Astronomy Daily S05E215 — The Star That Was Born Twice (Tuesday 27 October 2026). Hosts Anna and Avery. Lead: the Orion Nebula's age paradox. Its planet-forming discs and its expanding gas shell both look far younger than its million-year-old stars. Wilhelm, Trani and Portegies Zwart (Leiden / Universidad de Concepción) propose that the nebula's most massive star, θ1 Orionis C1, formed a few hundred thousand years ago when two stars merged, driven by their companion's gravity, suddenly switching on harsher radiation. We cover the caveats and the clean test that could falsify it. Also: WD J1328−1450, a white dwarf with two nested rings of gas that point to two hidden planetary bodies; China's AIMS telescope reads a solar flare's magnetic field directly in the mid-infrared; and a Yale study arguing giant impacts can help rocky planets keep hydrogen atmospheres. Quick hits: NASA's new Competition Model 2.0, Swift's mission formally ended ahead of re-entry, Hera's second braking burn, and the DPS meeting in Spokane. Skywatch: finding the Orion Nebula from Sydney, Los Angeles, New York and London; Saturn in the evening; Mercury's last call in the south; Mars and Jupiter converging before dawn. Sources · Wilhelm, Trani & Portegies Zwart, 'A triple-induced merger in θ1 Ori C solves the proplyd lifetime problem and the curiously young shell of M42', A&A, DOI 10.1051/0004-6361/202662018; arXiv 2609.38550 — https://arxiv.org/abs/2609.38550 · Leiden University release, 23 Oct 2026 — https://www.universiteitleiden.nl/en/news/2026/10/leiden-astronomers-explain-the-orion-nebulas-age-paradox · Gentile Fusillo, Lagos-Vilches, Koester et al., 'A double debris disc around a white dwarf sculpted by two planetary bodies', Nature Astronomy, 26 Oct 2026, DOI 10.1038/s41550-026-03004-3 — https://www.nature.com/articles/s41550-026-03004-3 · Deng, Bai, Wei et al., 'Apparent magnetic field decay in a solar flare from Zeeman splitting of the Mg I 12.32-µm line', Nature Astronomy, 23 Oct 2026, DOI 10.1038/s41550-026-03013-2; arXiv 2610.33502 — https://arxiv.org/abs/2610.33502 · Modirrousta-Galian & Korenaga, 'Retention of primordial atmospheres through giant impacts', Nature Astronomy, 26 Oct 2026, DOI 10.1038/s41550-026-03006-1 — https://www.nature.com/articles/s41550-026-03006-1 · NASA Science, 'Faster to Science: A Letter to the Science Community' — https://science.nasa.gov/directorates/smd/faster-to-science-a-letter-to-the-science-community/ · NASA Science, 'NASA's Swift Mission Ends Ahead of Re-Entry', 23 Oct 2026 — https://science.nasa.gov/blogs/swift/2026/10/23/nasas-swift-mission-ends-ahead-of-re-entry/ · ESA Hera mission updates — https://www.esa.int/Space_Safety/Hera
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This episode includes AI-generated content.
00:00:00 --> 00:00:02 Anna: Welcome back. And if you're new to our
00:00:02 --> 00:00:04 podcast, welcome aboard. In the middle of the
00:00:04 --> 00:00:07 Orion Nebula, there is a puzzle that has
00:00:07 --> 00:00:09 bothered astronomers since the first Hubble
00:00:09 --> 00:00:12 pictures came back in the 90s. The baby
00:00:12 --> 00:00:14 planetary systems there are being boiled away
00:00:14 --> 00:00:17 so fast they should be long gone. And yet
00:00:18 --> 00:00:19 there they are today.
00:00:19 --> 00:00:22 Avery: A team from Leiden and Chile thinks it knows
00:00:22 --> 00:00:24 why the star doing the boiling may have been
00:00:24 --> 00:00:27 born twice. Also coming up,
00:00:27 --> 00:00:29 a death star with two rings of planetary
00:00:29 --> 00:00:32 wreckage. The magnetic field of a solar flare
00:00:32 --> 00:00:35 measured directly, and a surprising idea
00:00:35 --> 00:00:37 about giant impacts.
00:00:37 --> 00:00:38 Anna: I'm Anna.
00:00:38 --> 00:00:39 Avery: And I'm Avery.
00:00:39 --> 00:00:42 This is astronomy AstroDailyPod Series 5,
00:00:42 --> 00:00:44 episode 215 for Tuesday
00:00:44 --> 00:00:47 27th October 2026.
00:00:47 --> 00:00:48 Let's get into it.
00:00:49 --> 00:00:51 Anna: Let's start with one of the most photographed
00:00:51 --> 00:00:54 objects in the sky. The orion Nebula,
00:00:54 --> 00:00:56 Messier 42, about
00:00:56 --> 00:00:59 1350 light years away. The
00:00:59 --> 00:01:02 glowing patch in Orion's sword. At
00:01:02 --> 00:01:04 its heart is a tight knot of young stars
00:01:04 --> 00:01:07 called the Trapezium. And the brightest of
00:01:07 --> 00:01:09 them is a massive star called Theta1
00:01:09 --> 00:01:12 Orionis C. It's roughly 33
00:01:12 --> 00:01:15 solar masses and it floods the whole nebula
00:01:15 --> 00:01:16 with ultraviolet light.
00:01:17 --> 00:01:19 Avery: And that's where the puzzle starts. In
00:01:19 --> 00:01:22 1994, Hubble images of the nebula showed
00:01:22 --> 00:01:25 dozens of little dark tadpole shapes. They
00:01:25 --> 00:01:28 turned out to be protoplanetary discs, the
00:01:28 --> 00:01:30 dusty discs around young stars where planets
00:01:30 --> 00:01:33 form, being blasted by that ultraviolet
00:01:33 --> 00:01:35 light. Astronomers called them problets.
00:01:36 --> 00:01:39 Anna: The problem is the arithmetic. The
00:01:39 --> 00:01:41 radiation strips material off those discs
00:01:41 --> 00:01:44 very quickly. Some are losing something
00:01:44 --> 00:01:46 like a millionth of a solar mass every
00:01:46 --> 00:01:49 year. The stars in the cluster are about a
00:01:49 --> 00:01:52 million years old. Run those numbers and the
00:01:52 --> 00:01:54 discs should have been gone long ago,
00:01:55 --> 00:01:57 unless they started out about as heavy as
00:01:57 --> 00:02:00 their own stars, which doesn't
00:02:00 --> 00:02:02 fit anything we know about discs. It's called
00:02:02 --> 00:02:05 the proplid lifetime problem, and it has
00:02:05 --> 00:02:07 been open for about 30 years.
00:02:08 --> 00:02:11 Avery: And there's a second puzzle that looks
00:02:11 --> 00:02:13 just like it. The nebula is surrounded by a
00:02:13 --> 00:02:16 huge shell of gas, inflated by the winds
00:02:16 --> 00:02:19 of the massive stars. Measurements published
00:02:19 --> 00:02:22 in 2019 and 2020 clock that shell
00:02:22 --> 00:02:25 expanding at about 13 kilometres per second.
00:02:25 --> 00:02:28 Run that backwards and the shell is only
00:02:28 --> 00:02:31 about 300 years old. So
00:02:31 --> 00:02:33 the discs look young, the shell looks young,
00:02:34 --> 00:02:35 and, um, the stars look old.
00:02:36 --> 00:02:39 Anna: Two clocks disagreeing with a third. And
00:02:39 --> 00:02:41 when two things disagree with the same
00:02:41 --> 00:02:43 reference in the same direction, it's worth
00:02:43 --> 00:02:46 asking whether one cause sits behind both.
00:02:47 --> 00:02:50 Avery: That's exactly what Micah Wilhelm and Simon
00:02:50 --> 00:02:52 Portugi Swart at Leyden Observatory with
00:02:52 --> 00:02:54 Alessandro Trani at the University of
00:02:54 --> 00:02:57 Concepcion in Chile have proposed. Their
00:02:57 --> 00:02:59 paper is impress at astronomy and
00:02:59 --> 00:03:02 astrophysics, and Leiden announced it on 23
00:03:02 --> 00:03:04 October, she said. The preprint first
00:03:04 --> 00:03:06 appeared in September, so this is published
00:03:06 --> 00:03:09 this month rather than brand new this week.
00:03:09 --> 00:03:12 Anna: Their suggestion is that Theta1 Orionis
00:03:12 --> 00:03:15 C1, the big star, wasn't always one
00:03:15 --> 00:03:18 star. It was two stars in a close pair
00:03:18 --> 00:03:20 that merged into one. And here's the key
00:03:20 --> 00:03:23 physics a uh star's power output rises
00:03:23 --> 00:03:26 much faster than its mass. So 1 star
00:03:26 --> 00:03:29 of 33 solar masses shines considerably
00:03:29 --> 00:03:31 brighter than 2 stars adding up to 33.
00:03:32 --> 00:03:35 Avery: So before the merger, the nebula had a
00:03:35 --> 00:03:38 dimmer central engine. The discs could
00:03:38 --> 00:03:40 survive for most of the cluster's life. Then
00:03:40 --> 00:03:43 a few hundred thousand years ago, the two
00:03:43 --> 00:03:45 stars collided. The radiation and the stellar
00:03:45 --> 00:03:48 wind jumped and and the discs started boiling
00:03:48 --> 00:03:50 off. The shell got a fresh shove at the same
00:03:50 --> 00:03:53 moment. Extrapolate today's fast rates
00:03:53 --> 00:03:55 backwards and you'd get the wrong age for
00:03:55 --> 00:03:56 both of them.
00:03:56 --> 00:03:59 Anna: And there's a nice detail in why the two
00:03:59 --> 00:04:02 stars merge. Theta 1 Orionis
00:04:02 --> 00:04:04 C already has a companion today, a star
00:04:04 --> 00:04:07 of about 12 solar masses on an 11 year
00:04:07 --> 00:04:10 orbit. The team suggests that companion
00:04:10 --> 00:04:13 once orbited an inner pair and its gravity
00:04:13 --> 00:04:16 slowly pumped up the inner pair's orbit,
00:04:16 --> 00:04:19 making it more and more elongated until
00:04:19 --> 00:04:21 the two inner stars touched.
00:04:21 --> 00:04:24 It's a three body effect with a long the
00:04:24 --> 00:04:26 von Zepol Litov Cosi mechanism.
00:04:27 --> 00:04:29 And it's the same physics that can push
00:04:29 --> 00:04:31 comets and planets onto extreme orbits.
00:04:32 --> 00:04:34 Avery: They ran Monte Carlo simulations of nearly
00:04:34 --> 00:04:37 2 possible starting triples. Outside the
00:04:37 --> 00:04:40 very tightest configurations, roughly half of
00:04:40 --> 00:04:43 them merged within a million years. So the
00:04:43 --> 00:04:45 starting arrangement isn't exotic. Most
00:04:45 --> 00:04:47 massive O type stars are in multiple systems
00:04:47 --> 00:04:48 anyway.
00:04:48 --> 00:04:51 Anna: And the merger story explains two other
00:04:51 --> 00:04:53 oddities about this star. It has a magnetic
00:04:53 --> 00:04:56 field tilted relative to its rotation
00:04:56 --> 00:04:59 and a hint of extra nitrogen in its light.
00:04:59 --> 00:05:02 Both are things you'd expect if two stars had
00:05:02 --> 00:05:05 been stirred together, mixing core material
00:05:05 --> 00:05:06 into the outer layers.
00:05:07 --> 00:05:09 Avery: Now this is where we slow down, because the
00:05:09 --> 00:05:12 authors are refreshingly upfront about what
00:05:12 --> 00:05:14 doesn't quite work yet.
00:05:14 --> 00:05:17 First, the size of the jump. To explain the
00:05:17 --> 00:05:20 shell's speed, the star's wind power needs to
00:05:20 --> 00:05:22 have risen by a factor of about 100.
00:05:23 --> 00:05:25 A simple merger of two stars gives you at
00:05:25 --> 00:05:26 most about 30.
00:05:27 --> 00:05:29 Anna: They propose the gap is closed by cool gas
00:05:30 --> 00:05:33 evaporating off those propolids and mixing
00:05:33 --> 00:05:35 into the hot bubble, which changes how
00:05:35 --> 00:05:38 efficiently the wind pushes. But that effect
00:05:38 --> 00:05:41 has to be strong enough, and they spell out
00:05:41 --> 00:05:43 the minimum strength needed. It hasn't been
00:05:43 --> 00:05:44 demonstrated.
00:05:44 --> 00:05:47 Avery: Second timing in their simulations, when
00:05:47 --> 00:05:50 these triples merge, they often merge too
00:05:50 --> 00:05:52 quickly within hundreds or thousands of years
00:05:52 --> 00:05:55 of forming. Not hundreds of thousands. The
00:05:55 --> 00:05:58 team suggests ways to delay it Stronger tides
00:05:58 --> 00:06:00 while the stars were still young and puffy.
00:06:01 --> 00:06:04 Rotation or a passing star nudging a
00:06:04 --> 00:06:07 stable triple into the unstable mode. They
00:06:07 --> 00:06:08 put the chance of that last one at, uh,
00:06:08 --> 00:06:11 roughly 10%. Plausible, not
00:06:11 --> 00:06:11 proven.
00:06:12 --> 00:06:14 Anna: Third, the best fitting version wants the two
00:06:14 --> 00:06:17 original stars to have been near twins in
00:06:17 --> 00:06:19 mass, and the authors estimate the odds of
00:06:19 --> 00:06:21 that at only 2 to 3%.
00:06:22 --> 00:06:24 And the nitrogen excess was measured from the
00:06:24 --> 00:06:26 combined light of C1 and its companion,
00:06:27 --> 00:06:29 so it still needs confirming in C1 alone.
00:06:30 --> 00:06:33 Avery: But here is what makes it good shrines it
00:06:33 --> 00:06:34 sticks its neck out.
00:06:34 --> 00:06:37 There's an earlier claim from spectra that
00:06:37 --> 00:06:40 Theta 1 Orionis C1 has a small
00:06:40 --> 00:06:43 third companion, about one solar mass
00:06:43 --> 00:06:45 on a 61 day orbit. The authors say
00:06:45 --> 00:06:48 plainly that if that companion is confirmed,
00:06:48 --> 00:06:51 their model is effectively falsified. When
00:06:51 --> 00:06:53 they simulated that configuration, it merged
00:06:53 --> 00:06:56 in a median of about 135 years.
00:06:56 --> 00:06:59 So they argue it can't exist. An observation
00:06:59 --> 00:07:01 from the Gravity Instrument on the Very Large
00:07:01 --> 00:07:03 Telescope didn't confirm it, uh, but the
00:07:03 --> 00:07:05 resolution was borderline, which
00:07:05 --> 00:07:08 Anna: is a lovely clean test. And
00:07:08 --> 00:07:11 because the cluster is so well studied, it
00:07:11 --> 00:07:12 may not take long.
00:07:12 --> 00:07:15 Avery: Co author Simon Porteghu Zwart says
00:07:15 --> 00:07:17 this would be the first time we've seen the
00:07:17 --> 00:07:20 consequences of a major stellar collision in
00:07:20 --> 00:07:23 our own cosmic neighbourhood, and that
00:07:23 --> 00:07:24 collisions may be more common than we
00:07:24 --> 00:07:27 thought. Similar triple mergers have been
00:07:27 --> 00:07:30 suggested for Eta Carinae and its great
00:07:30 --> 00:07:33 eruption in 1843, which is a southern
00:07:33 --> 00:07:36 sky object, and for the V838
00:07:36 --> 00:07:37 monocerratus outburst.
00:07:38 --> 00:07:40 Anna: And there's a planetary angle.
00:07:40 --> 00:07:43 Most stars, very possibly including the sun,
00:07:43 --> 00:07:46 formed in crowded clusters near massive
00:07:46 --> 00:07:48 stars. If a merger can switch on a much
00:07:48 --> 00:07:51 harsher radiation field partway through a
00:07:51 --> 00:07:54 cluster's life, then whether a young star
00:07:54 --> 00:07:56 keeps its planet forming disc depends
00:07:56 --> 00:07:59 not just on where it was born, but when.
00:07:59 --> 00:08:02 Avery: And one small housekeeping note the paper's
00:08:02 --> 00:08:05 acknowledgments. Thank Mordecai Mark Maclo
00:08:05 --> 00:08:08 for feedback. The same Maclo who co authored
00:08:08 --> 00:08:10 the Andromeda Superbubble paper we led with
00:08:10 --> 00:08:13 back in episode 199 Smallfield.
00:08:13 --> 00:08:16 Anna: And the best part, Orion is rising
00:08:16 --> 00:08:18 in the late evening right now for listeners
00:08:18 --> 00:08:20 everywhere. So you can go and look at the
00:08:20 --> 00:08:23 scene of the crime tonight. We'll tell you
00:08:23 --> 00:08:24 exactly when in the skywatch.
00:08:25 --> 00:08:28 Avery: From the birth of stars to the death of one,
00:08:28 --> 00:08:31 White dwarfs are the cores left behind when
00:08:31 --> 00:08:33 sun like stars die. And a few percent of them
00:08:33 --> 00:08:36 show something remarkable wreckage from their
00:08:36 --> 00:08:38 old planetary systems.
00:08:38 --> 00:08:40 Asteroids and comets that strayed too close
00:08:40 --> 00:08:43 and were torn apart by tides into a disc.
00:08:43 --> 00:08:45 Anna: Usually we see that as a single ring of gas
00:08:45 --> 00:08:47 and the giveaway is in the spectrum.
00:08:48 --> 00:08:51 Gas orbiting in a ring makes an emission line
00:08:51 --> 00:08:53 with two peaks. One from the side coming
00:08:53 --> 00:08:56 towards us and one from the side going away.
00:08:57 --> 00:08:58 Astronomers have found a couple of dozen of
00:08:58 --> 00:09:00 these over the past 20 years.
00:09:00 --> 00:09:02 Avery: Now a team led by Nicola Pietro Gentile
00:09:02 --> 00:09:05 Fusilio with Felipe Lagos Vilchas at the
00:09:05 --> 00:09:07 University of Warwick and Detlef Coaster at
00:09:07 --> 00:09:10 Kiel, has published a white dwarf that shows
00:09:10 --> 00:09:13 four peaks in its calcium lines instead of
00:09:13 --> 00:09:15 two. The paper came out in Nature Astronomy
00:09:15 --> 00:09:18 on 26 October and it's open access.
00:09:18 --> 00:09:21 Anna: The star is called WDJ
00:09:21 --> 00:09:24 1328 -1450,
00:09:24 --> 00:09:27 a white dwarf of about 17
00:09:27 --> 00:09:30 degrees and roughly half a solar mass.
00:09:30 --> 00:09:33 And four peaks means two separate rings of
00:09:33 --> 00:09:35 gas, one nested inside the other
00:09:35 --> 00:09:38 seem fairly close to edge on the
00:09:38 --> 00:09:40 scale is wonderful.
00:09:41 --> 00:09:44 Avery: The inner ring sits about 100
00:09:44 --> 00:09:47 to 240 kilometres from the star.
00:09:47 --> 00:09:49 The outer ring runs from roughly
00:09:49 --> 00:09:52 380 kilometres out to more
00:09:52 --> 00:09:55 than a million. So the outer ring is about
00:09:55 --> 00:09:57 three times the distance from the Earth to
00:09:57 --> 00:09:59 the Moon. All of it squeezed around a star
00:09:59 --> 00:10:00 smaller than Earth.
00:10:01 --> 00:10:04 Anna: And the two rings aren't the same. The
00:10:04 --> 00:10:07 outer one is brighter in calcium, but the
00:10:07 --> 00:10:09 team finds magnesium is strongly
00:10:09 --> 00:10:11 depleted in it compared with the inner ring.
00:10:12 --> 00:10:14 Different chemistry suggests different
00:10:14 --> 00:10:15 sources.
00:10:15 --> 00:10:17 Avery: That's the heart of their argument. The
00:10:17 --> 00:10:20 journal summarises it as two hidden planetary
00:10:20 --> 00:10:23 bodies shaping the two rings. One asteroid
00:10:23 --> 00:10:26 sized and one Pluto sized. Neither body
00:10:26 --> 00:10:28 is seen directly. They're inferred from the
00:10:28 --> 00:10:30 structure and the chemistry of the gas.
00:10:30 --> 00:10:33 Anna: Two honest notes. First,
00:10:34 --> 00:10:36 data age. The spectra come from the Very
00:10:36 --> 00:10:39 Large Telescope's X shooter instrument at
00:10:39 --> 00:10:42 Paranal in chile, taken in 2021
00:10:42 --> 00:10:44 and 2023. And Gentile
00:10:44 --> 00:10:47 Fusillo first presented this object at a
00:10:47 --> 00:10:48 conference in 2022.
00:10:49 --> 00:10:51 So the Star has been known as a puzzle for
00:10:51 --> 00:10:53 about four years. What's new is the published
00:10:53 --> 00:10:56 model. Second, the, uh, dust around
00:10:56 --> 00:10:59 the star is harder to pin down than the gas.
00:10:59 --> 00:11:01 And the authors note that several dust models
00:11:01 --> 00:11:02 fit equally well.
00:11:03 --> 00:11:05 Avery: There's a southern thread too. Besides
00:11:05 --> 00:11:07 Paranal, the team's brightness measurements
00:11:07 --> 00:11:10 draw on Skymapper, the survey telescope run
00:11:10 --> 00:11:12 by the Australian National University. At
00:11:12 --> 00:11:15 sighting Spring, the star sits at about minus
00:11:15 --> 00:11:17 14 degrees declination, so it's reachable
00:11:17 --> 00:11:19 from Both hemispheres. Though at around
00:11:19 --> 00:11:21 magnitude 17, it's a job for serious
00:11:21 --> 00:11:22 telescopes.
00:11:23 --> 00:11:25 Anna: And it fits a run of white dwarf stories
00:11:26 --> 00:11:28 from the second generation planet candidate
00:11:28 --> 00:11:31 we covered in episode 201 to the
00:11:31 --> 00:11:34 green white dwarf in 206. It's
00:11:34 --> 00:11:36 a reminder that our own solar system will one
00:11:36 --> 00:11:39 day be a white dwarf with a ring of
00:11:39 --> 00:11:40 whatever's left.
00:11:41 --> 00:11:43 Avery: Solar flares are magnetic events.
00:11:43 --> 00:11:46 Twisted magnetic fields above sunspots
00:11:46 --> 00:11:49 suddenly snap into a simpler shape and
00:11:50 --> 00:11:52 release energy. But measuring the magnetic
00:11:52 --> 00:11:55 field during a flare is genuinely hard.
00:11:55 --> 00:11:57 And most of what we know is inferred
00:11:57 --> 00:11:58 indirectly.
00:11:58 --> 00:12:01 Anna: Here's why. The main tool is
00:12:01 --> 00:12:04 the Zeeman effect. A magnetic field
00:12:04 --> 00:12:06 splits a spectral line into components.
00:12:06 --> 00:12:09 In visible light, the split is tiny compared
00:12:09 --> 00:12:12 with the line's width. So instruments measure
00:12:12 --> 00:12:14 the polarisation of the light and then infer
00:12:14 --> 00:12:17 the field with models and assumptions.
00:12:17 --> 00:12:19 Avery: But the splitting grows faster with
00:12:19 --> 00:12:21 wavelength than the line's own width does. Go
00:12:21 --> 00:12:24 far enough into the infrared and the split
00:12:24 --> 00:12:26 becomes wide enough to see directly. There's
00:12:26 --> 00:12:29 a magnesium line at 12.32 microns.
00:12:29 --> 00:12:31 That's one of the most magnetically sensitive
00:12:31 --> 00:12:33 lines known on the sun.
00:12:33 --> 00:12:36 Anna: China built a telescope specifically to use
00:12:36 --> 00:12:39 it. It's called ames, the Accurate
00:12:39 --> 00:12:41 Infrared Magnetic Field Measurements of the
00:12:41 --> 00:12:43 sun telescope at about 4
00:12:43 --> 00:12:46 metres altitude at Langhu in Qinghai
00:12:46 --> 00:12:49 Province, a high dry site that keeps
00:12:49 --> 00:12:52 water vapour out of the way. It was declared
00:12:52 --> 00:12:54 operational about a year ago as the world's
00:12:54 --> 00:12:57 first mid infrared solar magnetic field
00:12:57 --> 00:12:59 telescope, with accuracy better than 10
00:12:59 --> 00:13:00 gauss.
00:13:00 --> 00:13:03 Avery: And now the first big result in nature
00:13:03 --> 00:13:05 astronomy. On 23 October,
00:13:05 --> 00:13:08 Yongyang Deng, Xianyang, Bai,
00:13:08 --> 00:13:10 Sholin Wei and colleagues at the National
00:13:10 --> 00:13:13 Astronomical Observatories of China report
00:13:13 --> 00:13:16 AMES observations through an X 1.5
00:13:16 --> 00:13:19 flare. By measuring the Zeeman splitting
00:13:19 --> 00:13:21 directly, they saw the magnetic field weaken
00:13:21 --> 00:13:23 beneath the flare ribbons during the
00:13:23 --> 00:13:26 impulsive phase, the most violent part of the
00:13:26 --> 00:13:26 flare.
00:13:27 --> 00:13:29 Anna: That's exactly what you'd naively expect.
00:13:29 --> 00:13:32 A flare releases magnetic energy, so the
00:13:32 --> 00:13:35 field should drop somewhere. Seeing it in a
00:13:35 --> 00:13:37 direct measurement is a genuine step, but
00:13:37 --> 00:13:38 notice the word
00:13:38 --> 00:13:41 Avery: in the title apparent. The
00:13:41 --> 00:13:44 authors offer two the field may
00:13:44 --> 00:13:47 really have been rearranged locally, or the
00:13:47 --> 00:13:49 flare's heating may have lifted the layer
00:13:49 --> 00:13:52 where this magnesium line forms to a greater
00:13:52 --> 00:13:54 height, where the field is naturally weaker.
00:13:54 --> 00:13:56 In that case, you're measuring a different
00:13:56 --> 00:13:58 altitude, not a weaker field.
00:13:59 --> 00:14:02 Anna: And that distinction matters because flare
00:14:02 --> 00:14:04 models depend on knowing which it is.
00:14:04 --> 00:14:07 Separating the two will take more flares and
00:14:07 --> 00:14:09 comparisons with other instruments looking at
00:14:09 --> 00:14:11 different heights at the same time.
00:14:11 --> 00:14:14 Avery: This joins A solar thread we've followed for
00:14:14 --> 00:14:16 a while. From the superflare potential story
00:14:16 --> 00:14:19 in episode 192 to the radiation storm
00:14:19 --> 00:14:21 measured by weather balloons in 211,
00:14:22 --> 00:14:24 better magnetic measurements are the
00:14:24 --> 00:14:25 foundation for better space weather
00:14:25 --> 00:14:27 forecasting. And, um, that matters to
00:14:27 --> 00:14:30 everyone with a GPS receiver or a power grid.
00:14:31 --> 00:14:34 Anna: And one practical reminder, because we're
00:14:34 --> 00:14:37 talking about looking at the sun, never look
00:14:37 --> 00:14:39 at the sun directly or through binoculars or
00:14:39 --> 00:14:41 a telescope without proper protection.
00:14:42 --> 00:14:44 Eclipse glasses must meet the ISO
00:14:44 --> 00:14:47 123122 standard,
00:14:47 --> 00:14:50 and telescopes need certified solar filters
00:14:50 --> 00:14:51 over the front of the instrument.
00:14:52 --> 00:14:55 Avery: One of the clearest patterns in exoplanet
00:14:55 --> 00:14:57 science is a gap among small
00:14:58 --> 00:15:00 planets close to their stars. There are lots
00:15:00 --> 00:15:02 of rocky super Earths up to about
00:15:03 --> 00:15:05 one and a half times Earth size, and lots of
00:15:05 --> 00:15:08 puffier sub Neptunes around 2 to 3
00:15:08 --> 00:15:11 times Earth size, but surprisingly few
00:15:11 --> 00:15:14 in between. It's called the Radius
00:15:14 --> 00:15:17 Valley, first clearly measured from kepler
00:15:17 --> 00:15:18 data in 2017.
00:15:19 --> 00:15:22 Anna: The usual explanation is that many of these
00:15:22 --> 00:15:24 planets started with thick hydrogen
00:15:24 --> 00:15:26 atmospheres and some lost them,
00:15:27 --> 00:15:29 stripped by their star's radiation or
00:15:29 --> 00:15:31 boiled off by the heat of their own
00:15:31 --> 00:15:34 interiors, leaving bare rock planets
00:15:34 --> 00:15:37 that kept their hydrogen stay puffy.
00:15:37 --> 00:15:39 Avery: Hence two groups and giant
00:15:39 --> 00:15:42 impacts. Collisions between young planets
00:15:42 --> 00:15:44 are usually thought of as atmosphere
00:15:44 --> 00:15:47 strippers too. Smash two planets together
00:15:47 --> 00:15:49 and you'd expect to blast gas away.
00:15:50 --> 00:15:53 Anna: A new paper in nature astronomy on, um, 26
00:15:53 --> 00:15:55 October argues it can go the other way.
00:15:56 --> 00:15:59 Darius Modir Rastagalian and Yoon
00:15:59 --> 00:16:01 Kornaga at Yale find that giant
00:16:01 --> 00:16:04 impacts can help rocky planets retain
00:16:04 --> 00:16:07 hydrogen rich atmospheres. Their argument is
00:16:07 --> 00:16:09 that the mass the planet gains in a collision
00:16:09 --> 00:16:11 increases how tightly its gravity holds the
00:16:11 --> 00:16:14 atmosphere by more than the collision's heat
00:16:14 --> 00:16:15 dries gas away.
00:16:15 --> 00:16:18 Avery: In other words, it's a tug of war between a
00:16:18 --> 00:16:21 heavier planet and a hotter one.
00:16:21 --> 00:16:24 And they find heavier tends to win. And
00:16:24 --> 00:16:26 they suggest this could help explain why
00:16:26 --> 00:16:29 exoplanet sizes split into two groups in the
00:16:29 --> 00:16:29 first place.
00:16:30 --> 00:16:32 Anna: What's interesting is that the same pair
00:16:32 --> 00:16:35 argued in a preprint last year that very hot
00:16:35 --> 00:16:37 surfaces after an impact can drive off
00:16:37 --> 00:16:40 hydrogen remarkably fast. So
00:16:40 --> 00:16:42 this isn't a team that assumes impacts are
00:16:42 --> 00:16:44 gentle. They're weighing both sides of the
00:16:44 --> 00:16:45 ledger.
00:16:45 --> 00:16:48 Avery: The caveats? This is a theoretical result
00:16:48 --> 00:16:50 and we're reporting it at the level of the
00:16:50 --> 00:16:52 Journal's own summary.
00:16:52 --> 00:16:54 And there are several competing explanations
00:16:54 --> 00:16:57 for the Radius Valley, including the idea
00:16:57 --> 00:17:00 that some sub neptunes are water worlds
00:17:00 --> 00:17:02 rather than rocky planets wrapped in
00:17:02 --> 00:17:04 hydrogen. This adds a mechanism to that
00:17:04 --> 00:17:07 debate rather than settling it the test will
00:17:07 --> 00:17:10 Anna: come from planets, ages, masses and
00:17:10 --> 00:17:13 atmospheres measured together, which
00:17:13 --> 00:17:15 JWST and the coming PLATO mission
00:17:15 --> 00:17:17 are well placed to provide.
00:17:17 --> 00:17:20 Avery: A few quick updates. First, NASA is
00:17:20 --> 00:17:23 changing how it picks its science missions
00:17:23 --> 00:17:26 under what it calls Competition Model 2.0
00:17:26 --> 00:17:28 announced on the 21st. Proposals for
00:17:28 --> 00:17:31 discovery, New Frontiers and the new
00:17:31 --> 00:17:33 Astroprobes class compete on the science
00:17:33 --> 00:17:36 first with detailed engineering and cost
00:17:36 --> 00:17:39 deferred. NASA says selection time drops
00:17:39 --> 00:17:41 from about 35 months to about 19 and a half.
00:17:42 --> 00:17:44 Selected concepts then go through
00:17:44 --> 00:17:46 implementation studies with Goddard or jpl.
00:17:47 --> 00:17:49 And NASA warns that breaching cost or
00:17:49 --> 00:17:51 schedule thresholds could mean termination.
00:17:52 --> 00:17:54 The community town hall was held On Monday
00:17:54 --> 00:17:56 afternoon, US time. Um.
00:17:56 --> 00:17:58 Anna: Second Swift NASA
00:17:58 --> 00:18:01 formally closed the mission on the 23rd. The
00:18:01 --> 00:18:04 observatory reached about 275
00:18:04 --> 00:18:06 kilometres on the 20th, fell into safe
00:18:06 --> 00:18:09 mode and the team at Penn State sent the
00:18:09 --> 00:18:12 command that ended operations. Reentry
00:18:12 --> 00:18:14 is expected within the next few weeks. Its
00:18:14 --> 00:18:17 orbit is inclined about 21 degrees, so it
00:18:17 --> 00:18:19 will come down somewhere in the tropics, a
00:18:19 --> 00:18:22 band that includes northern Australia. Most
00:18:22 --> 00:18:23 of it should burn up.
00:18:23 --> 00:18:25 Avery: Third ESA's HERA spacecraft.
00:18:26 --> 00:18:29 ESA says the second braking burn on the 22nd
00:18:29 --> 00:18:31 was a success, cutting its speed relative to
00:18:31 --> 00:18:34 the Didymos asteroid system by 70 metres per
00:18:34 --> 00:18:36 second. Three smaller manoeuvres follow
00:18:36 --> 00:18:39 before arrival in November and the
00:18:39 --> 00:18:41 planetary science community is gathered in
00:18:41 --> 00:18:43 Spokane, Washington this week for the annual
00:18:43 --> 00:18:45 Division for Planetary Sciences meeting.
00:18:45 --> 00:18:47 We'll bring you the best of it.
00:18:47 --> 00:18:50 Anna: Let's go and find the Orion Nebula. And this
00:18:50 --> 00:18:53 is one of those lovely objects that genuinely
00:18:53 --> 00:18:56 belongs to everyone. Because it sits almost
00:18:56 --> 00:18:58 exactly on the celestial equator at about
00:18:59 --> 00:19:01 5 degrees south, it's well placed from
00:19:01 --> 00:19:02 both hemispheres.
00:19:03 --> 00:19:05 Avery: Here are the times for Wednesday night, the
00:19:05 --> 00:19:08 28th. In Sydney the nebula rises at
00:19:08 --> 00:19:10 about a quarter to 10 in the evening,
00:19:10 --> 00:19:13 AEDT by 2 in the morning it's
00:19:13 --> 00:19:15 about 50 degrees up in the northeast
00:19:16 --> 00:19:18 and it reaches its highest a, uh, very
00:19:18 --> 00:19:21 comfortable 61 degrees just after
00:19:21 --> 00:19:21 4am
00:19:22 --> 00:19:25 Anna: In Los Angeles it rises at about 10
00:19:25 --> 00:19:27 past 10 and in New York at about
00:19:27 --> 00:19:30 20 past 10. By 2am
00:19:30 --> 00:19:33 it's around 40 degrees up in the Southeast
00:19:33 --> 00:19:35 from LA and 36 from New York
00:19:36 --> 00:19:38 and London, where the clocks went back on
00:19:38 --> 00:19:41 Sunday, sees it rise at about half past
00:19:41 --> 00:19:43 nine, climbing to about 33 degrees at uh,
00:19:43 --> 00:19:44 best
00:19:44 --> 00:19:46 Avery: around 3am so the south
00:19:46 --> 00:19:49 gets the higher view this time about 60
00:19:49 --> 00:19:52 degrees against 30 to 50 in the north.
00:19:52 --> 00:19:55 But here's the fun part. From the north,
00:19:55 --> 00:19:57 Orion rises on its side with red
00:19:57 --> 00:20:00 Betelgeese leading from Sydney, blue
00:20:00 --> 00:20:03 white Rigel and The Sword lead the way and
00:20:03 --> 00:20:05 Orion appears upside down with the Sword
00:20:05 --> 00:20:08 above the belt from Sydney. The nebula
00:20:08 --> 00:20:11 is up nearly an hour before Betelgee's.
00:20:11 --> 00:20:13 Anna: Look below the three belt stars in the north
00:20:13 --> 00:20:16 or above them in the south for a short line
00:20:16 --> 00:20:19 of three fainter stars. That's the
00:20:19 --> 00:20:22 sword and the fuzzy middle one is the nebula.
00:20:22 --> 00:20:25 Binoculars show the glow. A small
00:20:25 --> 00:20:28 telescope shows four stars and a tiny
00:20:28 --> 00:20:29 trapezoid at its heart.
00:20:29 --> 00:20:32 And the brightest is Theta 1 Orionis
00:20:32 --> 00:20:34 C. Today's lead, you'll be looking at the
00:20:34 --> 00:20:37 star that may have been born twice.
00:20:37 --> 00:20:39 Avery: One complication the Moon.
00:20:40 --> 00:20:42 Monday was the full hunter's moon and this
00:20:42 --> 00:20:45 week it's a bright waning gibbous rising in
00:20:45 --> 00:20:48 the evening, about 90% lit. On Wednesday
00:20:49 --> 00:20:51 it'll wash out the faint outer nebula, but
00:20:51 --> 00:20:53 the bright core shows through moonlight.
00:20:53 --> 00:20:56 Well, last quarter is on the 1st of November
00:20:56 --> 00:20:58 and the darkest skies come with the new moon
00:20:58 --> 00:20:58 on the 9th.
00:20:59 --> 00:21:01 Anna: Now the planets in the evening.
00:21:01 --> 00:21:04 Saturn is the steady yellowish light
00:21:04 --> 00:21:07 high in the east to northeast after dusk
00:21:07 --> 00:21:10 at Ah, magnitude half. From Sydney it's about
00:21:10 --> 00:21:13 31 degrees up at the end of Twilight. From
00:21:13 --> 00:21:16 LA, about 19 from New York 16
00:21:17 --> 00:21:20 and from London 11 Southern
00:21:20 --> 00:21:20 listeners
00:21:20 --> 00:21:23 Avery: also get Mercury's Last Call from Sydney.
00:21:23 --> 00:21:26 It's about 9 degrees up in the west at the
00:21:26 --> 00:21:28 end of civil twilight around magnitude
00:21:28 --> 00:21:31 1.7. Find a flat western horizon
00:21:31 --> 00:21:34 in the next few evenings. From the north it's
00:21:34 --> 00:21:35 already lost in
00:21:35 --> 00:21:37 Anna: the glare and the north gets the pre dawn
00:21:37 --> 00:21:40 show. Mars and Jupiter are drawing together
00:21:40 --> 00:21:43 in the eastern morning sky at
00:21:43 --> 00:21:44 nautical dawn on Thursday.
00:21:44 --> 00:21:47 They're about 60 degrees up from Los Angeles
00:21:47 --> 00:21:50 and New York and about 50 from London, but
00:21:50 --> 00:21:52 only around 25 from Sydney.
00:21:52 --> 00:21:55 They're about 7 degrees apart now closing to
00:21:55 --> 00:21:58 under 6 by 1 November, 3 by the
00:21:58 --> 00:22:01 8th and just over 1 degree around the
00:22:01 --> 00:22:04 16th. Jupiter is the brilliant one at
00:22:04 --> 00:22:06 magnitude -1.9, with Mars
00:22:06 --> 00:22:08 much fainter and redder beside it.
00:22:09 --> 00:22:12 Avery: Venus passed between us and the sun last
00:22:12 --> 00:22:14 week and is still too close to it to see.
00:22:15 --> 00:22:17 Look forward to emerge low in the east before
00:22:17 --> 00:22:18 dawn over the coming weeks.
00:22:19 --> 00:22:22 Anna: And one standing reminder Swift may come
00:22:22 --> 00:22:25 down anytime in the next few weeks and re
00:22:25 --> 00:22:28 entries are sometimes seen as fireballs. If
00:22:28 --> 00:22:30 you see something spectacular in the tropics,
00:22:30 --> 00:22:32 note the time and direction and report it to
00:22:32 --> 00:22:34 your national space or meteor network.
00:22:35 --> 00:22:38 Avery: That's the show today. A possible
00:22:38 --> 00:22:40 stellar collision behind Orion's age
00:22:40 --> 00:22:43 paradox, a white dwarf ringed by
00:22:43 --> 00:22:46 two bands of planetary wreckage, a
00:22:46 --> 00:22:49 flare's magnetic field read directly in the
00:22:49 --> 00:22:52 infrared and a case that giant impacts
00:22:52 --> 00:22:53 can help planets keep their air.
00:22:54 --> 00:22:56 Anna: Full sources and links are in the show notes
00:22:56 --> 00:22:59 at, uh, astronomydaily IO, where you can also
00:22:59 --> 00:23:02 sign up for the newsletter and send us your
00:23:02 --> 00:23:04 questions through the contact form and keep
00:23:04 --> 00:23:04 them coming.
00:23:05 --> 00:23:07 Listener questions have shaped segments on
00:23:07 --> 00:23:07 this show before.
00:23:08 --> 00:23:11 Avery: Find us as astrodaily pod on social media
00:23:11 --> 00:23:14 and on, um, the bytes.com podcast network.
00:23:14 --> 00:23:17 And if you get a clear night, go and find the
00:23:17 --> 00:23:20 Orion Nebula. It's a remarkable thing to look
00:23:20 --> 00:23:22 at a star and wonder whether it used to be
00:23:22 --> 00:23:23 too.
00:23:23 --> 00:23:24 Anna: I'm Anna.
00:23:24 --> 00:23:25 Clear skies, everyone.
00:23:40 --> 00:23:49 Avery: Sam.