Kepler-444 / KOI-3158 : Five sub-Earth-sized planets orbiting a K dwarf

http://www.gizmag.com/earth-like-planets-milky-way-kepler-koi-3158/35378/

YouTube video of a presentation of the system given in October 2014.
https://www.youtube.com/watch?v=QEIokMsMUbY

Last year :
https://solar-flux.forumotion.com/t1420-second-kepler-science-conference-nov-4-8

The KOI 3158 system :

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Definitely a fascinating system. Some of the smallest planets discovered, around (I believe) the densest star with photometric astroseismology, in a tight triple system.

The companion binary lies at a projected separation of only 65 AU! That must have severely truncated the protoplanetary disk. The two should also cause a large RV trend, which I hope has been detected.

Any precise mass measurement for these planets are expected from ttv?

Towards the end of the talk video it's mentioned that the resonant TTV signal is marginally detected for the outermost pair of planets. I expect, due to Kepler's precision floor, the bottom table Led_Zep showed from last year hasn't been improved much.

Always nice when Kepler systems turn out to be Hipparcos stars. (KOI-3158 = HIP 94931)

Now on arXiv, listed as Kepler-444.

An ancient extrasolar system with five sub-Earth-size planets
http://arxiv.org/abs/1501.06227

New Scientist :
http://www.newscientist.com/article/dn26856-ancient-planets-are-almost-as-old-as-the-universe.html#.VMfQWHx0zIV

Strong HI Lyman-α variations from a 11 Gyr-old host star: a planetary origin ?
https://arxiv.org/abs/1703.00504

Quote :

Kepler-444 provides a unique opportunity to probe the atmospheric composition and evolution of a compact system of exoplanets smaller than the Earth. Five planets transit this bright K star at close orbital distances, but they are too small for their putative lower atmosphere to be probed at optical/infrared wavelengths. We used the Space Telescope Imaging Spectrograph instrument onboard the Hubble Space Telescope to search for the signature of the planet's upper atmospheres at six independent epochs in the Ly-α line. We detect significant flux variations during the transits of both Kepler-444e and f (~20%), and also at a time when none of the known planets was transiting (~40%). Variability in the transition region and corona of the host star might be the source of these variations. Yet, their amplitude over short time scales (~2-3 hours) is surprisingly strong for this old (11.2+-1.0Gyr) and apparently quiet main-sequence star. Alternatively, we show that the in-transits variations could be explained by absorption from neutral hydrogen exospheres trailing the two outer planets (Kepler-444e and f). They would have to contain substantial amounts of water to replenish such hydrogen exospheres, which would reveal them as the first confirmed ocean-planets. The out-of-transit variations, however, would require the presence of a yet-undetected Kepler-444g at larger orbital distance, casting doubt on the planetary origin scenario. Using HARPS-N observations in the sodium doublet, we derived the properties of two Interstellar Medium clouds along the line-of-sight toward Kepler-444. This allowed us to reconstruct the stellar Ly-α line profile and to estimate the XUV irradiation from the star, which would still allow for a moderate mass loss from the outer planets after 11.2Gyr. Follow-up of the system at XUV wavelengths will be required to assess this tantalizing possibility.

Mass, Density, and Formation Constraints in the Compact, Sub-Earth Kepler-444 System including Two Mars-Mass Planets
https://arxiv.org/abs/1703.03417

Quote :

Kepler-444 is a five planet system around a host-star approximately 11 billion years old. The five transiting planets all have sub-Earth radii and are in a compact configuration with orbital periods between 3 and 10 days. Here we present a transit-timing analysis of the system using the full Kepler data set in order to determine the masses of the planets. Two planets, Kepler-444 d (Md=0.036+0.065−0.020M⊕) and Kepler-444 e (Me=0.034+0.059−0.019M⊕), have confidently detected masses due to their proximity to resonance which creates transit timing variations. The mass ratio of these planets combined with the magnitude of possible star-planet tidal effects suggests that smooth disk migration over a significant distance is unlikely to have brought the system to its currently observed orbital architecture without significant post-formation perturbations.

Planet masses 0.036 and 0.034 earth mass ,radii 0.54 and 0.555 earth size? Three times lower than Mars? So close to host star? Density 1.27 g/cm and 1.08g/cm3 (almost pure water/ice? There must be something wrong with the measurement.Planet can not contain any volatiles and water being so close and assessing age of host star, bizarre

Still a fair amount of uncertainty in the error bars though, 95% limits on the mass extend into the rocky composition part of the diagram (figure 3).

Updated orbital elements for the planets and the A-BC orbit.

Stalport et al. "Global dynamics and architecture of the Kepler-444 system"
https://arxiv.org/abs/2209.06810

They also put constraints on the existence of an outer planet, which would have to be located at orbital periods between 12 and 20 days to avoid destabilising the system.

Another analysis of the system, finding a larger semimajor axis and lower eccentricity for the A–BC orbit, the result of which increases the periastron distance from ~5 au to ~23 au. This suggests the Kepler-444 A disc would have been truncated at ~8 au, which would have made it easier to build the planets.

Also has dynamical mass measurements of the Kepler-444 B and C stars.

Zhang et al. "The McDonald Accelerating Stars Survey (MASS): Architecture of the Ancient Five-Planet Host System Kepler-444"
https://arxiv.org/abs/2210.07252