We Know Surprisingly Little About Exoplanets

This is a map of all exoplanets we've discovered [1]. This whole region in space comprises billions
of stars, and yet, we've only found a few thousand exoplanets. Why so few? And why were so many of
them discovered inside of those two areas and not elsewhere?

I. Mysterious Region A

Part of the answer lies in interstellar distances. The closest star to the Sun is 4.2 light years
away. To get a sense of that scale, if we shrank the Sun down to the size of a tennis ball and
adjusted distances accordingly, then this other star would be 1900 km away, roughly the distance
between Beijing and Hong Kong [2].

Those distances make exoplanets difficult to observe, but we can rely on indirect methods to detect
them, like transit. A planet passing in front of its star blocks a part of its light, making it look
periodically dimmer. Of course, this only works if the planet happens to orbit at the right angle.
It's a number's game: if you want to see transits, you gotta scan as many stars as possible [3].
Another disadvantage is that exoplanets block a very small amount of light, so it's hard to tell if
you're looking at an actual transit or random flickering [4]. Despite this, transit detection was so
promising NASA decided to launch a space telescope just for that. Its name was Kepler [5].

It would stare at a single patch of stars for three and a half years looking out for transits; NASA
just had to decide where to stare at exactly. A logical choice would be toward the center of our
galaxy. The Milky Way is a barred spiral galaxy, meaning there's a high star density near the
central bulge and lower star density toward the edge. The Earth is in the middle. You would see a
lot of stars looking at the center, but it unfortunately lies in the ecliptic plane, that is, the
region in which planets orbit the Sun. Kepler would get blinded when looking in this direction, so
NASA ruled out any region within 55 degrees of the ecliptic. Kepler had to look elsewhere near the
galactic plane to get a decent star density, that leaves two regions. This one is observable from
the Northern hemisphere, where ground telescopes that supported the mission were located. This would
facilitate follow-up observations, and NASA selected this direction in particular [6]. The Kepler
mission started in 2009 and has found more than 2700 exoplanets [7]. That's why the map comprises
this cone: it's the planets found in the observational region of Kepler. Of course there are way
more planets there in reality, but only a few have the right orientation for transit detection to
work. Now, what about this other region?

II. Mysterious Region B

Well, there are other exoplanet detection methods, and perhaps the coolest one is gravitational
microlensing. Gravity bends the trajectory of light, and this can magnify objects. When the Earth, a
star system, and a background star happen to align, the light of the background star is deviated in
a particular way and astronomers look out for specific patterns to find exoplanets. This works even
for planets with a small mass and wide orbit, so microlensing isn't biased in favor of planets
orbiting close to their star like the transit method. Also, keep in mind that all these objects are
constantly moving across space, and when they are not longer aligned, it's impossible to repeat the
microlensing effect [8]. Astronomers constantly survey the sky to find microlensing alignments and
several hundreds have been found [9]. The center of the galaxy is particularly good microlensing
hunting ground because there are a lot of background stars, that explains why there's this other
cone in the map: most of these exoplanets were found through microlensing.

But these two methods don't give out that much information about exoplanets. Microlensing tells us
their mass relative to their star, but not much else [10]. Transit gives us the planet's radius,
orbital period, and even some data about the composition of its atmosphere, if there is one. But not
the mass of the planet. Plus, it only works for a minority of star systems [11]. That's why
scientists often combine multiple detection methods to get more data.

III. More Detection Methods

Radial velocity consists in measuring the small motion of stars. They don't stay perfectly in place:
the gravity of their planets makes them move a bit. Because of the Doppler effect, when a star moves
toward us, its light look more blue. When it moves away from us, it looks more red. We can measure
this tiny shift to detect exoplanets, and unlike the transit method, the planets don't need to be
perfectly aligned. Radial velocity can also measure the mass of planets [12]. Direct imaging is
another method that works better with planets that have wide orbits. This is not an artistic
depiction, it's a direct observation of exoplanets [11]. You can also mess things up: this picture
[13] shows what appeared to be a planet orbiting a star more than 1000 lightyears away from Earth,
but further research showed that this spot is probably just a background star [14].

There are even more detection methods, and they've been used to find thousands of exoplanets, but
they remain very hard to study. Let's take Alpha Centauri as an example, the nearest star system
from Earth. It comprises three stars. Proxima Centauri, the closest star to the Sun, has two
exoplanets, confirmed with radial velocity. The two big stars of that system, Rigil Kentaurus and
Toliman, are more difficult to investigate. We haven't confirmed any transit, so if there are
planets there, they probably don't have the right orientation for that method [15]. They are also
very difficult to observe with radial velocity because it's a binary system: when you observe one
star, the light from the other one contaminates your results [16]. They are very bright, which makes
direct imaging difficult [18]. And there are no object massive enough to use as a gravitational lens
to observe them. Or maybe there's one? More on that later. Alpha Centauri is the closest star
system to us, and we don't know that much about it because it doesn't have the right characteristics
for our observation methods. Star systems like TRAPPIST-1 are much farther away but have a better
orientation, so they are better studied in some ways [19].

The most obvious way to learn more about exoplanets is to make better telescopes. Kepler went
offline a few years ago, but TESS is an ongoing mission that surveys the entire sky in search of
transiting exoplanets. Future missions will focus on studying the Alpha Centauri star system [20],
terrestrial exoplanets [21], or investigate broader study objectives [22] [23]. A more extravagant
idea is to visit exoplanets. Breakthrough Starshot is a projet that proposes sending probes to the
Alpha Centauri system with solar sails. The destination is so far away that if we want to reach it
within a human lifetime, rockets won't cut it. An alternative is to beam light onto a sail to
accelerate a small payload at 15% the speed of light and reach Alpha Centauri in less than 30 years
[24]. You'll guess that an extremely expensive, decade-spanning mission is difficult to fund, and it
has indeed been put indefinitely on hold in 2025 [26]. Here's another extravagant idea: we could use
the Sun as a gravitational lens. If we put a telescope 542 astronomical units away from it, the Sun
would theoretically focus light onto the telescope and allow taking pictures of exoplanets with a 25
kilometer resolution, enough to see surface features like continents and signs of habitability [27].
But that's, like, very far and there's no serious project to do this at the moment. So for the next,
let's say, 30 years, we will probably remain limited to more or less the same observation techniques
we have now, but of course scientists will make better telescopes and find better ways to analyze
data, which could accelerate discoveries.

IV. Life

A big point of interest with exoplanets is the search for life. NASA released an astrobiology guide
in 2015 that identifies water as a necessary condition for life as we know it [28,page143].
Other types of life that would not need water are plausible and we'll talk about them, but for now
let's focus on what we're familiar with.

The habitable zone is the area around a star in which planets receive the right amount of
energy for water to exist in its liquid state on the surface of the planet. Just lying in that zone
is not enough though, you also need an atmosphere to provide some pressure, otherwise water would
just freeze or boil. And atmospheres, by themselves, tend to get stripped away by solar wind when
there's no magnetic field to shield them. On Earth, the movement of molten iron in the outer core
generates a magnetic field through a dynamo effect [28,page152]. Mars used to have a similar effect,
but it is so small that is internal heat dissipated and its dynamo weakened. On top of that, its
gravity is weak, so its atmosphere and water got knocked out [29]. There might also be such a thing
as too much water. Ocean worlds are hypothesized planets covered by water. If it's too deep, the
ocean might dissolve elements required for life, like phosphorus, at concentrations too low to make
life possible [30].

Stars themselves also impact habitability. Most of them in our galaxy are red dwarves, small stars
that emit much less light than our Sun [31]. Their habitable zone is thus very close to them.
Planets located there tend to get tidally locked, meaning that one side of the planet constantly
faces the star. This could be nefarious for life: without a day-night cycle, you get extreme
differences in temperature. But a sufficiently thick atmosphere and oceans could redistribute heat
and make the climate habitable. Life could adapt to these conditions, especially near the terminator
[32]. Another possibility is for an exomoon to orbit a tidally locked planet. No exomoon has been
detected so far, but they would have a day-night cycle even if their planet is tidally locked. They
would also probably experience intense volcanism as a result of tidal activity, like Io in our solar
system [33]. But some volcanism could contribute to a dynamo effect, thus shielding the moon [33].
Natural satellites like Enceladus and Europa are strongly suspected to host subsurface oceans thanks
to moderate tidal effects, and it might be possible for life to exist there [34]. Red dwarves have
other problems: their luminosity is highly variable, with flares making them very luminous for short
bursts [35]. They also emit less energy in the visible spectrum, which complicates photosynthesis
[36]. On the other hand, planets orbiting stars larger than our Sun could also harbor life, but
big stars emit more ionizing radiation, which could harm living beings, and they have shorter lives,
which leaves less time for life to develop [45].

Those were a few basic conditions for life as we know it. Some scientists have stated that the
specific way our solar system is organized is necessary for life. Jupiter supposedly shielded Earth
from large asteroid impacts by deviating their trajectories, but this idea has been challenged:
Jupiter might have caused more asteroid impacts than it prevented [37]. Another disputed point is
the Moon, which stabilizes the obliquity of the Earth. Without the Moon, climate would have
experienced more variations [38], but those would have occurred over millions of years, leaving
enough time for life to evolve adaptations [39]. Another idea is that our position in the Milky Way
matters. Too close to the center, and there is so much activity that it could disturb the solar
system are lead to more asteroid impacts; there's even a hypothesis that large scale extinction
events happened when the solar system was crossing areas of the galaxy that are denser in stars, but
this is not a proven fact [40]. Star systems too far away from the center of the galaxy, meanwhile,
might be too poor in heavy elements necessary for life, but again there's not enough evidence for
this [41].

Many of these points are kind of anthropocentric because they define habitability as similarity to
Earth. It's our single data point after all, but life might be more varied than we think. Maybe a
civilization on the outer rim of the Milky Way thinks it's impossible for life to arise as close to
the center of the galaxy as we are. Maybe another one lives on a natural satellite and thinks it's
impossible for life to develop on a planet. Besides, maybe life could use other biochemistries: some
scientists have proposed that it could rely on ammonia instead of water [42]. Even weirder lifeforms
could exist. Now, I'll share a highly speculative hypothesis, but it's a cool example: nuclear
phenomena akin to DNA replication could evolve inside of stars and resemble biological processes
[43]. Again, this is just a hypothesis, and it would be pretty hard to verify, but life could exist
in forms that look nothing like what we are familiar with.

NASA's astrobiology guide is not a manual on how to find life: it only raises questions to inspire
research [28,page9]. And the science has advanced a lot: NASA is working on releasing a new updated
guide to account for the progress they've made since [44]. But the search remains difficult: we
don't even know what's actually necessary for life and our observation techniques are limited by
the massive distances between stars.

References

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