What would be the consequences of an Earthlike Planet retaining helium in its atmosphere

Question

So I did the numbers using atomic rocket's page on worldbuilding and it appears my super-earth would retain the helium gas in its atmosphere from its early formation. What effects would this have on life and the planet itself? I don't know how much helium it would have either I am not sure what percentage of its atmosphere should be helium. I hope this is enough information. Am I correct that it would even still be a super earth and not a mini neptune? For further information the gravity of the planet is 1.75 times Earth's gravity, and its mass is 5.59 Earth masses and its radius is 1.79 that of Earth's. Its atmospheric pressure according to Artifexian's spreadsheet on planetary atmospheres is 1.7 atm.

Answer 1

It depends on what effects you are looking for. In terms of breathability, there would be none per se. That is to say that whether the atmosphere has helium in it or not is not important for oxygen metabolizing lifeforms. In fact, SCUBA divers frequently use a helium/oxygen mix called heliox for deep dives, because Nitrogen is a dangerously narcotic beyond certain pressures and helium is not (well...it's complicated, but for our purposes helium extends the depth you can dive at. That's not to say that it's 100% safe all the way to the bottom of the ocean). So if there is oxygen at an appropriate partial pressure, then oxygen breathing lifeforms are OK.

As the comments stated, helium is SUPER non-reactive, so as a gas in the atmosphere it is also unlikely to harm anything.

The real question is: what is the helium replacing, if anything. This will affect your atmosphere. One particularly important metric of an atmosphere is called the scale-height. A simplistic approximation to it is:

$$H = \frac{kT}{mg}$$

where $k$ is the Boltzmann Constant, T is the average atmospheric temperature, m is the average mass of an atmospheric molecule, and g is the acceleration due to gravity on the planet.

The scale height dictates how quickly your atmospheric pressure drops off, so a larger scale height means a slower drop-off.

If your atmosphere replaces Nitrogen with Helium, say, then the average mass of the molecules in the atmosphere will be lower, and your scale height would be higher. This has several practical consequences:

1. Mountains are more easily climbable from the perspective of air pressure only. Your larger planet likely has smaller mountains, but they'd be harder to climb because you are heavier

2. It would be harder to reach orbit - all else being equal, an atmosphere with a larger scale height would extend further beyond the surface of the planet and therefore you'd need to go higher to get into orbit. This is especially harder for your planet, since it is also bigger and more massive than Earth, so you'd have a big problem with space travel

3. Aircraft would be easier to fly at low speeds, but harder to fly at high speeds. At low speeds it is the density of the atmosphere that matters, while at high speeds it is the air drag that matters. You'd need to go higher to get away from the atmosphere, but at low speeds you'd have an advantage. This means that it would be easier to fly in the higher gravity, but harder to go fast.

Answer 2

You have to redesign most electronics. Crystal oscillators change frequency in the presence of helium. (Exposure to a helium leak will kill your cell phone for some hours because of this.)

Answer 3

Helium is quite inert, but it penetrates everything.

For example, your civilization will never have functional vacuum tube technology, as the helium will leak in. Ditto for cathode ray tubes.

This presents a serious obstacle in early electronic work. With no vacuum tubes, no CRT displays, early electronics, tv, radio, radar, x-ray generation and detection, are all greatly hampered. There are likely ways to work around that, but we have no idea how as the "Easy" pathway via vacuum tubes was available to us.

Then they develop semiconductor technology. And THAT also acts erratically. The smaller you make the junctions, the more the helium affects them. Resonant crystals change frequency, resistors change resistance. Airgap capacitors behave erratically.
https://www.phonearena.com/news/Helium-exposure-can-prevent-your-iPhone-from-working-right_id110568

There will be no such thing as utterly pure gases or liquids. Example: If you make a sealed glass container of 100.000% of Oxygen-17, then a few days later it will be a 99.95% Oxygen-17 contaminated with 0.05% helium! A few years later, the contamination will match the Helium gas percentage in the atmosphere.
https://pubs.acs.org/doi/abs/10.1021/ed006p108

And then there are biological effects: If you put an ordinary Earth creature in a high helium concentration, then the helium enters into the cells and wreak (small scale) havoc in there.
https://pubs.asahq.org/anesthesiology/article/112/6/1503/10306/Cellular-Effects-of-Helium-in-Different-Organs

Helium can also penetrate molten & semimolten metals during working, resulting in microbubbles in the material. This is almost never a problem for us, but who knows if the helium abundance is, say, 5%?

Most of these adverse effects only appear because organisms and devices from a non-helium environment are getting exposed to a high-helium environment. It will almost certainly not be a problem for native people and devices.

Answer 4

It'd be hard to light a fire on such a planet. Light gases (He and H2) are very good heat conductors. The difference is very noticeable already at lower contents.

Windchill might become problematic depending on your scenario.