restaurant at the end of the universe. There’s a link in the description. Now onto the episode
There are lots of reasons to search for planets around other stars—exoplanets.
A big one is to find other places in the universe that might harbor life. We only
know of one such planet and that's Earth. And so we get particularly excited when
we find Earth-mass planets at the right distance from their star to sustain liquid
water—also critical for life as we know it. We’ve found lots of those “habitable” worlds.
But we also don’t know what factors are really critical to the initial development of life.
Maybe star type? We get even more excited if that Earth-mass at the right distance
from a Sun-type star. We’ve found a few of those too, and we can infer that there are lots more.
But is that it? There may be other astronomical factors. For example, it’s been speculated that
Jupiter was critical for the development of life on Earth because it protected us from excessive
bombardment by comets and the like. So, how many Sun-like stars have we actually discovered
that are confirmed to have both habitable Earth-like and also more distant Jupiter-like
planets? Zero. Not a single one. There’s some evidence that such systems are actually truly rare,
but it’s also true that our current methods are just not good at finding these sorts of systems.
Fortunately, new work based on the Gaia satellite is about to fix that,
and hopefully it will tell us how common or exceedingly rare a solar system like ours really is.
But before we get to how we might figure out the true uniqueness of our solar system
let’s do a whirlwind tour through the history of exoplanet discovery. That’ll give us a good
understanding of the methods we’re going to need to search for our solar system’s twins.
Planets are not easy to see. They’re small and dim compared to their parent stars.
We’ve managed to snap pics of a few with sophisticated starlight blocking
and image analysis technology. But that’s relatively recent and very difficult—no good
for taking a large-scale census of planetary systems. For that, we need to use indirect
methods that rely on the near-invisible planet’s influence on its home star.
In 1992, radio astronomers announced the discovery of the first two exoplanets.
They orbit a millisecond pulsar, an ultra-compact stellar carcass that rotates 160 times a second.
Their crazy magnetic fields of this neutron star channel beams of high energy particles whose radiation sweeps
past the Earth with the regularity exceeding the best atomic clock. In this one pulsar,
a modulation in the timing of the pulse revealed the gravitational tug of two planets, with a third
following soon after. Well that's cool, but definitely not solar-system-like systems in any respect.
The next confirmed exoplanets all came from the radial velocity or doppler method. Planets don’t really
orbit stars. Stars and planets orbit their mutual center of mass—their barycenter—-which is always
close to and often inside the star because the star’s mass is so much larger than the planet.
But that means stars do wobble in response to their planets. That wobble can be seen as a
tiny change in the star’s velocity, in turn seen through Doppler shift. That’s the tiny stretching
or squishing of the light wavelength depending on whether it's moving away from or towards us.
A few planets were found this way through the 90s,
all of them giant and with small orbits because those are what the doppler method is
most sensitive to. The exoplanet gold rush really only started in the new millenium,
with tens of new planets a year. Most of these were via the radial velocity method thanks to
new, high resolution spectrometers like Keck-HIRES in Hawaii and HARPS at the European at La Silla
Observatory in Chile. Smaller and less hot planets were found, but still nothing like the Earth.
Most of these were gas giants orbiting close to their star and there was nothing like the earth.
I remember a time in the 90s when most planetary systems we knew of consisted of “inner
solar systems” filled with gas giants. We had no idea if other Earth-like planets even existed.
But the 2009 launch of the Kepler space telescope changed e verything. The spacecraft had one
job—to stare at a tiny patch, just 1/400th of the sky, and make precision measurements of the
brightnesses of the 150,000 stars in its view. Tiny, repeating changes in those brightnesses
revealed miniature eclipses—occultations—as exoplanets passed in front of their home star.
This only worked for planetary systems with convenient alignments,
but there were enough of those. By the mid 20-teens, we were averaging hundreds
of new exoplanets each year. And this method was finally sensitive to Earth-analogs—in principle,
anyway. We’ve found several Earth-ish size planets in their star’s habitable zones.
Most of these are around red dwarf stars, but also a handful around stars that are more like the Sun.
Extrapolating from Kepler’s discoveries, and taking account of planets that
Kepler probably missed, astronomers inferred that there are something like
11 billion Earth-like planets around Sun-like stars in the Milky Way alone.
This huge number of Earth-Sun-analogs is promising. But there’s a catch.
The majority of planetary systems cataloged by Kepler and other transit and
doppler surveys seem to look nothing like our solar system. Planets of a given system have
a surprising tendency to be all roughly the same size. Maybe you remember the Trappist-1 system,
with its string of 7 roughly Earth-mass planets orbiting close around a red dwarf.
More common are chains of super-Earths or mini-Neptunes.
The thing we don’t tend to find are a chain of terrestrial planets
in the inner system followed by a chain of gas giants in the outer.
The term “Peas in a Pod” describes this new picture in which planetary systems
tend to have similar-mass planets. If that’s how such systems tend to form ususally,
then it raises big questions about why our solar system didn’t form that way. There are solid
hypothesis for this—the most prominent being the Grant Tack hypothesis, which suggests that
Jupiter migrated into the inner solar system, disrupting the larger worlds that were developing there,
before retreating to the outer solar system again. The inner system then had to pull itself together from the depleted remnants
of Jupiter’s rampage. If this event resulted from some unusual gravitational interaction,
say with a passing star, then it might mean our planetary configuration is pretty rare.
But we just don’t know how rare. The main reason for that is a bit surprising: Jupiter-like worlds in
Jupiter-like orbits are actually hard to confirm from past surveys, despite those being so much more
massive than any terrestrial planet. The main challenge is the length of the year of such
an outer solar system body—12 years in the case of Jupiter. Now, the Doppler method can
identify a Jupiter-like companion from the wobble it induces in its star, it even measures
the properties of that orbit with not even having to observe a full orbit. However the doppler method is not well suited to the initial
discovery of exoplanets—it can’t track many many 1000s stars at once in order to identify candidates. Now the transit method
can do that—for example Kepler—but that method takes a lot longer to identify an exoplanet.
Each transit is seen as a little dip in the star’s light. We need to clock at least three of those dips to
even recognise a candidate exoplanet from the regular spacing of the dips. The Kepler mission
has no more than 4 years of observations on any given star. So it could potentially have flagged an
exo-Mars but definitely not an exo-Jupiter. .
Now, we could keep doing planetary transit observations until we have enough of a temporal
basis and enough stars to see long-period gas giants. And the TESS satellite has now picked
up where Kepler left off. But it’ll still take decades to confirm an exo-Jupiter.
Fortunately, we can do this a lot quicker by switching to yet another, completely
different method. As I mentioned, close planets cause their stars to wobble fast—good for the
Doppler method. But more distant planets cause their stars to wobble far. And this raises an intriguing
possibility. If we could actually see the stars moving—not just by their Doppler shifts—but
actual changes in position, then we might be able to catch these large-orbit exoplanets.
To do this we would need incredibly precise measurements of a star’s position. That’s
why we call this approach the astrometry method—where astrometry is just the science
of precision measurement of celestial positions. So let’s see how that could work. Both star and
planet orbit their mutual center of mass or “barycenter”. The location of the
barycenter depends only on the masses of both objects and their separation.
It’s always closest to the star, but the the distance of the barycenter from the
center of the star is proportional to two things: the relative mass of the planet, and the distance
of the star to the planet. Planets with larger orbits actually pull the barycenter further from
the star than those with smaller orbits. And that's perfect, because it makes this astrometry method
more sensitive to large orbits, whereas both the radial velocity and transit methods are better for
small orbits. And that's what we want for finding exo-Jupiters. We do still have the problem that these sorts of
wide-orbit exoplanets also have long orbital periods. But with the astrometry method we
actually see the orbit playing out and don’t have to wait even a full exo-year to start to
characterize the orbital parameters. Finally, astrometry can detect planetary systems with
any alignment, whereas the transit method only catches those that happen to be conveniently
oriented so as to produce said transits. And even the doppler method is insensitive to
systems flat against the plane of the sky. OK, so now we have a third method for
finding exoplanets that seems to shore up some of the weaknesses of
the other two. So what have we done with it? Well, this is exactly how astronomers first
confirmed that Sirius, the brightest star in the night sky, has a faint white dwarf companion,
Sirius B. That was way back in 1844. Plenty of other binary stellar companions have been
found this way, including things like companion black holes and neutron stars. But detecting an
actual exoplanet this way is much, much harder due to the tinyness of the star’s wobble and the
extreme resolution required to see that wobble. It wasn’t until 2010 that we detected our
first exoplanet via astrometry, but at that point Kepler was in full swing and just way,
way more productive. What we needed was a dedicated astrometry satellite. Enter Gaia.
Gaia was launched in 2013 and took data until this March this year, over which time
it scanned the entire sky over and over to make a census of a billion stars in the Milky Way.
The main superpower of Gaia is precision astrometry. It does this in a cool way.
Gaia has two telescopes on a spinning platform, and achieves precise position
measurements by converting the problem into a precise measurement of the time the star
crosses through the fields of view of the two telescopes. By combining 50-75 observations,
Gaia can achieve a position resolution of roughly the angular size of a US quarter on the moon.
So Gaia’s primary goal was to measure stellar parallax, that's the apparent shift in a star’s
position as Earth orbits the Sun. Gaia has enabled astronomers to calculate distances to
1.3 billion stars across the Milky Way. And combining this with the velocities also measured for many of
those stars, Gaia has given us this incredible dynamical map of a good chunk of the galaxy.
But if Gaia can see a star’s apparent wobble due to parallax, it should also be able to see an
actual wobble due the pull of the star's own planets. The first Gaia exoplanets found using astrometry are
Gaia-4b and 5b, both “Super-Jupiters” with twelve and 21 times the mass of Jupiter respectively,
both orbiting nearby low-mass stars. It was a painstaking task to sift these out of Gaia’s
third data release in 2023—first astronomers had to identify “wobble” consistent with exoplanets and then
rule out other causes and then follow up with radial velocity measurements to confirm.
But Gaia-4b and Gaia-5b are definitely there. These objects are unusual because they’re
super-Jupiters orbiting stars that are smaller and less massive than the Sun.
This combination of giant planet and small star was thought to be extremely rare, so we’ve already
learned something new here. But then again, the wide orbits of these guys make them hard
to see by other methods, so we never really knew that much about such systems in the first place. What
we do now know is that Gaia can spot gas giants far from their stars. Gaia-4b and 5b have proved that.
And in astronomy, where there’s one, there are many. These planets mean there is a whole
population of undiscovered planets hiding in the Gaia data waiting to be discovered,
and that’s going to happen following Gaia's Data Release 4 in December 2026.
The Gaia DR4 will be different because it will include, for the first time, Gaia’s
time-series measurements of stars’ positions on the sky, rather than just the parallaxes. DR4
will also span 5-and-a-half years, about twice as long as DR3. This enables the detection of
planets more distance from there stars
because these take years to complete an orbit.
Candidate exoplanets will be followed up with radial velocity - doppler observations -
and these will confirm cases of interest and better measure the
properties of the planets, but without Gaia we wouldn’t even know where to start looking in the first place.
Ok, so how many exoplanets do we expect Gaia to find?
Well, we talked to the astrophysicists who figured that out: Caleb Lammers and
Josh Winn who recently published a study on exactly this subject. Based on simulated Gaia data,
they predict that data release 4 will lead to 7,500 exoplanet detections, give or take a couple thousands.
This is likely to more than double the number of known exoplanets,
which currently stands at about 6,000 confirmed, around half of which are from Kepler.
Gaia DR4 will include only half of the data that the Gaia space telescope has collected. The
full dataset, DR5, will include astrometric measurements spanning 10.5 years and will
be released in the early 2030s. The number of exoplanets will then explode,
even over the DR4 figure. There are two main reasons for this. The longer timespan of
observations makes Gaia sensitive to planets with longer orbital periods—so just more planets. Also,
these longer orbital periods correspond to large, physically larger orbits and that means larger stellar wobbles.
And the larger the wobble, the further away it can be detected by Gaia, making the mission
sensitive over a larger volume of space. Lammers & Winn predict that DR5 will include a staggering
120,000 planet detections, give or take. That’s a 20-fold increase on current exoplanet numbers.
But will we detect our solar system, or one like it? Gaia will be sensitive to Jupiter-like worlds,
and those larger—so called super-Jupiters—with orbits within 1-5 times the Earth-Sun distance or
“astronomical units”. Jupiter itself is at 5 AU, so Gaia should be able to spot a Jupiter analog.
It won’t be sensitive enough to detect the wobble due to Earth-like planets,
but that’s fine, because TESS will observe a chunk of Gaia’s systems to get the inner systems of those.
This combination of astrometric detections from Gaia and transit discoveries from Kepler and TESS
will give us something close to a comprehensive census of possible planetary systems. That’s going
to allow us to really refine our understanding of the formation and evolution of these systems.
In the case of the gas giants for example, the ones that Gaia will discover, we may learn whether they form more
like terrestrial planets, from small bits clumping together, or more like stars, with a large cloud
collapsing. But it’s the combined knowledge from Gaia + TESS that will teach us the most
about our own solar system. It may be that we even find a true solar system analog out there,
with an exo-Earth discovered by transit and an exo-Jupiter in the same system found through the
wobble it produces. And if we don’t find such a system, or find very few, that will confirm what
some already suspect: that our solar system is really quite a rare beast, capable of nurturing
a uniquely habitable world amid rich planetary diversity that fills our galactic spacetime.