Fairly recently, we figured out that basically all stars have planetary systems.

But we also learned that our own solar system is actually quite weird compared to most.

In fact, we've never seen another system like it.

So, either we are extremely rare or we're about to discover that there are countless solar systemike systems in the next big data drop from GIA.

Could go either way, but the answer has deep implications for the rarity of life in the universe.

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 planet is at the right distance from a sun type star.

And we found a few of those two 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 sunlike stars have we actually discovered that are confirmed to have both habitable Earthlike and also more distant Jupiterike 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 those sorts of systems.

Fortunately, new work based on the Gaia satellite is about to fix that.

And hopefully it'll tell us how common or how 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.

They'll give us a good understanding of the methods we're going to need to search for our solar systems twin.

Planets are not easy to see.

They are 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 large scale senses of planetary systems.

For that, we need to use indirect methods that rely on the near invisible planet 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.

The crazy magnetic fields of this neutron star channel beams of high energy particles whose radiation sweeps past the Earth with a regularity exceeding the best atomic clocks.

In this one pulsar, a modulation in the timing of the pulse revealed the gravitational tug of two planets with a third following suit after.

That's cool, but it's definitely not a solar system-like system 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 barry center, which is always close to and often actually inside the star because the stars 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 stars velocity in turn seen through the Doppler shift.

That's the tiny stretching or squishing of light's wavelength depending on whether its source is moving away from us or towards us.

Now, a few planets were found this way through the '90s.

All of them giant with small orbits because those are what the Doppler method is most sensitive to.

The exoplanet gold rush really only started in the new millennium with tens of new planets each year.

Most of these were via the Doppler method thanks to new highresolution spectrometers like Kek Hyres in Hawaii and HARPS at the Lasia 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 '9s when most planetary systems we knew of consisted of inner solar systems filled with gas giants.

We had no idea if other Earthlike planets even existed.

But the 2009 launch of the Kepler space telescope changed everything.

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 stars.

This only worked for planetary systems with convenient alignments, but there were enough of those.

By the mid20s, we were averaging hundreds of new exoplanets each year.

And this method was finally sensitive to Earth analoges in principle.

Anyway, we have found several Earthish size planets in their stars habitable zones.

Most of these are around red dwarf stars, but also around a handful of stars that are more like the sun.

Now, extrapolating from Kepler's discoveries and taking into account planets that Kepler probably missed, astronomers inferred that there are something like 11 billion Earthlike planets around sunlike stars in the Milky Way alone.

This huge number of Earth sun analoges is promising, but there is 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 one system with a string of seven 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 an inner solar 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 usually, then it raises big questions about why our solar system didn't form that way.

There are solid hypotheses for this.

The most prominent being the grand tac hypothesis, which suggests that Jupiter migrated into the inner solar system in its early times, disrupting the larger worlds that were still developing there before retreating to the outer solar system again.

The inner solar 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 that 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 planets being so much more massive than any terrestrial planet.

The main challenge is the length of the year of such an outer system body 12 years in the case of Jupiter.

Now the Doppler method can identify Jupiter-like companions 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 just not well suited to the initial discovery of exoplanets.

It can't track many many thousands of 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 just a little dip in the stars light, and we need to clock at least three of those dips to even recognize a candidate exoplanet from the regular spacing of the dips.

The Kepler mission has no more than four years of observation on any given star.

So, it could potentially have flagged an exom Mars, but definitely not an exjupiter.

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 test satellite has picked up where Kepler has left off.

But it'll still take decades to confirm an exjupit.

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 by the 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.

And 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 this could work.

As I said, both star and planet orbit their mutual center of mass, their Barry center.

The location of that Barry center depends only on the masses of both objects and their separation.

It's always closest to the star.

But the distance of the Barry center 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 Barry center further from the star than those with smaller orbits.

And that's perfect because it makes the astrometry method more sensitive to large orbits where both the Doppler method and the transit methods are better for small orbits and that's what we want for finding exjupits.

We do still have the problem that these sort of wide orbit exoplanets also have long orbital periods.

But with the astrometry method, we actually see the orbit playing out and so we don't have to wait for even a full exoyear to start to characterize the orbital parameters.

Finally, astrometry can detect planetary systems with any alignment relative to us.

Whereas the transit method only catches those that happen to be conveniently oriented to produce said transits.

And even the Doppler method is insensitive to systems that are flat against the plane of the sky.

Okay, so we now 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 this?

Well, this is exactly how astronomers first confirmed that Sirius, the brightest star in the sky, has a faint white dwarf companion, Sirius B, and that was way back in 1844.

Plenty of other binary stellar companions have been found that way, including things like companion black holes and neutron stars.

But detecting an actual exoplanet this way is much harder due to the tininess of the stars 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.

Gao was launched in 2013 and took data until 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 Guyia is precision astrometry.

It does this in a cool way.

Guyia has two telescopes actually on a spinning platform and achieves precise position measurements by converting the problem into a precise measurement of time.

The time that a star crosses through the field of view of both telescopes.

Now by combining 50 to 75 observations, Gaia can achieve a position resolution 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.

And Gaia has enabled astronomers to calculate distances to actually 1.3 billion stars across the Milky Way.

And combining this with velocities also measured for many of those stars, Gaia has given us this incredible dynamical map of a good chunk of our galaxy.

But if Gia can see a stars apparent wobble due to parallax, he should also be able to see an actual wobble due to the pull of the stars own planets.

The first guy exoplanets found using astrometry are Gaia 4b and 5b, both super Jupiters with 12 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 wobbles consistent with exoplanets and then rule out other causes and then follow up the candidates with radial velocity measurements to confirm.

But Gaia 4b and Gaia 5b are definitely there.

These objects are unusual because they are super Jupiter's 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 much about such systems in the first place.

What we do 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 that there's 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'll 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 1/2 years about twice as long as DR3.

This enables the detection of planets more distant from their stars uh 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 GIA, we wouldn't even know where to start looking in the first place.

Okay, so how many exoplanets do we expect Gaia to find?

Well, we talked to the astrophysicists who figured that out, Caleb Lamers and Josh Wyn, 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 thousand.

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.

GIA DR4 will include only half of the data that the Gaia Space Telescope has collected.

The full data set 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 time span of observations makes Gaia sensitive to planets with longer orbital periods.

So just more planets.

Also, these longer orbital periods correspond to larger 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 well, a larger volume of space.

Lamar and wind predict the DR5 will include a staggering 120,000 exoplanet detections, give or take.

That's a 20fold increase on the 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 one to five times the Earth's sun distance or astronomical units.

Jupiter itself is 5 AU and so GIA should be able to spot a Jupiter analog.

It won't be sensitive enough to detect the wobble due to earthlike 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 tests 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 Gia 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 of gas collapsing.

But it's the combined knowledge from Gaia Plus test 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 exojupiter 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 the rich planetary diversity that fills our galactic spaceime,