Do We Live in the Rarest Solar System In The Universe? We're about to find out!

0:00 We'd like to thank Henen Shaving for

0:02 supporting PBS.

0:03 Fairly recently, we figured out that basically all stars

0:06 have planetary systems, but we also

0:08 learned that our own solar system is

0:10 actually quite weird compared to most.

0:12 In fact, we've never seen another system

0:14 like it.

0:15 So, either we are extremely

0:17 rare or we're about to discover that

0:20 there are countless solar systemike

0:22 systems in the next big data drop from

0:24 GIA.

0:25 could go either way, but the answer

0:27 has deep implications for the rarity of

0:30 life in the universe.

0:34 [music] We've got a couple of quick

0:39 announcements before we start.

0:41 First, our eternal battle against the algorithm

0:44 continues.

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0:57 yourself in the comments.

0:58 [music] We're friendly.

1:00 Now for the fun part.

1:01 It's Black Friday week at the Spacetime merch

1:04 store.

1:04 From now through Monday, you can

1:06 get 20 to 50% off most merch items from

1:10 all of 2025, including limited restocks

1:14 and three new releases.

1:16 First up, we have the LHC Higs Discovery desktop and

1:19 gaming mat.

1:20 This 32in x6in mat with

1:23 anti-slip design was inspired by the

1:25 discovery of the Higs at the Large

1:27 Hadron Collider.

1:28 Plus, we have our final

1:29 shirt and hoodie of the year.

1:31 Don't panic, it's only rocket science.

1:34 It's one of our rare full color designs

1:36 inspired by a little restaurant at the

1:38 end of the universe.

1:40 There's a link in

1:40 the description.

1:41 Now, onto the episode.

1:43 There are lots of reasons to search for

1:45 planets around other stars, exoplanets.

1:48 A big one is to find other places in the

1:51 universe that might harbor life.

1:53 We only know of one such planet and that's

1:56 Earth.

1:57 And so we get particularly

1:58 excited when we find Earth mass planets

2:02 at the right distance from their star to

2:04 sustain liquid water.

2:06 Also critical for life as we know it.

2:08 We've found lots of

2:10 those habitable worlds, but we also

2:12 don't know what factors are really

2:15 critical to the initial development of

2:16 life.

2:17 maybe star type.

2:19 We get even more

2:20 excited if that Earth mass planet is at

2:22 the right distance from a sun type star.

2:26 We found a few of those two and we can

2:28 infer that there are lots more.

2:31 But is that it?

2:32 There may be other astronomical

2:33 factors.

2:34 For example, it's been

2:35 speculated that Jupiter was critical for

2:38 the development of life on Earth because

2:40 it protected us from excessive

2:42 bombardment by comets and the like.

2:44 So, how many sunlike stars have we actually

2:47 discovered that are confirmed to have

2:50 both habitable Earthlike and also more

2:52 distant Jupiterike planets?

2:56 Zero.

2:56 Not a single one.

2:58 There's some evidence that such systems are actually

3:02 truly rare.

3:04 But it's also true that our

3:05 current methods are just not good at

3:08 finding those sorts of systems.

3:10 Fortunately, new work based on the Gaia

3:14 satellite is about to fix that.

3:17 And hopefully, it'll tell us how common or

3:20 how exceedingly rare a solar system like

3:23 ours really is.

3:25 But before we get to how

3:27 we might figure out the true uniqueness

3:29 of our solar system, let's do a

3:32 whirlwind tour through the history of

3:33 exoplanet discovery.

3:35 They'll give us a

3:36 good understanding of the methods we're

3:38 going to need to search for our solar

3:41 systems.

3:42 Twin planets are not easy to

3:44 see.

3:45 They are small and dim compared to

3:47 their parent stars.

3:48 We've managed to snap pics of a few with sophisticated

3:52 starlight blocking and image analysis

3:54 technology, but that's relatively recent

3:57 and very difficult.

3:58 No good for large

3:59 scale senses of planetary systems.

4:03 For that, we need to use indirect methods

4:05 that rely on the near invisible planet's

4:08 influence on its home star.

4:10 In 1992, radio astronomers announced the

4:13 discovery of the first two exoplanets.

4:16 They orbit a millisecond pulsar, an

4:19 ultra compact stellar carcass that

4:21 rotates 160 times a second.

4:24 The crazy magnetic fields of this neutron star

4:27 channel beams of high energy particles

4:29 whose radiation sweeps past the Earth

4:31 with a regularity exceeding the best

4:34 atomic clocks.

4:35 In this one pulsar, a

4:37 modulation in the timing of the pulse

4:39 revealed the gravitational tug of two

4:42 planets with a third following soon

4:45 after.

4:46 That's cool, but it's definitely not a

4:49 solar system-like system in any respect.

4:51 The next confirmed exoplanets all came

4:53 from the radial velocity or Doppler

4:56 method.

4:57 Planets don't really orbit

4:58 stars.

4:59 Stars and planets orbit their

5:00 mutual center of mass, their Barry

5:02 center, which is always close to and

5:05 often actually inside the star because

5:07 the stars mass is so much larger than

5:09 the planet.

5:10 But that means stars do

5:12 wobble in response to their planets.

5:15 That wobble can be seen as a tiny change

5:18 in the stars velocity in turn seen

5:21 through the Doppler shift.

5:23 That's the tiny stretching or squishing of light's

5:26 wavelength depending on whether its

5:28 source is moving away from us or towards

5:31 us.

5:31 Now, a few planets were found this

5:33 way through the '90s.

5:35 All of them giant

5:36 with small orbits because those are what

5:39 the Doppler method is most sensitive to.

5:42 The exoplanet gold rush really only

5:45 started in the new millennium with tens

5:48 of new planets each year.

5:50 Most of these were via the Doppler method thanks to

5:54 new highresolution spectrometers like

5:56 Kek Hyres in Hawaii and HARPS at the

5:59 Lasia Observatory in Chile.

6:01 Smaller and less hot planets were found, but still

6:05 nothing like the Earth.

6:06 Most of these were gas giants orbiting close to their

6:10 star and there was nothing like the

6:11 Earth.

6:12 I remember a time in the '9s when

6:15 most planetary systems we knew of

6:17 consisted of inner solar systems filled

6:20 with gas giants.

6:21 We had no idea if other

6:24 Earthlike planets even existed.

6:26 But the 2009 launch of the Kepler space

6:29 telescope changed everything.

6:31 The spacecraft had one job, to stare at a

6:34 [music] tiny patch, just 1/400th of the

6:37 sky, and make precision measurements of

6:39 the brightnesses of the 150,000 stars in

6:43 its view.

6:44 Tiny repeating changes in

6:47 those brightnesses revealed miniature

6:49 eclipses, occultations, as exoplanets

6:52 passed in front of their home stars.

6:55 This only worked for planetary systems

6:57 with convenient alignments, but there

7:00 were enough of those.

7:02 By the mid20s, we

7:04 were averaging hundreds of new

7:05 exoplanets each year.

7:07 And this method was finally sensitive to Earth analoges

7:12 in principle.

7:13 Anyway, [music] we have

7:15 found several Earthishsized planets in

7:17 their stars habitable zones.

7:19 Most of these are around red dwarf stars, but

7:22 also around a handful of stars that are

7:24 more like the sun.

7:26 Now, extrapolating from Kepler's discoveries and taking

7:29 into account planets that Kepler

7:31 probably missed, astronomers inferred

7:34 that there are something like 11 billion

7:37 Earthlike planets around sunlike stars

7:39 in the Milky Way alone.

7:41 This huge number of Earth sun analoges [music]

7:45 is promising, but there is a catch.

7:48 The majority of planetary systems cataloged

7:51 by Kepler and other transit and Doppler

7:53 surveys seem to look nothing like our solar

7:58 system.

7:59 Planets of a given system have a

8:01 surprising tendency to be all roughly

8:04 the same size.

8:05 Maybe you remember the

8:06 Trappist one system with a string of

8:09 seven roughly Earth mass planets

8:12 orbiting close around a red dwarf.

8:14 More common are chains [music] of super

8:17 Earths or mini Neptunes.

8:19 The thing we don't tend to find are a chain of

8:22 terrestrial planets in an inner solar

8:24 system followed by a chain of gas giants

8:27 in the outer.

8:28 The term peas in a pod

8:31 describes this new picture in which

8:33 planetary systems tend [music] to have

8:35 similar mass planets.

8:36 If that's how such

8:38 systems tend to form usually, then it

8:41 raises big questions about why our solar

8:43 system [music] didn't form that way.

8:46 There are solid hypotheses for this.

8:48 The most prominent being the Grand Tac

8:50 hypothesis, which suggests [music] that

8:53 Jupiter migrated into the inner solar

8:55 system in its early times, disrupting

8:58 the larger worlds that were still

9:00 developing there before retreating to

9:03 the outer solar system again.

9:05 The inner solar system then had to pull itself

9:07 together from the depleted remnants of

9:10 Jupiter's rampage.

9:12 If this event resulted from some unusual gravitational

9:16 interaction, say with a passing star,

9:18 then it might mean that our planetary

9:20 configuration is pretty rare, but we

9:23 just don't know how rare.

9:24 The main reason for that is a bit surprising.

9:27 Jupiter-like worlds in Jupiter-like

9:30 orbits are actually hard to confirm from

9:33 past surveys, despite those planets

9:36 being so much more massive than any

9:38 terrestrial planet.

9:39 The main challenge is the length of the year of such an

9:44 outer system body 12 years in the case

9:47 of Jupiter.

9:48 Now, the Doppler method can

9:51 identify Jupiter-like companions from

9:53 the wobble it induces in its star.

9:56 It even measures the properties of that

9:59 orbit with not even having to observe a

10:02 full orbit.

10:03 However, the Doppler method

10:05 is just not well suited to the initial

10:06 discovery of exoplanets.

10:09 It can't track many many thousands of

10:11 stars at once in order to identify

10:13 candidates.

10:13 [music] Now, the transit method can do that, for

10:17 example, Kepler, but that method takes a

10:20 lot longer to identify an exoplanet.

10:24 Each transit [music] is seen as just a

10:26 little dip in the stars light, and we

10:28 need to clock at least three of those

10:31 dips to even recognize a candidate

10:34 exoplanet [music] from the regular spacing of the dips.

10:37 The Kepler mission has no more than 4

10:40 years of observation on any given star.

10:43 So, it could potentially have flagged an

10:45 exo Mars, but definitely not an

10:48 exojupiter.

10:49 Now, we could keep doing

10:51 planetary transit observations until we

10:54 have enough of a temporal basis and

10:57 enough stars to see long period gas

11:01 giants and the test satellite has picked

11:04 up where Kepler has left off.

11:07 But it'll still take decades to confirm an

11:10 exjupit.

11:11 Fortunately, we can do this a

11:12 lot quicker by switching to yet another

11:15 completely different method.

11:17 As I mentioned, close planets cause their

11:20 stars to wobble fast.

11:23 Good for the Doppler method, but more distant planets

11:26 cause their stars to wobble far.

11:29 And this raises an intriguing possibility.

11:33 If we could actually see the stars

11:35 moving, not just by their Doppler

11:37 shifts, but by the actual changes in

11:39 position, then we might be able to catch

11:41 these large orbit exoplanets.

11:44 To do this, we would need incredibly

11:46 precise measurements of a stars

11:47 position.

11:48 [music] And that's why we call

11:50 this approach the astrometry method,

11:52 where astrometry is just the science of

11:55 precision measurement of [music]

11:56 celestial positions.

11:58 So let's see how

11:59 this could work.

12:00 As I said, both star

12:01 and planet orbit their mutual center of

12:03 mass, their Barry center.

12:04 The location of that Barry center depends only on the

12:08 masses of both objects and their

12:11 separation.

12:12 It's always closest to the

12:13 star.

12:14 But the distance of the Barry

12:16 center from the center of the star is

12:18 proportional to two [music] things.

12:20 The relative mass of the planet and the

12:24 distance of the star to [music] the

12:25 planet.

12:26 Planets with larger orbits

12:28 actually pull the Barry center further

12:30 from the star than those with smaller

12:33 orbits.

12:34 And that's perfect because it

12:36 makes the astrometry method more

12:39 sensitive to large orbits where both the

12:41 Doppler method and the transit methods

12:44 are better for small orbits and that's

12:47 what we want for finding exjupits.

12:49 We do still have the problem that these sort

12:51 of wide orbit exoplanets also have long

12:54 orbital periods.

12:56 But with the astrometry

12:57 method, we actually see the orbit

12:59 playing [music] out and so we don't have

13:01 to wait for even a full exo year to

13:04 start to characterize the orbital

13:06 parameters.

13:06 Finally, astrometry can detect planetary systems with any

13:10 alignment relative to us, whereas the

13:12 transit method only catches those that

13:14 happen to be conveniently oriented to

13:16 produce said transits.

13:18 And even the Doppler method is insensitive to systems

13:22 that are flat against the plane of the

13:24 sky.

13:24 Okay, so we now have a third method

13:27 for finding exoplanets that seems to

13:29 shore up some of the weaknesses of the

13:31 other two.

13:32 So what have we done with

13:34 this?

13:34 Well, this is exactly how

13:35 astronomers first confirmed that Sirius,

13:38 the brightest star in the sky, has a

13:41 faint white dwarf companion, Sirius B.

13:44 And that was way back in 1844.

13:46 Plenty of other binary stellar companions have

13:49 been found that way, including things

13:50 like companion black holes and neutron

13:52 stars, but detecting an actual exoplanet

13:55 this way is much harder due to the

13:56 tininess of the stars wobble and the

13:59 extreme resolution required to see that

14:02 wobble.

14:03 It wasn't until 2010 that we

14:05 detected our first exoplanet via

14:07 astrometry.

14:08 But at that point, Kepler

14:09 was in full swing and just way, way more

14:12 productive.

14:13 What we needed was a

14:14 dedicated astrometry satellite.

14:17 Enter Gaia.

14:19 Gaia was launched in 2013

14:22 and took data until March this year,

14:25 over which time it scanned the [music]

14:27 entire sky over and over to make a

14:29 census of 8 billion stars in the Milky

14:32 Way.

14:33 The main superpower of Guyia is

14:34 precision astrometry.

14:36 It does this in a

14:37 cool way.

14:38 Guyia has two telescopes

14:39 actually on a spinning platform and

14:42 achieves precise [music] position measurements by converting the

14:46 problem into a precise measurement of

14:49 time.

14:50 The time that a star crosses

14:52 through the fields of view of both

14:53 telescopes.

14:54 Now by combining 50 to 75

14:57 observations, Gaia can achieve a

14:59 position resolution roughly the angular size of a US quarter

15:05 on the moon.

15:06 So Gaia's primary goal was

15:09 to measure stellar parallax.

15:11 That's the apparent shift in a star's position as

15:15 Earth orbits the sun.

15:17 And Gaia has enabled astronomers to calculate

15:20 distances to actually 1.3 billion stars

15:22 [music] across the Milky Way.

15:25 And combining this with velocities also measured for many

15:29 of those stars, [music]

15:30 Gaia has given us this incredible

15:32 dynamical map of a good chunk of our

15:34 galaxy.

15:35 But if Gaia can see a stars

15:37 apparent wobble due to parallax, it

15:40 should also be able to see an actual

15:42 wobble due to the pull of the stars own

15:45 planets.

15:46 The first guy exoplanets found

15:48 using astrometry are Gaia 4b and 5b,

15:52 both super Jupiters with 12 and 21 times

15:56 the mass of Jupiter respectively, both

15:58 orbiting nearby low mass stars.

16:01 It was a painstaking task to sift these

16:04 out of Gaia's third data release in

16:06 2023.

16:07 First, astronomers had to identify

16:10 wobbles consistent with exoplanets and

16:12 then rule out other causes and then

16:16 follow up the candidates with radial

16:18 velocity measurements to confirm.

16:20 But Gaia 4b and Gaia 5b are definitely

16:24 there.

16:25 These objects are unusual because

16:27 they are super Jupiter's orbiting stars

16:30 that are smaller and less massive than

16:32 the sun.

16:33 This combination of giant

16:34 planet and small star was thought to be

16:37 extremely rare.

16:38 So, we've already learned something new here.

16:41 But then again, the wide orbits of these guys

16:45 make them hard to see by other methods.

16:48 So, we never really knew much about such

16:50 systems in the first place.

16:51 What we do know is that Gaia can spot [music] gas

16:56 giants far from their stars.

16:59 Gaia 4b and 5b have proved that.

17:02 And [music] in astronomy, where there's one, there are

17:06 many.

17:06 These planets mean that there's a

17:09 whole population of undiscovered planets

17:11 hiding in the Gaia data waiting to be

17:14 discovered.

17:15 And that's going to happen

17:17 following Gaia's data release 4 in

17:20 December 2026.

17:22 The Gaia DR4 will be

17:24 different because it'll include for the

17:26 first time Gaia's time series

17:28 measurements of stars positions on the

17:30 sky rather than just the parallaxes.

17:33 [music] DR4 will also span 5 1/2 years,

17:37 about twice as long as DR3.

17:39 This enables the detection of planets more distant

17:41 from their stars uh because these take

17:43 years to complete an orbit.

17:45 Candidate exoplanets will be followed up with

17:48 [music] radial velocity Doppler

17:50 observations and these will confirm

17:54 cases of interest and better measure the

17:56 properties of the planets.

17:58 But without Gaia, we wouldn't even know where to

18:02 start looking in the first place.

18:03 Okay, so how many exoplanets do we expect Gaia

18:06 to find?

18:07 Well, we [music] talked to the

18:09 astrophysicists who figured that out,

18:12 Caleb Lamas and Josh Wyn, [music] who

18:15 recently published a study on exactly

18:18 this subject.

18:20 Based on simulated Gaia

18:22 data, they predict that data release 4

18:24 will lead to 7,500 [music] exoplanet

18:28 detections, give or take a couple

18:30 thousand.

18:31 This is likely to more than

18:33 double the number of known exoplanets

18:36 which currently stands at about 6,000

18:38 confirmed around half of which are from

18:40 Kepler.

18:41 Gaia DR4 will include only half

18:45 of the data that the Gaia Space

18:47 Telescope has collected.

18:49 The full data set DR5 will [music] include astrometric

18:53 measurements spanning 10.5 years [music]

18:57 and will be released in the early 2030s.

19:00 The number of exoplanets will then

19:03 explode even over the DR4 figure.

19:06 There [music] are two main reasons for this.

19:09 The longer time span of observations

19:11 makes GIA sensitive to planets with

19:13 longer orbital periods.

19:15 So just more planets.

19:17 Also, these longer orbital

19:19 periods correspond to larger physically

19:22 larger orbits.

19:22 [music] And that means larger stellar wobbles.

19:26 And the larger the wobble, the further

19:28 away it can be detected by Gaia, making

19:31 the mission sensitive over well a larger

19:34 volume of space.

19:36 Lamar and wind predict

19:37 the DR5 will [music] include a

19:40 staggering 120,000 exoplanet detections, give or take.

19:47 That's a 20fold increase on the current

19:49 exoplanet numbers.

19:50 But will we detect

19:52 [music] our solar system or one like it?

19:55 Gaia will be sensitive to Jupiter-like

19:57 worlds and those larger so-called super

20:00 Jupiters with orbits within 1 to five

20:03 times the Earth's sun distance or

20:05 astronomical units.

20:07 Jupiter itself is 5

20:09 AU and so GIA should be able to spot a

20:12 Jupiter analog.

20:14 It won't be sensitive

20:15 enough to detect [music] the wobble due

20:17 to Earthlike planets, but that's fine

20:19 because TESS will observe a chunk of

20:22 Gaia's systems to get [music] the inner

20:25 systems of those.

20:27 This combination of astrometric detections from Gaia and

20:30 transit discoveries from Kepler and TESS

20:33 [music] will give us something close to

20:34 a comprehensive census of possible

20:38 planetary systems.

20:40 That's going to allow

20:41 us to really refine our understanding

20:43 [music] of the formation and evolution

20:45 of these systems.

20:46 In the case of the gas

20:48 giants, for example, the ones that Gia

20:50 will discover, we may learn whether they

20:52 form more like terrestrial planets from

20:55 small bits clumping together or more

20:57 like stars with a large cloud [music] of

21:00 gas collapsing.

21:01 But it's the combined

21:03 knowledge from Gaia Plus test that will

21:06 teach us the most about our own solar

21:08 system.

21:09 It may be that we even find a

21:13 true solar system analog out there with

21:16 an exo earth discovered by transit and

21:19 an exojupiter in the same system [music]

21:21 found through the wobble it produces.

21:24 And if we don't find such a system or

21:27 find very few, [music] that will confirm

21:29 what some already suspect that our solar

21:32 system is really quite a rare beast

21:35 capable of nurturing a uniquely

21:37 habitable world.

21:39 amid the rich planetary

21:41 diversity that fills our galactic

21:44 spaceime.

21:46 We'd like to thank Hensson Shaving for

21:48 supporting PBS.

21:49 Hensson [music] Shaving is the maker of the AL-13 safety razor.

21:53 When it comes to shaving, data probably

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22:04 That's why Henen partnered with

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22:08 It lets them quantify the impact of their razors and

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22:15 with different razors.

22:17 Plus, [music] the AL13 uses a plastic-free all aluminum

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22:27 To learn more, visit henssonshaving.com/pbs

22:32 and use the code spacetime to get 100

22:35 blades for free with your purchase of a

22:37 Hensson razor.

22:38 Just be sure to add it to

22:40 your cart.

23:05 [music] [music]