As telescopes like the James Webb look out across the Milky Way, you might have heard
the growing buzz around newly discovered exoplanets. It wasn’t that long ago – only
1992 – that we found the first planet outside our solar system, but since then over 5,500
have been confirmed in the relatively small area of space, only a few thousand light-years across,
that we reside in. When the Milky Way itself is 105,700 light years in diameter,
this offers us a tantalising promise of billions to trillions more across the rest
of our galaxy. Every star on average likely has at least one planet, and perhaps has several.
With all these exoplanets out there just waiting to be found, it’s only natural we would want to
turn our thoughts towards goldilocks zones , and planet habitability. After all,
humanity has its eye on colonising the stars, and an Earth 2.0 is surely waiting for us out there,
mathematically speaking. In fact, you might have already heard that phrase “Earth 2.0”
being delivered in headlines of articles or videos, claiming that a new planet has
been discovered that would be ideal for human life and colonisation.
However, don’t believe the hype. All the planets we’ve found so far have - to put
it scientifically - sucked. And once you hear what I’m about to tell you,
you’ll agree that moving to any of them will be the last thing any sane person would want to do.
I’m Alex McColgan, and you’re watching Astrum. Join with me today as I give you the grand
real-estate tour on the potential Earth 2.0’s we’ve discovered so far,
and explain why right now is the wrong time to get onto the planetary property ladder.
When choosing an Earth 2.0, just like with buying a house, it’s a good idea to start
by establishing your non-negotiables. Some of this is fairly simple to consider: for instance,
temperature. You want a nice, temperate climate, somewhere that’s not too warm,
and not too hot. Given that temperature drops the further you are from the Sun,
every planetary system has a sweet spot where the temperature of a planet is likely “just
right” – not too hot, and not too cold – similar to Goldilocks and her bowls of stolen porridge.
As a rough rule of thumb if a planet is at the right distance from a star, it may well have
a suitable temperature for us to set up shop. Of course, this is in no way a guarantee. After
all, Venus - a planet residing in this zone - in theory should be perfect, but its choking
atmosphere makes it the hottest planet in our solar system in spite of its relative distance
from the Sun compared with Mercury. So it's clear atmosphere also plays a part in all this
Speaking of atmosphere, that’s also something you’ll want to look for on Earth 2.0 – ideally
something with oxygen and water vapour, and without anything that would poison us,
but at least something that could trap heat in, and offer some protection from space radiation.
However, sometimes a non-negotiable is simply the neighbours that live close by. So, let’s
consider the first major reason why the galaxy’s exoplanets fall short – their neighbouring stars.
Our own star is known as a G-type yellow dwarf, a relatively fast-burning, short-lived sort of star,
at least compared to some of the other options out there. Stars like our Sun tend to live for
10 billion years before their cores collapse and they transform into a red giant. It is stable,
and that stability has been useful for the life that eventually flourished here . However,
yellow dwarfs are far from common in the Milky Way – they represent only 10% of the total stars. Far
more common in occurrence are the longer-lived red dwarfs, which make up 75% of the stars in
our galaxy, and can live for around 14 trillion years, longer than our universe has existed.
And unfortunately for the planets orbiting them, red dwarfs come with numerous drawbacks.
For starters, red dwarfs have a much closer goldilocks zone compared to their
yellow cousins. They burn less brightly – with luminosities between 10% and a measly 0.0125%
that of our Sun’s – so unless you want to live on a world shrouded in perpetual night (which
sounds cold and depressing to me, and will make growing plants a bit of a challenge),
you’d want an Earth 2.0 to be a lot nearer to its red dwarf to compensate.
This wouldn’t be in and of itself a problem, if it weren’t for the second
characteristic of red dwarfs that make them very unpleasant neighbours – their instability.
Our Sun is the main source of our space weather. It shoots out a frequent stream of solar winds,
flares, and coronal mass ejections. However, this doesn’t compare to the amount of space
weather created by red dwarfs. Flares from red dwarfs can be 100 to 1000 times more powerful
than those emitted by our Sun. The frequent bursts of plasma and radiation coming from
red dwarfs are powerful and dangerous enough to strip away the atmosphere,
and even boil liquid water on planets in the habitable zone. And given how
much closer planets are to their star in a red dwarf system, the odds of life getting
enough time to arise before being hit with a life-sanitising dose of x-rays is pretty
slim. If you want to live on a planet around a red dwarf, you’ll likely need some serious sun-cream .
This rules out many exoplanets, such as astronomer favourite Kepler 186 f, which
some scientists hoped might be habitable. While it may be possible to live for a
time in quite uncomfortable conditions in a deep underground bunker on such a planet,
the experience would hardly be pleasant. But even during periods between harsh solar weather events,
there’s another problem with planets around red dwarfs – their tendency to be tidally locked.
Being tidally locked means that the same side of the planet always faces the star as it orbits.
There is no rotation between day and night – one side of the planet is in perpetual heat,
and the other side is in permanent shadow. Due to gravitational constraints, planets
near their sun have a tendency towards being tidally locked – just take a look at Mercury,
with its days that last 176 Earth-days, or 2 Mercurian years. This lack of
rotation could render most of the planet uninhabitable – you’d either face scorching,
desert-creating heat on the day side, or freezing ice on the night side.
Even if humans tried to live in the narrow band of twilight that would ring a tidally locked planet,
liquid water might prove difficult to find. Any moisture in the air that found its way
to the night side of the planet would get locked there, solidified as ice. Overall,
the planet would be very dry; and even in the twilight zone, there would be no rain.
And that’s not to mention the low power of a magnetic field that would likely be found on
a tidally locked planet. The dynamo effect that powers the magnetic field on Earth is
thought to be influenced by the Coriolis effect – the rotation of the Earth imparting momentum
to rising and falling liquid metal in its core. Without this rotation,
such a magnetic field would be inevitably weaker.
So, I’m sorry to say that this rules out planets such as Proxima Centauri b, our closest exoplanet.
Proxima Centauri b is otherwise an interesting candidate – its closeness makes it one of the
easiest planets to get to for us, situated only 4.2 light years away. It’s thought to be a rocky,
Earth-like planet, which is convenient, as living on a gas giant could be challenging
to anyone with a fondness for solid floors beneath their feet. And most compelling,
its mass and radius are very similar to Earth’s, meaning it has a similar gravity.
Why is this important? Let’s compare it with another Earth 2.0 contender,
Kepler 452-b. The first planet to ever be referred to by that title,
and the planet described by a NASA scientist as “the closest twin to Earth”. Kepler 452-b
has much to recommend it. It is orbiting around a yellow dwarf, like us, at a similar distance.
This means it’s in the goldilocks zone that makes liquid water on the surface possible. However,
it is a Super-Earth. This means it has a mass and radius combination that makes
its gravity about twice that of Earth’s. When it comes to gravity, how much is too
much? Some scientists researching the question put the figure at 4 times Earth’s gravity,
this being about the limit of what a human could survive. However, even living in just twice
Earth’s gravity could get rather uncomfortable. To properly visualise this, or at least get a rough
approximation of it, imagine lying down, but then imagine a second person lying down on top of you,
putting all their weight onto you. This is your night, every night. Every breath being fought
for . Come daybreak, you must lift that extra person up, haul them onto your back, and then
carry them with you everywhere you go. Everywhere. And be careful not to fall, as suddenly it’s much
easier to break a bone. Falling down stairs is no longer painful, it’s likely lethal.
Everyone must do this. The elderly. The infirm. Small infants,
trying to take their first, wobbling step.
Keeping this in mind, it’s easy to grasp just how exhausting life in double gravity
might end up being. If you’re a body-builder or athlete you will likely be all right. But
for most people on Earth, it would be a slow torture. Your heart furiously beating to move
your blood around, making it feel like you’re running even when standing still.
And if you want to triple the gravity? Add an additional person on top of that.
And things don’t get much better going the other way. Sure, everything is suddenly much easier,
as that weight is lifted. However, living under reduced gravity comes with a host of health risks
that we can see from astronauts who’ve spent just a few months on the International Space
Station. Over time, the calcium bleeds out of your bones , causing them to become brittle. Your
muscles atrophy. Your heart develops issues. Your immune system suffers. All told, even if you’re
exercising every day, your long-term prospects aren’t looking good in a low-gravity environment
– a fact that is challenging NASA scientists as they try to figure out how to set up human
outposts on places like the Moon, or Mars, where astronauts might go to live for just a few years.
Humans have spent millions of years adapting to life on our planet. We’re very used to how things
are here. We’re just not suited to anywhere else – we’ve not evolved to face such conditions. In
some respects it’s a testament to how well we’ve acclimatised to the environment we were born in,
but it’s a big obstacle if we ever want to live anywhere else.
And so, the hunt for Earth 2.0 goes on. As I mentioned, there are likely billions of
undiscovered planets in our Milky Way galaxy. The odds of at least one being out there that
meets our criteria for gravity, temperature, lack of life-threatening radiation, and presence of
atmosphere and water, are high enough to be a certainty. Actually finding one that meets the
criteria is a more challenging task, particularly when most planets are still invisible to even our
strongest telescopes, except for a brief moment when they pass in front of a convenient star
and block out its light. If someone tells you that they’ve already found Earth 2.0,
I’d take it with a pinch of salt. We won’t ever really know that a planet is actually
suitable for human life until we somehow cross the gulf of space and go there to see it in person.
Of course, until we discover how to do that, it’s all a bit of a moot question anyway.
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