Earth days).⁵²

Its clouds constantly swirl with lightning, and the upper layers contain sulfuric acid droplets.⁵³ Acid rain does form, but it evaporates before reaching the ground due to the intense heat, kinda like a hellish version of Earth’s water cycle.⁵⁴ Most landers we’ve sent to Venus were crushed and fried within minutes to hours by the 465 °C heat and 92-bar pressure.⁵⁵

Reading everything I’ve mentioned so far, you’d think Venus was a definite “no-go” for life, but there’s a sliver of hope.

Trace amounts of water vapour in Venus’s atmosphere⁵⁸ and an elevated deuterium-to-hydrogen (D/H) ratio⁵⁹ suggest the presence of significantly more water in the past, possibly even surface oceans.⁶⁰ However, this evidence remains indirect and speculative, as there is no geological confirmation. The planet’s surface is geologically young and continuously reshaped by volcanism, erasing any potential signs of ancient water.

Life on the surface of Venus, meaning any lifeform that might exist directly on the crust, is extremely unlikely. If you were suddenly transported there, you would be crushed by immense pressure and burned by extreme heat. The atmospheric pressure at the surface is around 92 times greater than Earth’s, comparable to standing almost 900 m underwater.⁶¹ Temperatures reach about 465 °C, hot enough to melt lead.⁶² On top of that, thick clouds of sulphuric acid fill the atmosphere and would eat through most materials. These are not conditions that any known lifeform could survive.⁶³

But some scientists argue that microbial life could survive in Venus’s upper atmosphere. Between fifty and sixty kilometres above the surface, conditions become remarkably Earth‑like:⁶⁴

• Temperatures ranging from around 30 °C up to 70 °C
• Atmospheric pressure similar to that at sea level on Earth
• Trace water vapour alongside sulfur‐bearing compounds