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Exoplanet Stuff

 
 
 
The authors of this recent TRAPPIST-1 paper have found something remarkable about the star's seven planets, something they found with the help of their improved mass values.

They used a composition model inspired by the Solar System's planets: an iron core, an iron-magnesium-silicate mantle, and a possible super water ocean.

They first tried modeling with no ocean. That is a tolerable approximation for their closest approximation in the Solar System, the inner planets and the Moon. The inner planet with the most surface water is the Earth, but its ocean water has 0.023% of its mass, and its ocean contains nearly all of its surface water, The other inner planets and the Moon have less surface water than the Earth, including zero. The Earth's ocean has an average depth of 3.8 km, or a planetwide average of 2.6 km.

They did their modeling by varying the amount of iron in the planets, and by using Solar-System comparisons:
  • Earth: 32.5%
  • Mars: 30%
  • Stony meteorites (L/C1 chondrites): 18%
  • Moon: 10%
  • b: 25+-6%, c: 27+-5%, d: 20+-5%, e: 25+-5%, f: 20+-4%, g: 16+-4%, h: 16+-10%
They found that the planets approximately fit a line between innermost iron fraction of 25% and an outermost iron fraction of 15%.

They then considered an alternative hypothesis: that all the planets contain the same fraction of iron. They then estimated how much water that the planets then have. Using an Earthlike iron fraction, they found these water fractions:
  • b: 0, c: 0, d: 0, e: 2.9+-1.6%, f: 4.5+-1.5%, g: 6.4+-1.8%, h: 5.5+-3.8%
Meaning ocean depths of 300 - 700 km, ignoring compression.

The Earth's ocean pressure goes up by about 10^4 pascal/m, meaning that 100 km gives 1 gigapascal (GPa). From Water phase diagram, liquid water reaches 1.2 g/cm^3 at 2.2 GPa, and ice VII is at 1.5 GPa.

So about 200 km down, room-temperature water becomes Ice VII. The deepest of these possible planet oceans thus has 200 km of liquid water and 300 km of Ice VII.

The next phase transition up in pressure is to Ice X at 47 GPa.
 
Some Planets May Be Better for Life Than Earth: Researchers Identify 24 Superhabitable Exoplanets
The Best of All Possible Worlds
noting
In Search for a Planet Better than Earth: Top Contenders for a Superhabitable World | Astrobiology

Superhabitable exoplanets?

Authors Dirk Schulze-Makuch, René Heller, and Edward Guinan discuss a lot of features before settling on these ones:
  • In orbit around a K dwarf star
  • About 5–8 billion years old
  • Up to 1.5 times more massive than Earth and about 10% larger than Earth
  • Mean surface temperature about 5°C higher than on Earth
  • Moist atmosphere with 25–30% O2 levels, the rest mostly inert gases (e.g., N2)
  • Scattered land/water distributed with lots of shallow water areas and archipelagos
  • Large moon (1–10% of the planetary mass) at moderate distance (10–100 planetary radii)
  • Has plate tectonics or similar geological/geochemical recycling mechanism as well as a strong protective geomagnetic field
K dwarfs are 12% of stars in our Galaxy, compared with G dwarfs, 8% (the Sun is one of them: G2V)

The authors then used a habitable-zone extent roughly equivalent to 1.1 to 1.8 AU around the Sun, and they found 24 roughly Earth-sized planet candidates with 2 confirmed planets (Kepler 69c, Kepler 1126 b) among them.
A closer look at the 24 candidates in Fig. 2 reveals that 9 of them are orbiting around K stars, 16 of them are between about 5 and 8 billion years old (age estimates for the KOI samples are provided in Table 3 along with explanatory notes), and five of them are in the 10° range of the optimal temperature of a superhabitable planet as proposed by us (19°C; Table 3), with KOI-456.04 being the one with the most Earth-like temperature (Heller et al., 2020b). Only one of the candidates (KOI 5715.01) fits all three criteria, but it has a predicted lower global temperature than Earth when a gray atmosphere model is used that includes an approximation for a greenhouse effect (Table 3). However, if the greenhouse effect is stronger than on Earth, KOI 5715.01 could conceivably be superhabitable.
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Seeing Earth as a Transiting World
Imagine yourself a thousand light-years away, doing an exoplanet search with a Kepler-like telescope. Among the stars you look at is a G2V galactic-disk star about 5 billion years old, a star that is hard to distinguish from many other stars.

But your planetary system is close to the ecliptic, the plane of the Earth's orbit. Once every Earth year, the Sun gets fainter by 1 part in 10,000 for several hours, as the Earth crosses in front of it. But your telescope can measure this dip. You have seen our home, even if indirectly, and even if you can learn only a tiny bit about it.
Lisa Kaltenegger and John Pepper identify some 1000 stars closer than 100 parsecs (326 light years) where one can make such an observation. This is about 1/200 of all the stars that they considered.
 
[2010.14812] The Occurrence of Rocky Habitable Zone Planets Around Solar-Like Stars from Kepler Data - at least 1/2 of them have have a planet in their habitable zones. That is where water can be liquid, something necessary for our planet's biota, and likely necessary for any other biota.

About Half of Sun-Like Stars Could Host Rocky, Habitable-Zone Planets | NASA - less technical

ESA - Ariel moves from blueprint to reality - the Atmospheric Remote-Sensing Infrared Exoplanet Large-Survey Mission - it's scheduled to launch in 2029

It will observe transits of exoplanets in visible-light and IR wavelengths, constructing a spectrum with the planet's effective size in each wavelength band. Spectral lines in it can be used to identify components of the planet's atmosphere, since more extinction (absorption+scattering) means that the planet looks larger.

But that effect will be very, very tiny, and observing it will be a challenge.


Climate Stabilization on Distant Worlds

Our planet's surface temperature has been approximately stabilized by its carbonate-silicate cycle. A simplified account of it:
  1. Atmospheric carbon dioxide dissolves in rainwater, forming carbonic acid, which falls to the ground.
  2. Over long timescales, weathering from this weak acid dissolves silicate rocks, and the dissolved products are carried to the oceans, where they accumulate.
  3. Subduction of the seafloor carries the products to great depths, where they reform into silicates and gaseous carbon dioxide.
  4. The carbon dioxide is restored to the atmosphere by volcanism.
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The CO3-SiO4 cycle has kept the Earth's surface temperature in the liquid-water range for the last 2 billion years or more.

This cycle may operate on other Earthlike worlds. Waterworlds May Have Better Climate Buffering Capacities than Their Continental Counterparts - IOPscience - planets with only ocean on their surfaces and no exposed land.
 
Radioactive elements may be crucial to the habitability of rocky planets
The amount of long-lived radioactive elements incorporated into a rocky planet as it forms may be a crucial factor in determining its future habitability, according to a new study by an interdisciplinary team of scientists at UC Santa Cruz.

That's because internal heating from the radioactive decay of the heavy elements thorium and uranium drives plate tectonics and may be necessary for the planet to generate a magnetic field. Earth's magnetic field protects the planet from solar winds and cosmic rays.

...
What they found is that if the radiogenic heating is more than the Earth's, the planet can't permanently sustain a dynamo, as Earth has done. That happens because most of the thorium and uranium end up in the mantle, and too much heat in the mantle acts as an insulator, preventing the molten core from losing heat fast enough to generate the convective motions that produce the magnetic field.

With more radiogenic internal heating, the planet also has much more volcanic activity, which could produce frequent mass extinction events. On the other hand, too little radioactive heat results in no volcanism and a geologically "dead" planet.
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Radiogenic Heating and Its Influence on Rocky Planet Dynamos and Habitability - IOPscience
 
The planetary sweet spot: Abundance of elements in the Earth dictate whether plate tectonics can happen
According to Jackson, plate tectonics is a manifestation of the Earth trying to cool itself. Cold plates sink into the Earth and absorb heat, while volcanoes release heat where plates are spreading apart and forming. "Whether or not plate tectonics can happen actually depends on whether or not the Earth is too hot or too cold," he said. "If it's too hot, plate tectonics seizes up and if it's too cold, it freezes up."
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Radiogenic Heating and Its Influence on Rocky Planet Dynamos and Habitability - IOPscience


Stellar Flares Can Lead To The Diminishment Of A Planet's Habitability - Astrobiology
noting
Stellar flares versus luminosity: XUV-induced atmospheric escape and planetary habitability | Monthly Notices of the Royal Astronomical Society: Letters | Oxford Academic

That can be especially troublesome for planets of red dwarfs, like the TRAPPIST-1 planets.


Exoplanet survey spacecraft discovers two new warm exoplanets - around M dwarfs.
 
How Planets Can Save or Destroy Their Siblings
noting
How Jupiters Save or Destroy Inner Neptunes around Evolved Stars - IOPscience
In about 6 Gyr our Sun will evolve into a red giant and finally end its life as a white dwarf. This stellar metamorphosis will occur to virtually all known host stars of exoplanetary systems and is therefore crucial for their final fate. ... We report that the fate of the Neptune-mass planet, located closer to the star than the Jupiter-mass planet, can be very different from the fate of a single Neptune. The simultaneous effects of gravitational interactions, mass loss, and tides can drive the planetary system toward mean motion resonances. Crossing these resonances affects particularly the eccentricity of the Neptune and thereby also its fate, which can be engulfment, collision with the Jupiter-mass planet, ejection from the system, or survival at a larger separation.
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Radioactive Elements and Planetary Habitability - another article on it

On 300 Million Habitable Zone Planets

Water May Be Naturally Occurring On All Rocky Planets - Astrobiology
 
[2012.00764] The Exomoon Corridor: Half of all exomoons exhibit TTV frequencies within a narrow window due to aliasing

Planets of other stars ought to have moons, but where are they? Direct detection is likely to be very difficult, if the Solar System's moons are any guide.

With radial velocity, one can see Jupiter and Saturn, but not much else. With transits, the Earth and Venus are at the lower limit of detection, making it hard to see even the largest moons in the Solar System (Ganymede has 2/5 of the Earth's radius).

But with transit timing variations, one can see the moons' pulls on their planets. For instance, the Earth's Moon makes the two worlds' barycenter be about 3/4 of the Earth's radius away from its center. So if one has good enough measurements of the Earth's transit times, one can see the Moon's pull on it.

But there is a big problem. If a moon has a stable orbit around its planet, then its period must be much less than its planet's period around its star. That means that a moon can orbit several times from one transit to the next.

This is an effect called undersampling, and it makes aliasing.

In particular, Tp/Tm times, where Tp and Tm are the periods of the planet and the moon. Expressing as angular velocities, that ratio is wm/wp.

So one only observes the fractional part of wm/wp, meaning that wm = (fobs + n)*wp where n is an unobservable integer.

Let's see what wm/wp can be. I once tried to find that out for the Solar System's moons, and I found that Jupiter has the farthest moons by this measure.
  • Farthest retrograde: LIX −780.63 d - ratio 5.550
  • Farthest direct: LXII Valetudo +532.01 d - ratio 8.144
Using 4332.59 d for Jupiter's period

For comparison, this ratio for the Earth's Moon is 13.369

So if one could observe the Moon making the Earth wobble, one would see the Earth do so with a frequency of 0.369 year^(-1) or a period of 2.712 years.
 
A big problem is that it may be difficult to distinguish a moon-caused wobble from a wobble caused by another planet in the planet's system.

Back to David Kipping's paper [2012.00764] The Exomoon Corridor: Half of all exomoons exhibit TTV frequencies within a narrow window due to aliasing

He thought of an ingenious workaround. Instead of trying to detect individual exomoons, why not do it statistically? NASA Exoplanet Archive lists 4,324 planets, so that might work.

What we see is
fobs = abs(f - round(f))
with
f = wm/wp = Tp/Tm

0 <= fobs <= 1/2

He tried different distributions of moon periods, and he found that the distribution of observed relative orbit frequencies fobs is always approximately flat. That means that the distribution over observed relative periods Tobs = 1/fobs is

d(fobs) ~ d(Tobs)/(Tobs)^2

To capture a large range of data, logarithms are convenient: LTObs = log(Tobs) / L10Tobs = log10(Tobs). Thus

d(fobs) ~ 10^(-L10Tobs)*d(L10Tobs)

Most exomoons would be within about 2 to 4 planet periods, the "exomoon corridor". But known TTV periods peak at about 30 planet periods, with only a small tail extending into the exomoon corridor.
 
However, most transit-detected exoplanets have relatively short periods, and such short periods have a problem. If a moon orbits too close to its planet, it may get pulled apart. This gives a minimum orbit radius, the  Roche limit

a = 2.455 * rp*(dp/dm)^(1/3)

for a fluid moon, orbit radius a, planet radius rp, plant density dp, and moon density dm. The density scaling essentially scales the planet to what size it would have if it was the same density as that moon. The corresponding period is:

T = 3.847 * Ts

where Ts is the period of a surface satellite for the moon's density. For the Earth (mean density 5.517 g/cm^3), the surface-satellite period is 1.43 hours or 1h 26m. The Roche period for a same-density satellite is 5.51 hours or 0.229 days. For density 3.5 g/cm^3 (stony meteorite), it is 0.288 d, and for density 1 g/cm^3 (water), it is 0.538 d.

If the moon is solid and small enough, it may be rigid enough to avoid breaking up, so it can orbit closer in.

Nevertheless, these Roche periods are smaller than the orbit periods of most exoplanets - even very close ones.
 
Let's see what we find in the Solar System. The two nearest planets have no moons, so I looked for the closest asteroid to the Sun that has a moon.

 Minor-planet moon lists the asteroids that are known to have moons. The closest ones to the Sun are near-Earth asteroids.  Near-Earth object - as of 2018 Nov 25, there are 19,229 known ones. They fall into these families:
  • Comets (107)
  • Amor asteroids (8,120) - perihelion farther than the Earth's aphelion (1.017 AU)
  • Apollo asteroids (9,559) - perihelion closer than the Earth's aphelion, but major-axis length greater than 1 AU
  • Aten asteroids (1,411) - aphelion farther than the Earth's perihelion (0.983 AU), but major-axis length less than 1 AU
  • Atira asteroids (31) - aphelion closer than the Earth's perihelion
So the Apollo, Aten, and Atira asteroids can have perihelia that are arbitrarily close to the Sun. I looked for asteroids that get very close, and I found  List of near-Earth asteroids by distance from Sun. It listed asteroids by mean distance, but I found a better place to look:  List of Mercury-crossing minor planets

The first one discovered was 1566 Icarus, in 1949. It gets as close as 0.2 AU - its surface temperature gets up to 400 C. But it doesn't have a moon.

Even with this list I didn't have much patience for continuing, so I looked at the list by distance from the Sun, and one of the closest ones does indeed have a moon:

  • 163693 Atira
  • The asteroid's orbit: perihelion 0.5024 AU, aphelion 0.9798 AU, orbital period 233 d
  • The asteroid itself: diameter 4.8 km, rotation period 2.9745 h
  • The moon's orbit: distance 6 km, orbital period 15.504 h
  • The moon itself: diameter 1 km
 
Water on Exoplanet Cloud Tops Could Be Found with Hi-Tech Instruments - Astrobiology
noting
[2008.11464] Seeing above the Clouds with High Resolution Spectroscopy

By finding the apparent size of a transiting planet with a high-resolution spectroscope. A big problem will be collecting enough photons for getting good measurements, I'm sure.

Astronomers find evidence of an extragalactic exoplanet | Astronomy.com

In the Whirlpool Galaxy, some 23 million light years / 7 megaparsecs away. The planet, named M51-ULS-1b, was discovered from its eclipsing an X-ray binary.


To Separate Starspots from Planets

Can A Multi-layer Planet Be Approximated As A Homogeneous Planet? - Astrobiology

Coexistence Of CH4, CO2 And H2O In Exoplanet Atmospheres - Astrobiology
The authors went through a wide range of H, C, N, and O abundances, and they worked out what compounds of them would be in equilibrium. From reducing to oxidizing:
  • (A): H2O, CH4, NH3 and either H2 or N2, but only traces of CO2 and O2 -- outer planets, Titan
  • (C): H2O, CO2, CH4 and N2, but only traces of NH3, H2 and O2
  • (B): O2, H2O, CO2 and N2, but only traces of CH4, NH3 and H2 -- Venus, Earth, Mars
Our models show that graphite (soot) clouds can occur in type C atmospheres in addition to water clouds, which can occur in all types of atmospheres. Full equilibrium condensation models show that the outgassing from warm rock can naturally provide type C atmospheres. We conclude that type C atmospheres, if they exist, would lead to false positive detections of biosignatures in exoplanets when considering the coexistence of CH4 and CO2, and suggest other, more robust non-equilibrium markers.
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[2010.12241] Coexistence of CH4, CO2 and H2O in exoplanet atmospheres

The authors list Archean Earth as A, but it seems to me that it might instead be C, or else A then C. Before about 2.5 billion years ago (the Great Oxygenation Event), there wasn't enough molecular oxygen to make it B.
 
Exoplanet discovered orbiting a dead star
notes
Planet discovered transiting a dead star

A planet was recently discovered orbiting white dwarf WD 1856+534

The planet is roughly Jupiter-sized -- and ten times larger than its star. It has a period of 1.4 days, something that raises the question of how it got so close to its star. If that star was once a red giant, then the planet would have orbited well inside its star.

Exocast: The Exoplanet Podcast on Twitter: "It's time for the #ExoCup2020 FINAL!
And what a battle do we have for you: #WD1856b The one & only White Dwarf transiting planet, a cold super-Jupiter that out-lived its star.
#OB161928 The smallest rogue planet yet detected, an earth-mass object drifting the galaxy. Good luck!" / Twitter
  • The white-dwarf planet: WD 1856+534 b - 59.8%
  • The rogue planet: OGLE-BLG-2016-1928 - 40.2%
 
About Half of Sun-Like Stars Could Host Rocky, Habitable-Zone Planets | NASA
noting
[2010.14812] The Occurrence of Rocky Habitable Zone Planets Around Solar-Like Stars from Kepler Data
They estimated the occurrence of planets with radii from 0.5 to 1.5 Earth radii in the habitable zones of stars with surface temperatures between 4800 K and 6300 K. Spectral types F8 - K3 from Spectral Types The Sun is G2 with 5780 K

They used habitable-zone estimates from this paper: Kopparapu 2014 The numbers:
  • 6300 K - light flux 0.4 - 1.2
  • 4800 K - light flux 0.3 - 1.0
The Sun at the Earth is 1.0 -- for in-between temps, was interpolated linearly.

Kepler didn't get good data for higher temps than the limit, while lower temps are associated with luminosity low enough to make a habitable-zone planet orbit close enough to become tidally locked.

By doing a lot of statistics on the Kepler results, the authors found that about 1/2 of all stars in that range have rocky planets in their habitable zones.
 
Hubble pins down weird exoplanet with far-flung orbit that behaves like the long-sought 'Planet Nine'
The 11-Jupiter-mass exoplanet called HD 106906 b was discovered in 2013 with the Magellan Telescopes at the Las Campanas Observatory in the Atacama Desert of Chile. However, astronomers did not know anything about the planet's orbit. This required something only the Hubble Space Telescope could do: Collect very accurate measurements of the vagabond's motion over 14 years with extraordinary precision. The team used data from the Hubble archive that provided evidence for this motion.

The exoplanet resides extremely far from its host pair of bright, young stars—more than 730 times the distance of Earth from the Sun, or nearly 6.8 billion miles. This wide separation made it enormously challenging to determine the 15,000-year-long orbit in such a relatively short time span of Hubble observations. The planet is creeping very slowly along its orbit, given the weak gravitational pull of its very distant parent stars.

The Hubble team was surprised to find that the remote world has an extreme orbit that is very misaligned, elongated and external to the debris disk that surrounds the exoplanet's twin host stars. The debris disk itself is very unusual-looking, perhaps due to the gravitational tug of the wayward planet.
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New Insights Into Temperature-dependent Ice Properties And Their Effect On Ice shell Convection For Icy Ocean Worlds - Astrobiology
noting
[2011.12502] New insights into temperature-dependent ice properties and their effect on ice shell convection for icy ocean worlds
From the abstract,
We propose a new model for thermal conductivity that spans the temperature range relevant to the ice shells of ocean worlds. This increases the thermal conductivity at low temperatures near the surface by about a fifth. We show that such an increase in thermal conductivity near the cold surface can stabilizes the ice shell of Europa. Furthermore, we show that including temperature dependent specific heat capacity decreases the energy stored in the conductive lid which reduces the response timescale of the ice shell to thermal perturbations by approximately a third. This may help to explain surface features such as chaotic terrains that require large additions of energy to the near-surface ice.
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A Distinct Population of Small Planets: Sub-Earths - Astrobiology

There is a well-known gap in planet sizes at 2 Earth radii, dividing super-Earths and mini-Neptunes. There is another gap that was recently discovered at about 1 Earth radius. It may separate planets formed from gaseous planetary disks from planets that formed later in terrestrial-planet fashion.

Assessing the habitability of planets around old red dwarfs - flares can be a problem even for these stars.

About Half of Sun-Like Stars Could Host Rocky, Potentially Habitable Planets – Exoplanet Exploration: Planets Beyond our Solar System - another article on that subject

Radioactive elements may be crucial to the habitability of rocky planets
The amount of long-lived radioactive elements incorporated into a rocky planet as it forms may be a crucial factor in determining its future habitability, according to a new study by an interdisciplinary team of scientists at UC Santa Cruz.

That's because internal heating from the radioactive decay of the heavy elements thorium and uranium drives plate tectonics and may be necessary for the planet to generate a magnetic field. Earth's magnetic field protects the planet from solar winds and cosmic rays.
Click to expand...
The researchers adjusted the amount of radioisotopes up and down.
What they found is that if the radiogenic heating is more than the Earth's, the planet can't permanently sustain a dynamo, as Earth has done. That happens because most of the thorium and uranium end up in the mantle, and too much heat in the mantle acts as an insulator, preventing the molten core from losing heat fast enough to generate the convective motions that produce the magnetic field.

With more radiogenic internal heating, the planet also has much more volcanic activity, which could produce frequent mass extinction events. On the other hand, too little radioactive heat results in no volcanism and a geologically "dead" planet.
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Also
Radioactive Elements and Planetary Habitability
and
[2011.04791] Radiogenic Heating and its Influence on Rocky Planet Dynamos and Habitability
 
One of The Blackest Planets in The Galaxy Is Headed For a Fiery Death - "hot Jupiter" WASP-12b
It is spiraling in, and it will reach its end in 2.9 million years.

Its albedo is 6%, meaning that it reflects only 6% of the light that it receives. That makes it darker than fresh asphalt.


Lava Worlds: From Early Earth to Exoplanets - Astrobiology
noting
[2012.07337] Lava Worlds: From Early Earth to Exoplanets

Oceans of liquid rock.  Magma ocean  Lava planet - our home planet may have had some magma oceans early in its history, like after large impacts, like the impact that produced the Moon.

The asteroid 4 Vesta was completely melted very early in the Solar System's history, as a result of its aluminum-26.

 Chthonian planet - a stripped gas giant - some of them are close enough to their stars to be lava planets.
 
The ‘Cold Jupiter’ Factor
noting
The New Generation Planetary Population Synthesis (NGPPS). III. Warm super-Earths and cold Jupiters: A weak occurrence correlation, but with a strong architecture-composition link | Astronomy & Astrophysics (A&A)
We find a difference in the bulk composition of inner super-Earths with and without cold Jupiters. High-density super-Earths point to the existence of outer giant planets in the same system. Conversely, a present cold Jupiter gives rise to rocky, volatile-depleted inner super-Earths. Birth environments that produce such dry planet cores in the inner system are also favorable for the formation of outer giants, which obstruct inward migration of icy planets that form on distant orbits. This predicted correlation can be tested observationally.

…low-mass solid disks tend to produce only super-Earths but no giant planets. Intermediate-mass disks may produce both super-Earths and cold Jupiters. High-mass disks lead to the destruction of super-Earths and only giants remain.
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Astronomers discover rare, 10 billion year old 'Super-Earth' outside of our solar system - ABC7 Los Angeles
The new discovery is about 50% bigger than Earth and is three times its mass, which is why astronomers call it a 'Super-Earth.'

It only takes less than half an earth day to go around its own sun.

And it is very hot, with an average temperature of 3,140 degrees Fahrenheit.

This super-earth is about 10 billion years old, making it one of the oldest rocky planets ever to be discovered.
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Another article on this issue:
Warning: Too Much Radiogenic Heat Could Hurt Exoplanet Habitability
noting
[2011.04791] Radiogenic Heating and its Influence on Rocky Planet Dynamos and Habitability
The authors concede "Because the qualitative outcomes of our 1D model are strongly dependent on the treatment of viscosity, further investigations using fully 3D convection models are desirable."
 
Featured Image: A Wide Planetary-Mass Binary
noting
A Wide Planetary-mass Companion to a Young Low-mass Brown Dwarf in Ophiuchus - IOPscience

From aasnova: "Fontanive and collaborators show that this binary system consists of two bodies of perhaps 15 and 8 Jupiter masses, separated by around 200 au (for reference, Pluto’s semimajor axis is just 39 au!)."

Holding the System of HR 8799 Together
noting
An Exact, Generalized Laplace Resonance in the HR 8799 Planetary System - IOPscience

So if the four planets are in that resonance, they would not be likely to eject any of them.
 
Planetary Sleuthing Finds Triple-Star World | NASA
and
Recovering a Triple Star Planet with New Data
From the latter:
Caltech’s David Ciardi, who discussed this elusive system at the recent virtual meeting of the American Astronomical Society, says this about it:

“KOI-5Ab got abandoned because it was complicated, and we had thousands of candidates. There were easier pickings than KOI-5Ab, and we were learning something new from Kepler every day, so that KOI-5 was mostly forgotten.”

Ciardi, who is chief scientist at NASA’s Exoplanet Science Institute, has now pulled KOI-5Ab back to vibrant life. Almost certainly a gas giant, the planet orbits one of three stars in the system in a misaligned orbit that calls into question the nature of its formation. Making original confirmation of the planet difficult was the inability to completely distinguish it from the possible effects of the other two stars in the system. Untangling the question would involve TESS data. The object appears in TESS terminology as TOI-1241b, showing a five-day orbit that matches the Kepler result.
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Another article on that planet of a very old star:
Rocky Planet Found Around 10 Billion-Year-Old Star - Sky & Telescope - Sky & Telescope
Astronomers have discovered three planets orbiting a star about 10 billion years old — one of them rocky. The star, TOI 561 (meaning it was the 561st object of interest from the Transiting Exoplanet Survey Satellite), is in our galaxy’s older thick disk, which means its planets have a nice view from on high of the Milky Way’s spiral.

The star has three planets, with diameters 1.5, 3, and 2 times Earth’s. The innermost one is rocky, with three times Earth’s mass, but on a period of 0.44 days, it’s anything but Earth-like. Its dayside surface temperature is around 2500K (4,000°F). That’s almost twice as hot as Earth’s magma, and it’s surely molten. What it actually looks like is uncertain, because as lead scientist Lauren Weiss (University of Hawaii, Manoa) notes, “It exceed temperatures where geophysicists have made lava in the lab.”
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