Show notes A cloud on Mars that forms only if textbook-theoretical physics really happens; Webb maps a hot Jupiter's edge angle by angle; multi-planet systems turn out to come in two families tied to their stars; and the Sagittarius dwarf galaxy's original disc is rebuilt from its own debris. Plus Hera's first braking burn, the station's final solar arrays, NASA ADS hands over to SciX, and a both-hemispheres skywatch. The Cloud That Shouldn't Exist Every morning in Mars's southern spring and summer, the Arsia Mons Elongated Cloud stretches up to 1,800 km from the 20 km volcano and vanishes within the day. A team led by Jorge Hernández-Bernal (LMD/CNRS/Sorbonne Université) could only reproduce it by adding homogeneous nucleation, in which vapour freezes with no dust seed, at supersaturation more than 100,000 times everyday Earth levels. A model result: sufficient, not yet observed directly. Nature Geoscience, 7 October 2026. A Map Around a Hot Jupiter's Edge Using JWST's NIRISS, NIRSpec and MIRI and the transmission strings method, Lili Alderson, Hannah Wakeford and colleagues mapped WASP-17 b's limb in 20° steps: ~860 K at the morning edge, ~1,520 K at the evening edge, the largest limb contrast yet. Data from March 2023. Nature Astronomy, 14 October 2026. Planetary Systems Come in Two Families Diego Turrini (INAF), Camilla Danielski and colleagues find 298 well-measured multi-planet systems split into a calm, compact low-mass family and a heavyweight family around metal-rich stars with about ten times more dynamical excitation. Nature Astronomy, 13 October 2026. Rewinding a Shredded Galaxy Zixi Guo, Xiang-Xiang Xue (NAOC) and colleagues reproduce a vertical metallicity gradient across the Sagittarius streams only if the dwarf began as a disc with an inverted-V metallicity profile, seen in 3 of 217 MaNGA dwarfs. Nature Astronomy, 13 October 2026. Quick hit Hera's 93-minute braking burn (BRM-1); CRS-35 and the ISS's final roll-out solar arrays; NASA ADS closes in November and becomes SciX. Skywatch Mercury and Venus in the southern evening sky; Venus inferior conjunction 24 Oct; crescent Moon in Ophiuchus; Sagittarius and WASP-17 high from Australia; Saturn all night; Mars and Jupiter closing pre-dawn toward 1.2° on 16 Nov; Orionids peak 21–22 Oct. Links · ESA: Mars's oddest cloud may be even odder than we thought (7 Oct 2026): https://www.esa.int/Science_Exploration/Space_Science/Mars_Express/Mars_s_oddest_cloud_may_be_even_odder_than_we_thought · Hernández-Bernal et al., Nature Geoscience (2026), DOI 10.1038/s41561-026-02089-9: https://doi.org/10.1038/s41561-026-02089-9 · Preprint: arXiv 2609.37259: https://arxiv.org/abs/2609.37259 · Alderson et al., A spatial map of the atmospheric limb of a hot Jupiter, Nature Astronomy (2026): https://www.nature.com/articles/s41550-026-02986-4 · Turrini, Danielski et al., Two families of multi-planet systems, Nature Astronomy (2026): https://www.nature.com/articles/s41550-026-02990-8 · INAF Media: Due famiglie di sistemi planetari (13 Oct 2026): https://www.media.inaf.it/2026/10/13/famiglie-esopianeti-stella-madre/ · Guo, Xue et al., A ∧-like metallicity profile in the disk of the Sagittarius dwarf galaxy, Nature Astronomy (2026): https://www.nature.com/articles/s41550-026-02996-2 · ESA: Brake! Time for ESA's Hera asteroid mission to go slow (7 Oct 2026): https://www.esa.int/Space_Safety/Hera/Brake!_Time_for_ESA_s_Hera_asteroid_mission_to_go_slow · Spaceflight Now: CRS-35 launch (13 Oct 2026): https://spaceflightnow.com/?p=74541 · Nature Astronomy editorial: Farewell NASA ADS (12 Oct 2026): https://www.nature.com/articles/s41550-026-03009-y astronomydaily.io · @AstroDailyPod · Bitesz.com Podcast Network
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00:00:00 --> 00:00:02 Every morning [music] in Mars' southern
00:00:02 --> 00:00:04 spring, a thin white line peels away
00:00:04 --> 00:00:06 from a giant volcano [music] and
00:00:06 --> 00:00:09 stretches up to 1 km across the
00:00:09 --> 00:00:12 planet. By afternoon, it's gone. For 8
00:00:12 --> 00:00:14 years, nobody could make that cloud
00:00:14 --> 00:00:16 happen in a computer. This month,
00:00:16 --> 00:00:18 somebody did, but only by switching on
00:00:18 --> 00:00:20 physics that had never been seen working
00:00:20 --> 00:00:22 in any planetary [music] atmosphere.
00:00:22 --> 00:00:25 Also today, a hot Jupiter mapped around
00:00:25 --> 00:00:28 its edge [music] angle by angle with a
00:00:28 --> 00:00:31 650° gap between its morning and its
00:00:32 --> 00:00:34 evening. Planetary systems that come in
00:00:34 --> 00:00:37 two distinct families. A shredded
00:00:37 --> 00:00:39 galaxy's chemistry rewound from its own
00:00:39 --> 00:00:42 debris. And a European spacecraft
00:00:42 --> 00:00:43 hitting the brakes on its way to the
00:00:43 --> 00:00:46 asteroid we once hit on purpose.
00:00:46 --> 00:00:49 >> This is Astronomy Daily for Thursday,
00:00:49 --> 00:00:52 the 15th of October, 2026. Series 5,
00:00:52 --> 00:00:55 episode 205. I'm Anna.
00:00:55 --> 00:00:58 >> And I'm Avery. Let's get into it.
00:00:58 --> 00:01:01 >> We start on Mars on the flank of a
00:01:01 --> 00:01:04 volcano called Arcia Mons. It's the
00:01:04 --> 00:01:06 southernmost of the three great Tharsus
00:01:06 --> 00:01:09 volcanoes, sitting just south of the
00:01:09 --> 00:01:12 Martian equator. And it rises about 20
00:01:12 --> 00:01:15 km. That's more than twice the height of
00:01:15 --> 00:01:18 Everest. And in the mornings during the
00:01:18 --> 00:01:21 southern spring and summer, it grows a
00:01:21 --> 00:01:21 tail.
00:01:21 --> 00:01:25 >> It does. A thin streak of water ice
00:01:25 --> 00:01:27 cloud forms on the downwind side around
00:01:27 --> 00:01:30 sunrise. And over a few hours, it
00:01:30 --> 00:01:34 stretches out to as much as 1 km.
00:01:34 --> 00:01:36 Describes that as roughly twice the
00:01:36 --> 00:01:38 length of the United Kingdom. For our
00:01:38 --> 00:01:41 North American listeners, think Los
00:01:41 --> 00:01:44 Angeles to Denver. Then it detaches from
00:01:44 --> 00:01:47 the mountain, drifts and evaporates.
00:01:47 --> 00:01:50 Next morning it does it all again day
00:01:50 --> 00:01:52 after day for months.
00:01:52 --> 00:01:53 >> And it has a name.
00:01:53 --> 00:01:57 >> The Arcia Mons elongated cloud or Amech
00:01:57 --> 00:02:00 to the people who studied it. Mars
00:02:00 --> 00:02:02 Express Orbiter drew attention to it in
00:02:02 --> 00:02:05 2018 and the team later found it had
00:02:05 --> 00:02:07 been partly photographed by Viking 2
00:02:07 --> 00:02:10 back in the 1970s. So the cloud isn't
00:02:10 --> 00:02:13 new. It's been there every Martian year.
00:02:13 --> 00:02:15 What's new is that somebody can finally
00:02:15 --> 00:02:16 explain it.
00:02:16 --> 00:02:19 >> Why has that been so hard? Clouds over
00:02:19 --> 00:02:21 mountains are hardly exotic. We get them
00:02:21 --> 00:02:22 here.
00:02:22 --> 00:02:26 >> We do. Lee clouds, wave clouds, the
00:02:26 --> 00:02:28 lenticular clouds that look like flying
00:02:28 --> 00:02:31 saucers. Air flows over a mountain, gets
00:02:31 --> 00:02:33 pushed upward, cools, and the water
00:02:33 --> 00:02:36 vapor condenses. The standard assumption
00:02:36 --> 00:02:39 was that Mars follows the same recipe
00:02:39 --> 00:02:41 with one key ingredient, something for
00:02:41 --> 00:02:44 the ice to form on. On Earth, that's
00:02:44 --> 00:02:48 dust. Sea salt, pollen, soot. On Mars,
00:02:48 --> 00:02:51 it's thought to be dust. Scientists call
00:02:51 --> 00:02:54 that heterogeneous nucleation. Ice grows
00:02:54 --> 00:02:56 on a seed.
00:02:56 --> 00:02:58 >> And that recipe didn't work.
00:02:58 --> 00:03:00 >> The weather models reproduce the waves
00:03:00 --> 00:03:04 over aramans. They reproduced the cold,
00:03:04 --> 00:03:06 but they would not reproduce the cloud
00:03:06 --> 00:03:09 Mars Express kept photographing, not its
00:03:09 --> 00:03:12 shape, not its behavior. That matters
00:03:12 --> 00:03:14 because these are the same models we
00:03:14 --> 00:03:17 lean on to understand the whole Martian
00:03:17 --> 00:03:18 water cycle.
00:03:18 --> 00:03:20 >> So what did the new study change? The
00:03:20 --> 00:03:23 paper is led by Jorge Hernandez Bernol
00:03:23 --> 00:03:26 of LMD, the meteorology laboratory of
00:03:26 --> 00:03:30 CNRS at Sorban University in Paris with
00:03:30 --> 00:03:35 Anie Matnan, Imic Spiega and France for
00:03:35 --> 00:03:37 it was published on the 7th of October
00:03:37 --> 00:03:39 in nature geoscience.
00:03:39 --> 00:03:42 They added a process called homogeneous
00:03:42 --> 00:03:43 nucleation
00:03:44 --> 00:03:47 >> which means no seed at all.
00:03:47 --> 00:03:50 >> Exactly. water vapor going straight to
00:03:50 --> 00:03:53 ice crystals in open air with nothing to
00:03:53 --> 00:03:56 grow on. It's in the textbooks, but it
00:03:56 --> 00:03:58 needs air that is wildly super
00:03:58 --> 00:04:00 saturated, holding far more vapor than
00:04:00 --> 00:04:03 it should be able to. So, it's normally
00:04:03 --> 00:04:06 treated as theory, possible on paper,
00:04:06 --> 00:04:08 and assumed not to happen in a real
00:04:08 --> 00:04:11 atmosphere. When the team switched it
00:04:11 --> 00:04:13 on, the cloud appeared. Hernandez
00:04:13 --> 00:04:15 Bernal's line was that once they
00:04:15 --> 00:04:17 included this physics, the cloud emerged
00:04:17 --> 00:04:19 just as they'd hoped.
00:04:19 --> 00:04:21 >> How super saturated are we talking?
00:04:21 --> 00:04:24 >> ESA says the relative humidity in the
00:04:24 --> 00:04:26 model reaches levels more than a 100
00:04:26 --> 00:04:28 times what we're used to in daily life
00:04:28 --> 00:04:31 on Earth. That number is really the
00:04:31 --> 00:04:33 whole story. Nowhere on our planet comes
00:04:33 --> 00:04:34 close.
00:04:34 --> 00:04:36 >> Give me an everyday picture of why the
00:04:36 --> 00:04:38 seed matters.
00:04:38 --> 00:04:41 >> Think about cloud seeding. When people
00:04:41 --> 00:04:43 try to coax rain from a cloud on Earth,
00:04:43 --> 00:04:45 they spray in tiny particles, silver
00:04:45 --> 00:04:48 iodide, for example, because water vapor
00:04:48 --> 00:04:50 finds it far easier to freeze or
00:04:50 --> 00:04:52 condense onto a surface than to organize
00:04:52 --> 00:04:55 itself from scratch. Building a crystal
00:04:55 --> 00:04:57 from nothing means a handful of
00:04:57 --> 00:04:58 molecules have to lock together by
00:04:58 --> 00:05:01 chance and survive long enough to grow.
00:05:01 --> 00:05:03 In ordinary air, that almost never
00:05:03 --> 00:05:05 happens because long before the vapor
00:05:05 --> 00:05:07 gets that desperate, it has already
00:05:07 --> 00:05:09 found a speck of dust and used it. So
00:05:10 --> 00:05:12 homogeneous nucleation isn't forbidden.
00:05:12 --> 00:05:14 It's just always beaten to the punch.
00:05:14 --> 00:05:17 >> Unless there's no time to find the dust.
00:05:17 --> 00:05:19 >> Exactly. And that's what makes this
00:05:19 --> 00:05:21 volcano special.
00:05:21 --> 00:05:24 >> Why RCMs, though? Why not every mountain
00:05:24 --> 00:05:25 on Mars?
00:05:25 --> 00:05:28 >> It's the combination. The Martian
00:05:28 --> 00:05:32 atmosphere is very thin, and RCA Mons is
00:05:32 --> 00:05:35 very tall. Wind blowing past the volcano
00:05:35 --> 00:05:38 sets up a strong atmospheric wave that
00:05:38 --> 00:05:40 lifts moist air several kilometers in a
00:05:40 --> 00:05:43 matter of minutes. In the model, the
00:05:43 --> 00:05:46 temperature drops by about 30° in 10
00:05:46 --> 00:05:49 minutes. Air that cools that fast never
00:05:49 --> 00:05:51 gets the chance to use the dust. The
00:05:51 --> 00:05:54 humidity simply rockets until the vapor
00:05:54 --> 00:05:56 freezes on its own.
00:05:56 --> 00:05:59 >> Okay, so the mountain is effectively a
00:05:59 --> 00:06:02 giant cloud chamber. That's a nice way
00:06:02 --> 00:06:06 to put it. A natural laboratory that
00:06:06 --> 00:06:10 happens to be 20 km tall. Hernandez
00:06:10 --> 00:06:12 Bernal made a lovely point in the
00:06:12 --> 00:06:16 Spanish press. If Earth had a 20 m
00:06:16 --> 00:06:18 in the middle of Siberia, we might
00:06:18 --> 00:06:20 occasionally see something similar,
00:06:20 --> 00:06:23 though not nearly as long.
00:06:23 --> 00:06:26 >> Let's be careful here. Has anyone
00:06:26 --> 00:06:29 actually watched this happen on Mars?
00:06:29 --> 00:06:31 >> No. And that's the caveat we need to
00:06:31 --> 00:06:34 keep. Nobody has measured individual ice
00:06:34 --> 00:06:37 crystals forming without dust. What the
00:06:37 --> 00:06:39 team has shown is that a model only
00:06:39 --> 00:06:42 reproduces this cloud when that physics
00:06:42 --> 00:06:45 is switched on. The paper's own title is
00:06:45 --> 00:06:48 careful. It says homogeneous nucleation
00:06:48 --> 00:06:50 is suggested by these clouds, not
00:06:50 --> 00:06:54 proven. And ESA notes that some features
00:06:54 --> 00:06:56 of the modeled cloud still don't exactly
00:06:56 --> 00:06:58 match the observations.
00:06:58 --> 00:07:01 So, it's sufficiency, not proof.
00:07:01 --> 00:07:04 >> It's the same distinction we drew back
00:07:04 --> 00:07:07 in episode 197 with the little red dot
00:07:07 --> 00:07:10 simulations. A model that reproduces an
00:07:10 --> 00:07:12 observation tells you a mechanism can do
00:07:12 --> 00:07:15 the job. It doesn't rule out every other
00:07:15 --> 00:07:18 mechanism, but the team did try the
00:07:18 --> 00:07:20 conventional physics first and it
00:07:20 --> 00:07:23 failed. That counts for something,
00:07:23 --> 00:07:25 >> Avery. And how fresh is it,
00:07:26 --> 00:07:29 >> Barry? The preprint went up on the 29th
00:07:29 --> 00:07:31 of September, about a week before the
00:07:31 --> 00:07:33 journal version. So, this is genuinely
00:07:34 --> 00:07:35 new work.
00:07:35 --> 00:07:37 >> Who should care beyond the people who
00:07:37 --> 00:07:39 study Martian weather?
00:07:39 --> 00:07:42 >> Three groups, I'd say. First, Mars
00:07:42 --> 00:07:45 modelers. If one cloud needs this
00:07:45 --> 00:07:47 physics, other high cold Martian clouds
00:07:47 --> 00:07:50 might too, and that changes how water
00:07:50 --> 00:07:52 moves around the planet. Second, people
00:07:52 --> 00:07:55 who study Earth. The authors say the
00:07:55 --> 00:07:57 result is relevant to certain clouds
00:07:57 --> 00:08:00 here as well. And third, exoplanet
00:08:00 --> 00:08:02 scientists. When we build models of
00:08:02 --> 00:08:04 atmospheres around other stars, we tend
00:08:04 --> 00:08:06 to prune out processes we think are too
00:08:06 --> 00:08:09 unlikely to matter. Mars Express project
00:08:09 --> 00:08:12 scientist Colin Wilson put it neatly.
00:08:12 --> 00:08:14 Understanding an exotic Martian cloud
00:08:14 --> 00:08:17 took exotic physics, and that may be
00:08:17 --> 00:08:19 true elsewhere in the cosmos, too.
00:08:19 --> 00:08:21 >> And it took an old spacecraft to catch
00:08:21 --> 00:08:24 it. Mars Express launched in 2003. Its
00:08:24 --> 00:08:26 orbit is highly elliptical, which lets
00:08:26 --> 00:08:29 it see Mars in the morning, and very few
00:08:29 --> 00:08:32 orbiters can do that. ESA's ExoMars
00:08:32 --> 00:08:35 Trace Gas Orbiter is one of the others.
00:08:35 --> 00:08:37 This study used three Mars Express
00:08:37 --> 00:08:38 instruments, the visual monitoring
00:08:38 --> 00:08:41 camera, nicknamed the Mars webcam, the
00:08:41 --> 00:08:43 highresolution stereo camera, and the
00:08:43 --> 00:08:46 Omega spectrometer. It's also the first
00:08:46 --> 00:08:48 time ESA has released stereo camera
00:08:48 --> 00:08:50 imagery of this cloud. It's a good
00:08:50 --> 00:08:52 reminder that old missions, watching
00:08:52 --> 00:08:54 patiently, are how you catch things that
00:08:54 --> 00:08:56 only exist for a few hours a day.
00:08:56 --> 00:08:58 >> And a southern angle to finish.
00:08:58 --> 00:09:01 >> A small one, but true. This is a
00:09:02 --> 00:09:04 southern spring cloud. It only appears
00:09:04 --> 00:09:06 in spring and summer in Mars' southern
00:09:06 --> 00:09:08 hemisphere. The same season we're
00:09:08 --> 00:09:11 enjoying here in Sydney right now, just
00:09:11 --> 00:09:13 on a Martian calendar that runs nearly
00:09:13 --> 00:09:15 twice as long as ours. from a cloud on
00:09:15 --> 00:09:18 Mars to the edge of a planet around
00:09:18 --> 00:09:21 another star. WASP 17b is a hot Jupiter
00:09:21 --> 00:09:24 in the constellation Scorpius. It has
00:09:24 --> 00:09:26 about half of Jupiter's mass, but nearly
00:09:26 --> 00:09:28 twice its width, so it's enormously
00:09:28 --> 00:09:33 puffed up at around 1°,
00:09:33 --> 00:09:34 >> which makes it a gift for transit
00:09:34 --> 00:09:36 studies.
00:09:36 --> 00:09:39 >> Exactly. When a planet crosses its star,
00:09:39 --> 00:09:41 a thin ring of atmosphere around its
00:09:41 --> 00:09:43 edge lets starlight filter through and
00:09:43 --> 00:09:45 different molecules absorb different
00:09:45 --> 00:09:47 colors. Usually, astronomers average
00:09:47 --> 00:09:50 that whole ring into one spectrum.
00:09:50 --> 00:09:51 Lately, they've split it into two
00:09:51 --> 00:09:54 halves. The morning side, where gas is
00:09:54 --> 00:09:56 arriving from the night, and the evening
00:09:56 --> 00:09:58 side, where it's leaving the day.
00:09:58 --> 00:10:00 >> And this team went further,
00:10:00 --> 00:10:03 >> much further. The paper published
00:10:03 --> 00:10:05 yesterday in Nature Astronomy is led by
00:10:05 --> 00:10:08 Lily Alderson with Hannah Wakeford and
00:10:08 --> 00:10:10 David Grant of the University of Bristol
00:10:10 --> 00:10:12 and colleagues including Tiffany Qataria
00:10:12 --> 00:10:15 at JPL. They used a technique called
00:10:15 --> 00:10:17 transmission strengths which Grant and
00:10:17 --> 00:10:20 Wakeford developed in 2023. Instead of
00:10:20 --> 00:10:21 assuming the planet silhouette is a
00:10:21 --> 00:10:24 perfect circle, it lets the outline
00:10:24 --> 00:10:26 bulge and dip and then reads off a
00:10:26 --> 00:10:28 separate spectrum at any angle around
00:10:28 --> 00:10:30 the edge. They stepped from the morning
00:10:30 --> 00:10:32 equator over the pole to the evening
00:10:32 --> 00:10:35 equator in 20° increments
00:10:35 --> 00:10:37 >> using which instruments
00:10:37 --> 00:10:39 >> three of webs
00:10:39 --> 00:10:43 near spec and me covering 6 to 12
00:10:43 --> 00:10:46 microns and the headline is a
00:10:46 --> 00:10:48 temperature gradient around the limb.
00:10:48 --> 00:10:51 The morning edge comes out near 860
00:10:51 --> 00:10:54 Kelvin, the evening edge near 1520
00:10:54 --> 00:10:58 Kelvin. That's a contrast of about 650
00:10:58 --> 00:11:00 degrees, the largest limb contrast
00:11:00 --> 00:11:04 measured so far. For comparison, WASP
00:11:04 --> 00:11:06 39B showed about 180.
00:11:06 --> 00:11:09 >> Does the chemistry change, too?
00:11:09 --> 00:11:10 >> Water and carbon dioxide stay roughly
00:11:10 --> 00:11:13 constant all the way around, which is
00:11:13 --> 00:11:15 what circulation models predict. There
00:11:15 --> 00:11:16 are hints of more methane toward the
00:11:16 --> 00:11:19 poles, but the authors say plainly that
00:11:19 --> 00:11:20 isn't well constrained. And the
00:11:20 --> 00:11:22 temperatures agree with web's separate
00:11:22 --> 00:11:24 maps of the dayside and the night side.
00:11:24 --> 00:11:27 So, WASP 17b is now one of the most
00:11:27 --> 00:11:29 completely mapped exoplanets there is.
00:11:29 --> 00:11:31 >> Caveats
00:11:31 --> 00:11:33 >> two, they assumed the northern and
00:11:33 --> 00:11:35 southern halves of the planet match to
00:11:36 --> 00:11:38 keep the computation manageable and each
00:11:38 --> 00:11:40 wavelength fit still took about 2 weeks
00:11:40 --> 00:11:44 on 40 processors. And a disclosure, the
00:11:44 --> 00:11:45 web observations were made in March
00:11:45 --> 00:11:48 2023. The paper is new this week, but
00:11:48 --> 00:11:51 the light is 3 and a half years old. And
00:11:51 --> 00:11:54 for anyone wanting to find the star,
00:11:54 --> 00:11:57 >> it sits at minus28°,
00:11:57 --> 00:11:59 so it's a southern object. Tonight, it's
00:11:59 --> 00:12:03 about 40° up at sunset from Sydney. From
00:12:03 --> 00:12:06 Los Angeles, it's 14° up and gone by
00:12:06 --> 00:12:09 full dark. And from London, it barely
00:12:09 --> 00:12:12 clears the horizon. It's 11th magnitude,
00:12:12 --> 00:12:14 so you'll need a telescope. But it's a
00:12:14 --> 00:12:17 nice thought that the best mapped edge
00:12:17 --> 00:12:19 of any alien world is hanging over
00:12:19 --> 00:12:20 Australia this evening.
00:12:20 --> 00:12:23 >> Next, a big picture look at planetary
00:12:23 --> 00:12:25 systems. We now know of more than a
00:12:25 --> 00:12:26 thousand stars with more than one
00:12:26 --> 00:12:29 planet, and they look wildly different
00:12:29 --> 00:12:30 from one another. The question has
00:12:30 --> 00:12:32 always been whether that variety is
00:12:32 --> 00:12:36 baked in at birth or sculpted later. And
00:12:36 --> 00:12:37 the answer is
00:12:37 --> 00:12:40 >> according to a team led by Diego Tini of
00:12:40 --> 00:12:42 INAF, the Italian National Institute for
00:12:42 --> 00:12:45 Astrophysics with Camila Danielski of
00:12:45 --> 00:12:48 the University of Valencia, the answer
00:12:48 --> 00:12:52 is both. They studied 298 multilanet
00:12:52 --> 00:12:54 systems whose masses and orbits are well
00:12:54 --> 00:12:57 measured and published on Tuesday in
00:12:57 --> 00:12:59 Nature Astronomy. When they sorted them
00:12:59 --> 00:13:01 by total planet mass relative to the
00:13:01 --> 00:13:04 star, the system split into two clear
00:13:04 --> 00:13:06 groups.
00:13:06 --> 00:13:07 Two families.
00:13:07 --> 00:13:10 >> Two families. The lower mass family is
00:13:10 --> 00:13:12 compact. Planets of similar size sit in
00:13:12 --> 00:13:14 tight orbits, often far closer to their
00:13:14 --> 00:13:17 star than Mercury is to ours. That's the
00:13:17 --> 00:13:19 so-called peas in a pod pattern. Kepler
00:13:19 --> 00:13:21 revealed. Their host stars are the sun's
00:13:22 --> 00:13:24 mass or smaller with the sun's chemistry
00:13:24 --> 00:13:26 or less. And dynamically, they're fairly
00:13:26 --> 00:13:28 calm, comparable to our own solar
00:13:28 --> 00:13:30 system.
00:13:30 --> 00:13:32 >> And the other family,
00:13:32 --> 00:13:35 >> heavy weights. Total mass is typically 2
00:13:35 --> 00:13:38 to 10 times our solar systems. Around
00:13:38 --> 00:13:40 stars that are more massive and richer
00:13:40 --> 00:13:43 in heavy elements than the sun.
00:13:43 --> 00:13:45 Combining stellar mass and metallicity,
00:13:45 --> 00:13:47 their birth discs held roughly three
00:13:47 --> 00:13:50 times more dust and ice. And their
00:13:50 --> 00:13:52 orbits are about 10 times more stirred
00:13:52 --> 00:13:54 up, more eccentric, and more tilted than
00:13:54 --> 00:13:56 in the calm family,
00:13:56 --> 00:13:58 >> which suggests what?
00:13:58 --> 00:14:02 >> Violence. D. Poly Crony of INAF led the
00:14:02 --> 00:14:04 simulations and the systems that best
00:14:04 --> 00:14:06 match the heavy family are the ones that
00:14:06 --> 00:14:08 were unstable from the start. Too much
00:14:08 --> 00:14:10 planet building material makes planets
00:14:10 --> 00:14:13 too big to sit peacefully. They scatter,
00:14:13 --> 00:14:16 collide, and fling each other out. In
00:14:16 --> 00:14:18 those runs, most systems loft at least
00:14:18 --> 00:14:20 one planet.
00:14:20 --> 00:14:21 >> That rings a bell.
00:14:21 --> 00:14:26 >> It does. Back in episode 195, we covered
00:14:26 --> 00:14:28 Betigan's work on how our own solar
00:14:28 --> 00:14:31 system could be destabilized far in the
00:14:31 --> 00:14:35 future. This is the same physics running
00:14:35 --> 00:14:37 at the start of a systems life instead
00:14:37 --> 00:14:39 of the end. Pini's phrase for the link
00:14:40 --> 00:14:43 between star and planets is genetic one.
00:14:43 --> 00:14:45 What you're born with shapes what you
00:14:45 --> 00:14:46 become.
00:14:46 --> 00:14:48 >> Caveats.
00:14:48 --> 00:14:51 >> Danielski herself flagged the big one.
00:14:51 --> 00:14:53 The team's main worry was whether the
00:14:53 --> 00:14:55 split was real or just a product of how
00:14:55 --> 00:14:58 we find planets because transits favor
00:14:58 --> 00:15:01 compact systems and radial velocities
00:15:01 --> 00:15:04 favor massive ones. They tested four
00:15:04 --> 00:15:06 independently selected samples and the
00:15:06 --> 00:15:09 split held in all of them. But we still
00:15:09 --> 00:15:12 can't easily see wide systems of modest
00:15:12 --> 00:15:15 planets. So a middle population could be
00:15:15 --> 00:15:18 hiding. And a small disclosure, the
00:15:18 --> 00:15:20 planet catalog was downloaded in October
00:15:20 --> 00:15:21 last year.
00:15:21 --> 00:15:23 >> And where does that leave us?
00:15:23 --> 00:15:27 >> Somewhere in between. Our solar systems
00:15:27 --> 00:15:29 calm history matches the lower mass
00:15:29 --> 00:15:32 family, but it's much more spread out
00:15:32 --> 00:15:35 than those compact systems. It's another
00:15:35 --> 00:15:37 reminder not to treat the sun and its
00:15:37 --> 00:15:40 planets as the universal template.
00:15:40 --> 00:15:44 >> Our third story is galactic archaeology.
00:15:44 --> 00:15:46 The Sagittarius dwarf galaxy was only
00:15:46 --> 00:15:49 discovered in 1994, hiding behind the
00:15:49 --> 00:15:51 crowded center of the Milky Way, and
00:15:51 --> 00:15:54 it's being torn apart. Its stars now
00:15:54 --> 00:15:56 stream around our galaxy in long ribbons
00:15:56 --> 00:15:58 that wrap the sky.
00:15:58 --> 00:16:01 >> So, can you work out what it looked like
00:16:01 --> 00:16:02 before it was shredded?
00:16:02 --> 00:16:05 >> That's the challenge. And positions and
00:16:05 --> 00:16:07 speeds of the debris alone don't get you
00:16:07 --> 00:16:10 very far. A team led by Zji Guo and
00:16:10 --> 00:16:12 Shang Shangghu at the National
00:16:12 --> 00:16:14 Astronomical Observatories of China
00:16:14 --> 00:16:17 added chemistry. Using stream members
00:16:17 --> 00:16:19 from China's Lamos Survey Telescope,
00:16:19 --> 00:16:21 they looked at how the abundance of
00:16:21 --> 00:16:24 heavy elements, the metalicity, changes
00:16:24 --> 00:16:26 across the width of the streams.
00:16:26 --> 00:16:28 >> And it changes.
00:16:28 --> 00:16:31 >> It does. There's a vertical metallicity
00:16:31 --> 00:16:34 gradient across the streams. The team
00:16:34 --> 00:16:36 ran large computer simulations of the
00:16:36 --> 00:16:38 dwarf galaxy falling into the Milky Way.
00:16:38 --> 00:16:40 And the only way to reproduce that
00:16:40 --> 00:16:42 gradient was if Sagittarius started out
00:16:42 --> 00:16:45 as a disc whose metallicity had an
00:16:45 --> 00:16:48 unusual shape. Normally a galaxy disc is
00:16:48 --> 00:16:50 most metal rich at the center and gets
00:16:50 --> 00:16:53 poorer as you go outward. Sagittarius
00:16:53 --> 00:16:55 instead looks like an inverted V.
00:16:55 --> 00:16:58 Metalicity rises from the center, peaks
00:16:58 --> 00:17:00 partway out, then falls again.
00:17:00 --> 00:17:02 >> Is that weird?
00:17:02 --> 00:17:07 rare but not unique. They searched 217
00:17:07 --> 00:17:10 dwarf galaxies in the manga survey and
00:17:10 --> 00:17:12 found three with the same shape. And
00:17:12 --> 00:17:15 intriguingly, the Milky Way's own
00:17:15 --> 00:17:17 integrated metalicity profile looks
00:17:17 --> 00:17:19 similar. So, the authors raised the
00:17:19 --> 00:17:21 question of whether the two galaxies
00:17:21 --> 00:17:24 have in some sense evolved together.
00:17:24 --> 00:17:26 >> Caveats,
00:17:26 --> 00:17:29 >> it's a reconstruction from simulations.
00:17:29 --> 00:17:31 So, it shows a disc like that can
00:17:31 --> 00:17:33 reproduce the streams, not that nothing
00:17:34 --> 00:17:37 else could. And a disclosure on timing,
00:17:37 --> 00:17:39 the paper was published this week, but
00:17:39 --> 00:17:41 it was first submitted in July last
00:17:41 --> 00:17:43 year, so it's been more than a year in
00:17:44 --> 00:17:45 review.
00:17:45 --> 00:17:48 >> There's a call back here, too.
00:17:48 --> 00:17:51 >> There is. In episode 195, we covered
00:17:51 --> 00:17:54 Reuben-GC1,
00:17:54 --> 00:17:56 a newly found globular cluster that
00:17:56 --> 00:17:59 Reuben data traced back to Sagittarius.
00:17:59 --> 00:18:01 Every piece of this galaxy we recover
00:18:01 --> 00:18:04 helps rebuild the hole. And the southern
00:18:04 --> 00:18:06 Andle is literal. The core of
00:18:06 --> 00:18:09 Sagittarius sits at -30°,
00:18:09 --> 00:18:12 about 60° up from Sydney at full dark
00:18:12 --> 00:18:15 tonight, against 22 from Los Angeles and
00:18:15 --> 00:18:18 five from London. Its nucleus is the
00:18:18 --> 00:18:21 globular cluster Messier 54, an easy
00:18:21 --> 00:18:24 telescope target just below the teapot.
00:18:24 --> 00:18:27 >> Three quick ones. First, ASA's Hera
00:18:27 --> 00:18:29 mission was due to fire its thrusters
00:18:30 --> 00:18:32 for 93 minutes in the early hours of
00:18:32 --> 00:18:35 today, European time. It was the first
00:18:35 --> 00:18:37 of its breaking burns on approach to
00:18:37 --> 00:18:40 Ditimus and Dimorphice. The asteroid
00:18:40 --> 00:18:43 pair NASA's Dart spacecraft struck in
00:18:43 --> 00:18:45 2022, shortening the Moonletit's orbit
00:18:45 --> 00:18:47 by about 33 minutes.
00:18:47 --> 00:18:49 >> As we record, issa hasn't yet confirmed
00:18:49 --> 00:18:52 the result. Its new Nora antenna in
00:18:52 --> 00:18:53 Western Australia is among the stations
00:18:54 --> 00:18:55 tracking it, and we'll bring you the
00:18:55 --> 00:18:57 outcome tomorrow. Arrival at the
00:18:57 --> 00:18:59 asteroids is due in November.
00:18:59 --> 00:19:01 Second, a cargo dragon launched from
00:19:01 --> 00:19:04 Florida on Tuesday on NASA's CRS35
00:19:04 --> 00:19:06 mission, carrying more than three tons
00:19:06 --> 00:19:09 of supplies and the final pair of
00:19:09 --> 00:19:10 rolledout solar arrays for the
00:19:10 --> 00:19:13 International Space Station. It's due to
00:19:13 --> 00:19:15 dock at 11:00 a.m. Eastern time today,
00:19:15 --> 00:19:17 which is 2:00 tomorrow morning in
00:19:17 --> 00:19:18 Sydney.
00:19:18 --> 00:19:21 >> And third, a farewell. After more than
00:19:21 --> 00:19:23 33 years, NASA's astrophysics data
00:19:24 --> 00:19:26 system, the digital library almost every
00:19:26 --> 00:19:28 astronomer uses closes its doors in
00:19:28 --> 00:19:30 November. It's being replaced by a
00:19:30 --> 00:19:32 broader successor called Science
00:19:32 --> 00:19:35 Explorer or SCX. Nature astronomy marked
00:19:35 --> 00:19:36 the occasion with an editorial this
00:19:36 --> 00:19:38 week. The papers aren't going anywhere.
00:19:38 --> 00:19:40 They're just moving house.
00:19:40 --> 00:19:42 >> Time to look up. And tonight, the
00:19:42 --> 00:19:44 evening western sky splits the
00:19:44 --> 00:19:47 hemispheres neatly. Mercury reached its
00:19:47 --> 00:19:48 greatest distance from the sun on Monday
00:19:48 --> 00:19:50 and is still well placaced for southern
00:19:50 --> 00:19:52 observers. From Sydney tonight, it's
00:19:52 --> 00:19:55 about 24 degrees up at sunset and sets
00:19:55 --> 00:19:57 roughly 2 hours after the sun.
00:19:57 --> 00:19:59 >> That's a terrific apparition.
00:19:59 --> 00:20:01 >> One of the best of the year for the
00:20:01 --> 00:20:04 south. Venus is below it, about 14° up
00:20:04 --> 00:20:06 at sunset from Sydney and setting an
00:20:06 --> 00:20:09 hour and a quarter after the sun. In
00:20:09 --> 00:20:11 North America, it's a different story.
00:20:11 --> 00:20:14 From Los Angeles, Mercury is 10° up at
00:20:14 --> 00:20:16 sunset and sets within the hour, while
00:20:16 --> 00:20:19 Venus sets with the sun. From New York,
00:20:19 --> 00:20:21 Mercury is about 7° up and Venus is
00:20:21 --> 00:20:24 already gone. From London, Mercury
00:20:24 --> 00:20:27 barely clears the horizon,
00:20:27 --> 00:20:28 >> and Venus is about to leave us
00:20:28 --> 00:20:29 altogether.
00:20:29 --> 00:20:32 >> It reaches inferior conjunction on the
00:20:32 --> 00:20:35 24th of October, passing between us and
00:20:35 --> 00:20:39 the sun. Right now, it's a thin crescent
00:20:39 --> 00:20:43 about 4% lit, but a large one, merely a
00:20:43 --> 00:20:45 full arc minute across.
00:20:45 --> 00:20:47 >> Which brings us to our standing safety
00:20:47 --> 00:20:50 note. And this week, it matters. As
00:20:50 --> 00:20:52 Venus slides toward the sun, please
00:20:52 --> 00:20:54 don't go hunting for it with binoculars
00:20:54 --> 00:20:56 or a telescope while the sun is above
00:20:56 --> 00:20:59 the horizon. Sweeping optics across the
00:20:59 --> 00:21:01 sun can damage your eyes instantly and
00:21:01 --> 00:21:04 permanently. Never look directly at the
00:21:04 --> 00:21:06 sun. For any solar viewing, use only
00:21:06 --> 00:21:11 eclipse glasses certified to ISO12312-2
00:21:11 --> 00:21:13 and for telescopes and binoculars with
00:21:13 --> 00:21:15 proper certified solar filters over the
00:21:15 --> 00:21:18 front. The crescent moon is in the
00:21:18 --> 00:21:21 evening sky, too. In Ofucus tonight,
00:21:21 --> 00:21:24 about a quarter lit from Sydney, it's
00:21:24 --> 00:21:27 more than 50° up at sunset. From New
00:21:27 --> 00:21:30 York, it's about 17. First quarter falls
00:21:30 --> 00:21:33 on Sunday the 18th in the Americas and
00:21:33 --> 00:21:35 early Monday morning here in Australia.
00:21:35 --> 00:21:38 >> And two of today's stories are overhead
00:21:38 --> 00:21:39 for the south.
00:21:39 --> 00:21:42 >> They are. The teapot of Sagittarius is
00:21:42 --> 00:21:45 high after dark from Australia with the
00:21:45 --> 00:21:48 shredded Sagittarius dwarf galaxy
00:21:48 --> 00:21:51 sitting behind it about 60° up at full
00:21:51 --> 00:21:55 dark from Sydney. And WASP 17 is still
00:21:55 --> 00:21:58 about 24° up in Scorpius at full dark.
00:21:58 --> 00:22:01 The honest balance is that this has been
00:22:01 --> 00:22:04 a southern evening sky all week.
00:22:04 --> 00:22:06 >> So, what does the north get?
00:22:06 --> 00:22:08 >> The mornings.
00:22:08 --> 00:22:11 Saturn is just past opposition, rising
00:22:11 --> 00:22:13 in the east at sunset, shining at
00:22:13 --> 00:22:17 magnitude 0.4 and visible all night
00:22:17 --> 00:22:19 everywhere. Its rings are tilted only
00:22:19 --> 00:22:23 about 7° towards us this year. So, in a
00:22:23 --> 00:22:25 small telescope, they look like a thin
00:22:25 --> 00:22:27 bright blade rather than the wide
00:22:27 --> 00:22:30 ellipse in the textbook pictures, but
00:22:30 --> 00:22:33 the real show is before dawn. Mars and
00:22:33 --> 00:22:37 Jupiter are about 12° apart and closing.
00:22:37 --> 00:22:39 From New York at nautical dawn tomorrow,
00:22:40 --> 00:22:43 Mars is about 56° up and Jupiter about
00:22:43 --> 00:22:48 46. From Sydney, they're 23 and 19.
00:22:48 --> 00:22:50 They'll be just 1.2° 2° apart on the
00:22:50 --> 00:22:53 16th of November. So, watch the gap
00:22:53 --> 00:22:55 shrink week by week.
00:22:55 --> 00:22:58 >> And a meteor shower on the way.
00:22:58 --> 00:23:00 >> The Orionids peak on the night of the
00:23:00 --> 00:23:04 21st into the 22nd of October. Their
00:23:04 --> 00:23:07 dust from Hal's comet, the same comet
00:23:07 --> 00:23:10 that gives us the Eta Aquarids in May.
00:23:10 --> 00:23:13 The radiant is in Orion and rises around
00:23:13 --> 00:23:15 midnight, so the pre-dawn hours are
00:23:15 --> 00:23:18 best. The moon will be about 3/4 lit,
00:23:18 --> 00:23:20 but setting in the early hours, which
00:23:20 --> 00:23:23 help. The north has the edge here, too.
00:23:23 --> 00:23:26 At dawn, the radiant is about 64° up
00:23:26 --> 00:23:29 from New York against 40 from Sydney.
00:23:29 --> 00:23:31 Southern observers still see a
00:23:31 --> 00:23:33 respectable show, just with fewer
00:23:33 --> 00:23:34 meteors per hour.
00:23:34 --> 00:23:36 >> Evenings for the south, mornings for the
00:23:36 --> 00:23:38 north.
00:23:38 --> 00:23:40 >> That's the shape of it this week. Some
00:23:40 --> 00:23:43 weeks it runs the other way.
00:23:43 --> 00:23:46 That's Astronomy Daily for today. We
00:23:46 --> 00:23:49 started with a 20 km volcano, making
00:23:49 --> 00:23:52 clouds the textbook said shouldn't form,
00:23:52 --> 00:23:54 then mapped the edge of a planet in
00:23:54 --> 00:23:57 Scorpius, sorted planetary systems into
00:23:57 --> 00:23:59 two families, and rewound a shredded
00:24:00 --> 00:24:01 galaxy.
00:24:01 --> 00:24:03 >> Tomorrow, we'll have the outcome of
00:24:03 --> 00:24:06 Hera's first breaking burn. And next
00:24:06 --> 00:24:09 Monday, we'll preview Japan's Martian
00:24:09 --> 00:24:13 moons exploration mission, MMX, ahead of
00:24:13 --> 00:24:14 its launch.
00:24:14 --> 00:24:16 >> You'll find the show notes, links to
00:24:16 --> 00:24:18 every paper, and the full back catalog
00:24:18 --> 00:24:21 at astronomyaily.io.
00:24:21 --> 00:24:22 That's also where you can sign up for
00:24:22 --> 00:24:24 the newsletter or send us a question
00:24:24 --> 00:24:26 through the contact form. Several of our
00:24:26 --> 00:24:28 best segments have started that way. We
00:24:28 --> 00:24:32 are astroailyaily pod on social media
00:24:32 --> 00:24:34 and astronomy Daily is part of the
00:24:34 --> 00:24:37 byes.com podcast network. If you're
00:24:37 --> 00:24:40 enjoying the show, a rating or review
00:24:40 --> 00:24:42 really does help other people find us.
00:24:42 --> 00:24:44 Thanks for listening. I'm Anna
00:24:44 --> 00:24:48 >> and I'm Avery. Clear skies
00:24:48 --> 00:24:58 [music and singing]
00:24:58 --> 00:25:06 >> stories [music] told
00:25:06 --> 00:25:08 stories