Scientists, cosmologists, astronomers and more have been watching the stars for centuries now,
trying to uncover their secrets. But, for all we have realised about the true nature
of the universe, there’s still so much that we haven’t. There’s still an endless stream
of cosmic information that we don’t yet know or haven’t yet proven. For instance, while we do of
course know that other planets exist, we’ve so far only proven them to exist beyond doubt within the
Milky Way. But that could be about to change. This is Unveiled, and today we’re answering the
extraordinary question: Did scientists just finally discover a planet beyond our galaxy?
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Whenever we speak of planets that aren’t Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus,
or Neptune - that is, planets that are outside the solar system - we refer to them as exoplanets.
And, for a long time, scientists have been confident that the universe should be full of
exoplanets. Some predictions claim that almost all stars should have at least one orbiting
around them… while others say there should be lots of stars hosting more than one planet,
like our own sun does. In either case, while the estimates do vary,
it’s thought there could be trillions of planets in the Milky Way galaxy alone.
And, considering that the highest estimates reckon at two trillion or more galaxies in the universe,
we’re potentially talking trillions of trillions of planets, out there in total.
It’s perhaps surprising then that we’ve officially catalogued only a few thousand of them, so far.
As of 2021, there are still less than 5,000 exoplanets officially confirmed by science
and observation. And all the ones that have been recorded can be found comfortably within
our own galaxy, the Milky Way. The first was in 1992, when the exoplanets known as
Phobetor and Poltergeist were discovered orbiting around a pulsar known as PSR B1257+12,
around 2,300 light years away from us. More than 2,500 of the other exoplanets we know about so
far were detected by the ground-breaking Kepler Space Telescope, over an almost ten-year mission
run by NASA between 2009 and 2018. In general, scientists expect the number of known exoplanets
to sharply rise over the coming years, however, especially following the launches of missions like
the James Webb Space Telescope (led by NASA), the Nancy Grace Roman Space Telescope (also by NASA)
and the PLATO Space Telescope (by the European Space Agency) - which are all scheduled over the
next decade. When it comes to the universe, we’re expanding our horizons faster and more
efficiently than ever before… which brings us to the key discovery for today’s video.
In late October 2021, news broke of a potential sighting of an exoplanet
not in the Milky Way… and literally millions of lightyears further away from us than anything else
we’ve ever found. The discovery was made through a study by a team at the Center for Astrophysics,
a joint venture by Harvard University and the Smithsonian, with scientists using data
from NASA’s Chandra X-Ray Observatory - an x-ray space telescope that’s been running since launch
in 1999. And, indeed, the x-ray capabilities of Chandra are crucial to this story.
Typically, when astronomers search for new planets, the detecting of
transits is one of the best and most reliable ways of finding what they need.
A transit is when a planet moves between its host star and the observer, which is us, here on Earth.
When this happens, we can measure a slight dimming in the brightness of the host star, a change in
the optical light that’s coming from it, thereby confirming that a planet has passed between us
and it… and therefore confirming that, yes, a planet does exist there. When astronomers gaze
through their telescopes searching for exoplanets, then, they’re not really looking for the planets
themselves, but rather the distinct effect any planet can have on the star that it’s closest to.
The problem is that this doesn’t (and will likely never) work when looking out at greater distances
that just a few thousand lightyears. The dimming of a star’s optical light during a regular transit
can only ever carry so far through space, which means that we can’t expect to employ
quite the same method when searching for planets that are not in the Milky Way - at
distances hundreds of thousands of lightyears away from us, or more. It’s a different story
with x-ray emissions, though, and here’s where the Chandra Observatory comes in.
Chandra is designed to detect x-ray sources in the universe. That is non-optical light,
with a high frequency and short wavelength. It has to be a space-based telescope, orbiting the Earth,
because x-rays are generally lost within our planet’s atmosphere… meaning that ground-based
telescopes just cannot pick them up. It’s also why the data that passes through Chandra offers
astronomers something different to analyse, though, painting the universe (as it does) in
literally a different light. And the crucial thing here is that x-ray transits - that is,
the disruption of an x-ray emission caused by a passing planet - are, theoretically,
possible to capture at far greater distances away from Earth. Extragalactic distances,
even. Which is what scientists, in this case, are claiming that Chandra has managed to do.
The study published in October 2021, by the Center for Astrophysics, focussed in on the
spiral galaxy, Messier 51 (also known as M51, or the Whirlpool Galaxy). M51 is thought to
be around 76,000 lightyears in diameter, but also about 28 million lightyears away from us.
Inside M51, however, scientists know of a specific binary system - catchily named M51-ULS-1.
And it’s inside this system where our potential first-exoplanet-detected-outside-the-Milky-Way
resides. It’s thought the binary at the centre
of the system is made up of two things: a massive star, and either a neutron star or a black hole.
And it’s the dramatic interactions that occur between these huge cosmic structures which then
produce the x-ray emissions that the Chandra Observatory can register in the first place.
The really interesting bit came, however, when scientists noticed that, during a moment when
Chandra was focussed on M51-ULS-1 specifically, there unfolded a period of around three hours
when the x-rays coming out of it dimmed - and for a short time even disappeared. This
means that something was blocking them, and the suggestion is that that “something” is a planet.
Researchers crunched the numbers, and according to a report on the Center for Astrophysics website,
estimate that the proposed planet should be about the same size as Saturn. And it should orbit
its binary host at about double the distance that our Saturn does from our star, the sun.
The bad news here is that, if those calculations ring true, then it’ll be another seventy years
or so before the same planet will transit again between us and its x-ray heart. This means that
scientists will not be able to definitely confirm what they saw until that time. Or until such time
as we develop a new, non-transit, but equally reliable way of detecting far off planets. And, of
course, it is possible that the x-ray dimming that was measured from M51-ULS-1 could’ve been caused
by something else - a passing gas or dust cloud, perhaps - although the scientists behind the study
have said that they think this to be unlikely. Up until this apparent breakthrough, most of all
confirmed exoplanets have fallen within a range of around 3,000 lightyears from
Earth. At this distance it’s much more possible to measure potential transits via optical light.
But now, thanks to alternative x-ray tactics, the astronomical landscape may have changed forever,
and the search for new planets may have slipped into overdrive. If what scientists have detected
inside the M51 galaxy is a planet, then it’s a few million times further away from us than most
other planets we’ve ever encountered. Science has predicted for a long time that planets like
this should be out there (and in huge numbers) but now we’re beginning to really see them.
So, how should this story make us feel? It’s perhaps a little daunting whenever we try to
contemplate the true scale of the universe. For the modern human, the idea of just Mars
or Venus can seem alien enough… and they’re the closest possible other planets to us.
Consider, then, the incomparably vast expanse that exists between us and the M51,
and all the possibilities that could play out in the space from here to there.
It’s incredible to think about, but also it’s exciting.
For so long we’ve looked up to the sky with wonder, but gradually we’re coming to terms
with it. And that’s how scientists may have just discovered a planet beyond our galaxy.
What do you think? Is there anything we missed? Let us know in the comments, check out these other
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