When humans first began exploring Earth and migrating to new locations, a complete map
of the world (the like of which we have now) was still a far-off dream. Let alone an instant,
detailed picture of almost any place on the planet, as we also have thanks to satellite
technology. We’re now at a similarly early stage with the universe, however…
and the prospect of plotting and mapping every planet out there feels impossible. But naturally,
NASA is trying its best, anyway. This is Unveiled and today we’re
taking a closer look at the 5,000-plus exoplanets that NASA has discovered so far.
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For most of our history humans have only been aware of one planet in the universe, Earth.
It feels strange to modern minds but knowing that there are other worlds out there is still
a relatively new sensation for us. It’s only been over the last few thousand years that we’ve really
gotten to grips with it, identifying the likes of Mercury, Venus, Mars, Jupiter, and Saturn some
four millennia ago… before a long gap in which our astronomical technology and knowhow improved,
until our telescopes could finally pick out Uranus for the first time in 1781, Neptune in 1846,
and the dwarf planet Pluto in 1930. But, of course, today we know that all
those planets, dwarf planets, and their surrounding moons and other objects
are all quite close to us since they’re in our own solar system. So, exoplanet discovery,
revealing planets in other star systems, is an even more recent phenomenon. In 1992,
the first two confirmed exoplanets (nicknamed “Poltergeist” and “Phobetor”) were found
revolving around a pulsar. And ever since then, exoplanet discovery has grown and grown.
It's not as simple as just pointing your telescope in the right region of the sky and then spotting
one, however. Most of the time, astronomers don’t even see the planet they identify at first.
Instead, a variety of methods are used to predict the presence of a massive body nearby.
For example, most exoplanets are discovered via something called the transit method. This is the
technique of watching a usually much larger star… to see if its intensity dims at all
due to it being obscured by an object in orbit around it. Based on how much light is covered,
scientists can then calculate how massive that orbiting object is, and thereby determine
whether it is (or isn’t) a planet. Another technique used is called the
radial-velocity method, where astronomers again watch stars first and foremost, to see if their
light becomes red or blueshifted, presenting a kind of wobble which could be the result
of a nearby planet’s gravitational influence. Further methods for exoplanet detection include
direct imaging and gravitational microlensing – which we saw, in a recent video, was also
used to detect perhaps the most distant star ever found. But, back to planets, and although
some are used more than others, all methods have their own advantages and can be used to verify
the findings of another. Nowadays, then, we have plenty of bases covered… and, although all these
techniques require super-advanced telescopes, we have an ever-growing fleet of cutting edge,
specifically designed facilities, both on the ground and in space itself.
Using all these instruments and methods, NASA reached a milestone in exoplanet discovery
midway through 2022, when it officially added the 5,000th discovered world to its archive. To break
it down, many of those 5,000 were found by the Kepler Space Telescope, an iconic design launched
in 2009 for the express purpose of discovering planets that revolve around distant stars.
Kepler spent an eventful nine-and-a-half years in space and uncovered thousands of planets
(and thousands more potential planets) before it finally ran out of fuel in 2018. In many ways,
its mission is a model for how the search for exoplanets has unfolded, in general. Because,
although 5,000 have now been confirmed by NASA, there are still many more suspected discoveries
that haven’t been verified yet. The verification process is long and intensive, with any one
potential planet having to be shown by at least two methods of observation. The findings must then
be published in a peer-reviewed journal, which can also often take years. The Transiting Exoplanet
Survey Satellite (or TESS), for example, has so far confirmed the discovery of more than 200
planets, but has discovered more than twenty times as many planet candidates that are currently under
review. So, although 5,000 is the official total number only just passed, we know that thousands
more have likely already been found – we’re just waiting for them to be rubber-stamped.
If there’s one thing that astronomers have learned from cataloging all these other worlds, it’s that
there’s a huge amount of diversity in the types of planets that form. NASA has a categorization
system that tries to break exoplanets up into four distinct types: gas giants, Neptunians,
super-Earths, and terrestrials. Which category a planet ends up in is mostly according to its size
and mass, but other factors are taken into consideration as well. Gas giants, in general,
are massive, unsurprisingly gaseous planets about the size of Jupiter
or larger… but there are various more distinct sub-categories, too. Hot Jupiters, for instance,
are particular types of gas giant found so close to their home star that, according to NASA,
“their temperatures soar into the thousands of degrees”. Neptunian planets are of varying
composition but are usually around the size of Neptune. They have solid cores
and atmospheres that consist of only the most basic elements but, again, there are variations.
Mini Neptunes are much like other Neptunian planets except that they’re much smaller. Next,
super-Earths are planets found to be very like our own but they’re much more massive. And, lastly,
terrestrials are those smaller, rocky worlds (like Earth) where
it’s thought that there might be water and therefore a greater potential for life.
There have been some even more distinct planets identified, as well. The Unified Astronomy
Thesaurus – an open-source database for exoplanet information - has further categories including
pulsar planets, extrasolar ice giants, free floating planets, and Chthonian planets – which
are gas giants stripped of their atmosphere and outermost layers. Some of the strangest exoplanets
ever discovered have included unique features like glass rain, sideways rain, coal-black surfaces,
100% lava surfaces, and even diamond cores. But there are patterns to be found, too. For one,
planets seem to have a preference when it comes to their overall size. There are very few that exist
in the range of between 1.5 to 2 times the size of Earth. This apparent gap is called the “Radius
Valley” or the “Fulton Gap”, and researchers think that it could exist because there’s a critical
size that determines how a planet evolves. If it grows large enough then it quickly moves
beyond the gap, gathers an atmosphere and becomes a gas giant… but, if it doesn’t grow large enough,
it never pushes through that gap at all, and remains on the other side of it.
And, what of the search for life? The number of confirmed exoplanets that are located within the
habitable zone around their home star, with potentially perfect temperatures and Earth-like
conditions, has mounted up and up. Using information gathered from these worlds, scientists
can today better estimate the likelihood of alien life in the universe. An important measure
on the Drake Equation, for example, is how many habitable planets there are out there… and now,
according to projections made following confirmed observations, it’s thought that there could be
as many as 300 million habitable planets at least, and in just the Milky Way. We’re a
long way from officially registering all of them… but the first steps, it seems, have been made.
Moving forwards, the technology is always changing. The Kepler is no longer in service,
but more planets are continually being discovered by still-operational telescopes like the TESS.
And researchers are also looking to new technology to help in the search, including
the recently launched James Webb Space Telescope – which is capable of studying even the precise
atmospheric conditions of these faraway worlds. For now, 5,000 officially discovered exoplanets
(and counting) is certainly something to celebrate. But, compared to the number
that are estimated to be out there – with suggested hundreds of billions (perhaps
trillions) in the universe as a whole – the work has really only just begun. We still have many,
many more to find, especially as most of what we’ve spotted so far have also been the planets
that are closest to us in space – and often just a few hundred light-years away. Each new planet
registered, however, is another subject to study and to learn from. Another new world to explore.
It’s been a long and exciting road to this point, but there’s plenty more to come.
What do you think? Is there anything we missed? Let us know in the comments, check out these other
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