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Kepler-88’s Planetary Dance Grows More Complicated

Transit timing variations are useful to astronomers trying to learn what forces are acting upon a known exoplanet. They could eventually help us ferret out the existence of a sufficiently large moon, for example, though we have yet to confirm one. But they also show us how much impact other planets in the same system can have upon the planet being observed.

All this is why the Kepler-88 system has been high on the list of interesting targets for astronomers. Before the recent discovery of a new gas giant, we knew about Kepler-88 b and c, one of them (the outer world Kepler-88 c) about 20 times more massive than Kepler-88 b, a planet less massive than Neptune. The story here was the mean motion resonance, in which planet c, a Jupiter-mass world, orbits the star in 22 days while Kepler-88 b orbits in 11: Two orbits of b in the time it takes c to make a single orbit. Planet b is the only transiting planet in this system; Kepler-88 c was confirmed by radial velocity methods.

The mass differential is telling, so that the more massive Kepler-88 c causes notable transit timing variations in Kepler-88 b. In fact, the Kepler mission was able to detect TTVs of up to half a day forced by Kepler-88 c. These are to my knowledge the largest transit timing variations yet observed. All of that makes the system interesting in its own right, but now we have word of another planet here, Kepler-88 d, which turns out to be about three Jupiter masses.

The radial velocity discovery, which also confirmed the existence, mass and orbital period of Kepler-88 c, was made by a team led by Lauren Weiss (University of Hawaii Institute for Astronomy), using the High-Resolution Echelle Spectrometer (HIRES) instrument on the 10-meter Keck I telescope in Hawaii. The discovery paper appears in The Astronomical Journal.

Image: An artist’s illustration of the Kepler-88 planetary system. Credit: W. M. Keck Observatory / Adam Makarenko.

“At three times the mass of Jupiter, Kepler-88 d has likely been even more influential in the history of the Kepler-88 system than the so-called king, Kepler-88 c, which is only one Jupiter mass,” says Weiss. That influence is a reference to the effects of a gas giant in a Jupiter-like orbit (the planet is in an elliptical orbit around Kepler-88 with a period of four years). Jupiter is assumed to have affected cometary orbits in the early days of the Solar System, drawing materials rich in volatiles into the inner system where they would have been part of the mechanism for producing oceans on Earth.

But this is a stellar system that will have a much different fate than our own. The host star Kepler-88 is a massive B-class object, highly luminous and with a lifetime lasting only in the low millions of years. No habitable zone worlds to nourish with oceans or, at least, no such worlds with a billion-year timeframe for life to flourish.

ADDENDUM See andy’s comment below, and also Mike Fidler’s — the B-class statement in the paragraph above is is incorrect, and the result of discrepancies between the sources. It looks as though this is a G-class object and I’m trying to confirm that.

Later: I’m going with andy’s assessment — this is a G-class star, as given by the paper’s data on it: 5466 K, 0.985 solar masses and 0.900 solar radii. Other sources also peg it as a G, so I think we have to assume that the B-class identification that appears in some online sources is incorrect.

Even so, it’s friendly to planet formation. According to the paper, finding multiple giant planets is not surprising, because of the high metallicity of the star. The paper goes on to speculate about planet formation and the likelihood of early migration:

Since both planets c and d are gas giants, they must have formed early in the disk lifetime, when gas was abundant… Perhaps additional giant planets were present earlier, or are still present. Planets c and d likely underwent viscous (Type I) migration in the proto-planetary disk. As the gas disk dissipated, planet-planet scattering would likely have increased, and low and high-eccentricity migration likely became important at this time. The high eccentricity of planet d probably arose due to a significant exchange of angular momentum with another gas-giant planet.

The smaller Kepler-88 b, then, may have formed when gas was less abundant in the early disk, and the authors believe that the world could have been caught in its current mean motion resonance with Kepler-88 c during the period of early inward migration of planet c.

The paper is Weiss et al., “The Discovery of the Long-Period, Eccentric Planet Kepler-88 d and System Characterization with Radial Velocities and Photodynamical Analysis,” Astronomical Journal Vol. 159, No. 5 (29 April 2020). Abstract / Preprint.


Comments on this entry are closed.

  • Michael Fidler May 1, 2020, 15:56

    This is really confusing, the star is showing B class from several sources but the listed characteristics is G? I was interested in the fate of the planets after the star turns into a pulser and how many of these type of pulsar systems must exist in the Galaxy thru the billions of year. Then looking up the mass found it to be the same as our sun? Even giant or dwarf B’s do not fit the bill??? Hmm.

    • Paul Gilster May 1, 2020, 20:43

      Mike, confusing indeed. Thanks to you and andy noticing the problem, I’ve made an addendum to the original post. There are several sources citing a B-class, but all the evidence points to a G, and so does SIMBAD.

  • torque_xtr May 1, 2020, 15:58

    There is some misinformation about Kepler-88 spectral class – it’s indicated as B on Wiki page, but other parameters tell that it’s sunlike. In the preprint, https://arxiv.org/pdf/1909.02427.pdf, the mass and radius are revised further downwards, at M = 0.99sol and R = 0.900 sol; so it’s the system of a sunlike star with high-metallicity.

    PS there’s a beautiful TTV visualization in the preprint – lightcurves stacked vertically and forming clear sine-like curve with some irregularities. And what a huge amplitude – if it was applied to Earth, it would mean shifts for equinoxes, solstices and New Years by more than two weeks from average!

  • andy May 1, 2020, 18:16

    The host star Kepler-88 is a massive B-class object, highly luminous and with a lifetime lasting only in the low millions of years.

    I’m guessing you got this from the Extrasolar Planets Encyclopaedia, which (at the time of writing) lists it as a B-class, but I’ve got no idea where they got that from. SIMBAD lists it as a G8IV, the NASA catalogue gives G6V, while the paper gives 5466 K, 0.985 solar masses and 0.900 solar radii. This is not a B-type star at all.

    • Paul Gilster May 1, 2020, 20:16

      Yes, I did pull that from the Extrasolar Planets Excyclopedia. Thanks for the SIMBAD reference. I need to straighten this out — you’re certainly right re mass, etc.

  • Robin Datta May 2, 2020, 2:39

    There should be some clearing place in the astronomical community where all confusing and conflicting data should be parked for review, analysis, clarification and consensus.

    Such data may be more likely to contain outliers that may be interesting: unexpected needles in the haystack.

  • Ashley Baldwin May 2, 2020, 18:28

    I don’t know how often or who updates these data bases . I’ve known SINBAD to be wrong too. Jim Kalers “Stars” is a good touchstone site. The authors to be fair don’t refer to the spectral class of Kepler 88. Though perhaps they should .

    B class stars are only mentioned in the text once in relation to being used as adjacent spectra – in section 2.1. They are used as comparator template on either side of the target star as one of several ways described for confirming the RV signal constrained for planet d is legitimate and not a systematic error.

    With a four year period and highly eccentric orbit even ultra massive planet d would only impart a very small and variable radial velocity change on its host star spectrum. In other words this planet is right on the edge of what can be constrained from the ground even by this method even using the gold standard HIRES instrument . Indeed the only reason it can be even then is because of the big pointer provided for its orbital position from the c planet TTVs.

    B class stars rotate extremely rapidly so have a high baseline radial velocity, pushing their spectral lines close together. Tight. Any changes induced in their RV signal from even a large mass and very close planet would be tiny and hard to tease out of the tight spectrum .With a life numbered in just millions of years its unlikely they would have the time to accrete any planet let alone three, with one three times bigger than Jupiter ! These tight spectral lines is why RV spectroscopy doesn’t work for larger stars earlier than F class. Other than for some nearby, relatively old and bright A stars.

    The B class spectra used are analogous here to the alias signals injected into the Kepler and TESS photometry data stream as a control to show how sensitive the investigators and/or their programmes are at pulling out genuine signals . Another such comparator control but this time for the RV technique involved the prolonged photometry of Proxima Centauri by the Pale Red Dot team to confirm Proxima b’s RV signature was real and not as a result of photospheric activity of an active and noisy M star.

    In this study, if the team can pull out – “deconvolve” – a planetary RV signal from the noise created by template B class spectra -as well as that of the background of Earth’s own atmospheric spectrum , then they can confident that signal is a real planet.

    This was a very sophisticated piece of work. A real graft .

    Hats off to the authors .

  • wdk May 2, 2020, 19:06

    The other day I was mulling a textbook true or false question:

    Eccentric Jupiters would probably eject any terrestrial planet from the planetary system where they are found.
    Answer: TRUE

    Well, aside from the syntax, I did hesitate to decide on the answer or to
    impose it on anyone else. But in the case in question, if not ANY terrestrial planet, the most terrestrial like planets at Earth vicinity are not to be found. Could Kepler-88d have hidden any evidence?

  • Michael Fidler May 3, 2020, 10:31

    Kepler 88d would be pretty close to the snow line in its orbit but with such a high eccentricity 0.4342 any earth size satellites would have large variations in the 4 year orbital period. Has anybody found a good chart of the habitable zone with the orbit of d shown???

  • Laintal May 17, 2020, 4:08

    Thanks for the comments the size was confusing as well

    Cheers Laintal