Podcaster: Steve Nerlich
Title: Dear Cheap Astronomy: Ep. 122: Other Planets
Organization: Cheap Astronomy
Links: http://cheapastro.com
Description: Cheap Astronomy sizes up some exoplanets and gardens Mars.
Dear Cheap Astronomy – How big can rocky planets and how small can gas giants get?
Well there is some data, so we don’t have to talk in hypotheticals. There’s a rocky planet with about 40 times Earth mass and about 3 and a half times Earth’s diameter, which is about 85% of Neptune’s diameter. So, it’s a mighty big rocky planet that’s approaching gas giant scale.
Dear Cheap Astronomy – Will we grow crops in Martian regolith?
So, firstly you can’t really grow any plants from Earth on Mars since Earth plants need oxygen. While photosynthesis can make oxygen, plants don’t have vascular system that can move the oxygen around. So if there isn’t enough oxygen in the atmosphere to start with, then the roots die and the plant dies.
Bio: Cheap Astronomy offers an educational website where you’re only as cheap as the telescope you’re looking through.
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Transcript:
Hi, this is Steve Nerlich Why why why why why why why why cheap astronomy? Yeah, why and this is dear cheap astronomy Episode 122, Other Planets. Apart from the one we live on, there are 7 planets we know a bit about, and over 5,000 exoplanets we can make some wild guesses about.
So let’s get started. Dear Cheap Astronomy, how big can rocky planets get, and how small can gas giants get? Well, there is actually some data, so we don’t have to talk in hypotheticals.
There’s a rocky exoplanet that’s about 40 Earth masses and about 3.5 times the Earth’s diameter, which is about 85% of Neptune’s diameter. So it’s a mighty big rocky planet that is within gas giant scales. As we’ll learn later, Neptune is a bit of an exoplanet yardstick, and we’ll also learn that there’s a reason for that.
Anyhow, the planet TOI 849b, discovered in 2020, orbits very close to a sun-like star, completing each orbit in just 18 hours. Being this close to its star, it is presumably tidally locked, and the surface that’s facing its star is estimated to have a surface temperature of 1500 degrees Celsius, which is pretty darn hot. The general view is that this is one bizarre planet.
Our admittedly limited understanding of planet formation has it that anything in a protoplanetary disk that’s more than about 10 Earth masses will engage in a kind of runaway accretion process where it should inevitably end up as a gas giant. For example, we think Jupiter was formed around a seed object, probably composed of rocks and metals, that was about 15 Earth masses, which subsequently wrapped a huge gas envelope around it. It’s possible that TOI 849b used to be a gas giant, but subsequently lost its surrounding gas envelope, leaving only its bare core behind.
This may have been the result of an earlier collision, or it may have been the result of prolonged exposure to the stellar wind of its star, which blew all the gas away, although there are other gas giant exoplanets that are also in close proximity to their star and are still hanging onto their gas envelope. So perhaps TOI 849b first suffered a collision, and then whatever gas was left over got photon blasted away due to its close proximity to its star. So currently it is unusual and bizarre, but if we find 10 more TOI 849b like planets in the next 10 years, we’ll probably stop saying it’s bizarre and just add it to the growing list of possible planetary outcomes that we keep finding across the cosmos.
As we often point out here at Cheap Astronomy, the known population of exoplanets is a biased sample, we just detect what we are able to detect. It’s easy to detect large planets orbiting close to small stars, and it’s hard to detect small planets orbiting at a distance from large stars. Moving on though, are there small gas giants?
And the answer is yes, indeed they are plentiful enough to have been granted their own category, gas dwarfs. A gas dwarf is any gas planet smaller than Neptune. In the current population of known gas giant exoplanets, there are actually a lot more gas dwarfs than gas giants.
And as we said before, Neptune plays a particular role as a yardstick here. So there’s lots and lots of sub-Neptune exoplanets, while planets bigger than Neptune are comparatively rare. It seems we just got lucky having two big gas giants in our solar system, and maybe three if you count Uranus, which is very slightly bigger than Neptune, although it has less mass.
The theory behind the Neptune radius cliff is that any Neptune-sized body that accumulates more gas begins to generate enough internal gravity so that its gas envelope starts being worked into the rocky iron core, so there’s quite a long phase where such a planet can accumulate more gas without growing in size. Any extra gas just keeps mixing into the solid core. It’s only when the core material is fully saturated with gas, which is mostly hydrogen, that the surrounding gas envelope starts growing further, so that the planet’s radius starts growing further.
And at least from the exoplanet data we have available, it seems that occurrence is quite rare, because there’s just not enough free gas to be able to build up a gas giant to a size much larger than Neptune. Of course it’s worth repeating that this is just current thinking based on our biased sample of observable exoplanets, so this whole thinking may shift as our resolving power improves and we start spotting more exoplanets.
[Speaker 3]
This is the middle bit.
[Speaker 1]
We do have a fairly patchy exoplanet picture at this point in history. There are definitely exoplanets, and we can estimate their size, mass and density, and their proximity to their star, but that is about it. There is a slowly growing number, for which we have additional data on atmospheric chemistry drawn from spectroscopic observations, but we still have a long way to go.
More locally, Mars is our best known other planet, since we have landed on it, and also driven vehicles on it, and even flown a helicopter on it. And indeed we are likely to land people on it one day, although predictions of it happening in the 2030s look extremely ambitious. But one day we will be able to consider the following.
Dear Cheap Astronomy, will we ever grow crops in Martian regolith? So firstly you can’t really grow any crops from Earth on Mars, since Earth’s plants need oxygen. While photosynthesis can make oxygen, plants don’t have vascular systems that can move oxygen around.
So if there isn’t enough oxygen in the atmosphere to start with, then the roots die, and the plant dies. You might have more luck with single cell algae, but they need a lot more water and warmth than they will ever get on the surface of Mars. It might be warmer and wetter underground on Mars, but then you can’t have photosynthesis.
So really the only way to grow crops on Mars is to first land astronauts or robots and have them build a sealed, oxygenated and warmed greenhouse. With that in place, we can then talk about the growth potential of Martian regolith. Unlike lunar regolith, which is composed of sharp and spiky particles, mostly fractured shrapnel from small and large meteorite collisions, Martian regolith has undergone weathering, so its particles are smoother and rounder, some components having been shaped by water from billions of years ago when water flowed on the planet.
But otherwise, the Martian winds have done most of the work. Speaking of winds, there’s always some exchange between Martian regolith and Martian dust, where dust may stay aloft in the atmosphere for long periods, remembering that even though the atmosphere is less dense than Earth, there’s also less gravity and no rain. As a consequence, there’s always some dust in the atmosphere, which is what makes the Martian sky red, but there’s also dust storms that arise during Martian spring and summer, because you get more atmospheric gas from polar ice melts, and there’s more energy in the atmosphere from solar heating, so what might normally be regolith becomes dust lofted upwards for a temporary period.
And this cycling of regolith to dust to regolith again also adds to the weathering. But anyway, once the regolith is back on the ground and in a greenhouse, could we actually grow Earth crops in it? Well, not easily.
Our landers and rovers have found a lot of perchlorate in soil samples. Although we have only tested a few sites, its consistent finding at all those sites suggests that the perchlorate is a planet-wide phenomenon. And unfortunately, perchlorate is pretty toxic stuff, not only to plants, but also to people.
So while you might be able to grow some slow-growing perchlorate-resistant plants, there’s still going to absorb that perchlorate, so you wouldn’t be able to eat them anyway. Optimists have suggested that we could first seed the land with bacteria that can metabolise perchlorates, which might work if those bacteria didn’t need water and weren’t bothered by the high UV radiation on the Martian surface. Unfortunately, neither of those things are true.
And sure, you could bring the soil into a protected environment and grow the bacteria there, but if you’re going to go to that sort of trouble, you might as well just build a processing plant and chemically leach the perchlorate out. Indeed, if you are going to go to that sort of trouble, you might consider that it would be a whole lot easier just to grow everything hydroponically. So, as always, anything is possible, but whether it’s worth it, economically or existentially, remains to be seen.
If we ever do decide it’s really worthwhile to colonise such a harsh environment as Mars, then sure we could have industrial-scale greenhouses and industrial-scale soil leaching refineries, after which we could add organic nutrients in the usual way, which is the one part of the whole process that might be cheap. On the bright side though, the leached perchlorates could be converted into oxygen and also rocket fuel, so at least you’d be able to take holidays from your smelly old greenhouse, probably to somewhere that isn’t red.
[Speaker 3]
This is the end bet.
[Speaker 1]
So, there you go, technology may make it possible to live on the surface of Mars. We might also live in gondolas floating in the high clouds of Venus, or even build some giant rotating O’Neill cylinders at Lagrange points 4 and 5. Trips to interstellar exoplanets do seem out of the question without something like warp drive, which doesn’t look likely to be ever invented, because, you know, physics.
But that’s it for another episode of Dear Cheap Astronomy. If you’ve got a space science question, or you just want to go off-world, why not write to cheapastro at gmail dot com and we’ll work up an itinerary for you. Thanks for listening.
Steve Nellick, Cheap Astronomy. You are listening to the 365 Days of Astronomy Podcast. Cool.
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365 Days of Astronomy
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