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.