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Jupiter is the boss.
Well, in terms of planets in our Solar System it is. It has played a very important role in the configuration of the Solar System due to its mass and gravity. Here are a few ways it has shaped our system:
- Most of the orbits of the planets in the Solar System are closer to the orbital plane of Jupiter than to the equatorial plane of the Sun.
- It has driven an entire population of asteroids to its LaGrangian points, and they are called Trojan asteroids.
- Jupiter has created spaces in the asteroid belt.
- It may have been responsible for the late heavy bombardment when it migrated.
That’s just a sample of the effects that massive Jupiter has on the Solar System. It is all due to Jupiter’s mass, which is approximately 2.5 times greater than all other planets.
Now astronomers have found an exoplanet that is three times more massive than Jupiter. How has that planet formed its own Solar System?
The newly discovered planet is called Kepler-88 d. As the name implies, it orbits the star Kepler-88, a Sun-like star in the constellation Lyra. Astronomers already knew of the presence of two planets: Kepler-88 by Kepler-88 c. The new Kepler-88 d was discovered after six years of observation.
A new article featuring the discovery has just been published in The Astronomical Journal. Its title is “The discovery of the eccentric Kepler-88 planet of long period and characterization of the system with radial velocities and photodynamic analysis”. The lead author is Dr. Lauren Weiss, a postdoctoral fellow at the Institute of Astronomy at the University of Hawaii.
The discovery was made with the W. M. Keck Observatory in Hawaii. His Echelle high-resolution spectrometer instrument (HIRES), found on the 10-meter Keck telescope, was key to the observations. The six years of observations revealed that K88-d has a four-year orbit. And the orbit is not circular, it is elliptical.
The Kepler-88 system was already of interest to astronomers. You already know 88b and 88c, two planets that orbit very close to the star. But finding the third planet, the massive Kepler-88 d, has even more curious astronomers.
When objects orbit the same star, they experience what is called an orbital resonance. That means that the bodies will exert a regular and periodic gravitational influence on each other. When two objects are in orbital resonance, their orbits are a ratio of two integers. For example, 2: 1 or 1: 1, or even 4: 1. It makes sense when you think about it. All bodies exert a gravitational force on each other.
Orbital resonance can keep orbits stable or unstable. In our Solar System, Pluto’s orbit passes within Neptune’s orbit, and we sometimes call Pluto a “transneptunal object”. But its orbital resonance means that although their paths intersect, Pluto and Neptune do not collide. Pluto completes two orbits each time Neptune completes three.
And there are different types of orbital resonance, and that’s where the Kepler-88 system becomes interesting.
When the first two planets were discovered in the Kepler-88 system, they had some strange features. They orbited in something called medium motion resonance. That means the two planets orbit like a clock, energy efficient. The orbital periods of each planet are a simple ratio of integers to each other.
88 b is a sub-Neptune, and orbits the star in just 11 days. That’s almost exactly half the orbit of 88 c, a planet the size of Jupiter, which lasts about 22 days. Every two times planet b orbits the star, it is pumped by gravity by the largest 88 c. Since Kepler-88 c is almost 20 times more massive than Kepler-88b, its gravitational pull has a dramatic effect on the smallest planet, creating huge changes in the orbit of the smallest planet.
Kepler-88 d has three times the mass of Kepler-88 c, making the newly discovered planet the most massive known in this system. ANIMATION CREDIT: W. M. KECK / ADAM MAKARENKO OBSERVATORY
The orbital changes caused by resonance in the system are called transit time variations, or TTV. The Kepler spacecraft detected them and was able to identify the precise time that Kepler-88b spent between the spacecraft and the star. Other planetary systems display TTV as well, but the TTVs in this system are some of the largest ever found, up to half a day before or after. The Kepler-88 system has the nickname “King of TTVs”.
Now, with the discovery of an even larger planet, Kepler-88d, the system has another dimension.
“At three times the mass of Jupiter, Kepler-88 d has probably been even more influential in the history of the Kepler-88 system than the so-called King, Kepler-88 c, which is just a mass of Jupiter,” said author leader Dr. Lauren Weiss in a press release. “So perhaps Kepler-88 d is the new supreme monarch of this planetary empire: the empress.”
One of the interesting things about this system is that only one of the planets, the little Kepler-88 b, transits its star from our point of view. As the document says, “The system has a single planet in transit, Kepler-88 b (KOI-142.01), a planet the size of Neptune with an orbital period of 10.95 days. Kepler-88 b is disturbed by a giant non-transit planet with a period of 22.26 days, Kepler-88 c (KOI-142.02.) ”The presence of the other two planets, Kepler-88 cyd, is only known to through its observed effects on the smallest planet. Years of monitoring radial velocity and photometry observations confirmed their presence.
Exactly how our Solar System got to the stage it is in now, and how planets formed and migrated, remains largely a mystery. There are theories and there is evidence, but there is no general conclusion for many aspects of the history of our system.
But we do know that, as the largest planet, Jupiter had and still has a very important role in everything. What role does Kepler-88 d play in your system? Are there more planets in the system yet to be discovered?
In their article, the authors write: “Perhaps additional giant planets were present before or are still present. Planets c and d probably underwent viscous migration (Type I) in the protoplanetary disk. As the gas disk dissipated, planet-planet dispersion 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 angular momentum exchange with another gas giant planet. “
Perhaps there is another gas giant orbiting Kepler-88, waiting to be discovered.
In any case, the Kepler-88 system, while interesting, is not exactly unique. It joins a group of other known systems that host small transiting planets and larger gas giants, such as WASP-47 and 55 Cancri. As the authors say in their conclusion, “Kepler-88 joins the ranks of metal-rich stars that host small transiting planets and two or more giant planets.”
