Two new papers in the Astronomical Journal show that in some multiplanet systems, giant planets tend to kick their smaller neighbors out of orbit and wreak havoc on their climates.
Illustration depicting one interpretation of the exoplanet Gliese 357d. Image credit: Chris Smith / NASA’s Goddard Space Flight Center.
In one of the two papers, University of California, Riverside astrophysicist Stephen Kane detailed how the pull of massive planets in the HD 141399 system is likely to toss their Earth-like neighbors out of the habitable zone.
HD 141399 is an early K dwarf star located 121 light-years away in the constellation of Boötes.
The star harbors four giant planets with masses in the range between 0.45 and 1.36 Jupiter masses.
Unlike most other known planetary systems, these four planets are farther from their parent star.
“The HD 141399 system of four giant planets is exceptionally rare among known exoplanetary architectures,” Professor Kane said.
“According to the NASA Exoplanet Archive, HD 141399 is one of only two known planetary systems with at least four planets that are all more massive than Saturn.”
“The other system is HR 8799, with four wide-separation planets that were detected via direct imaging.”
“Thus, the HD 141399 system is an incredible opportunity to study the formation, dynamics, and evolution of an unusual planetary architecture.”
Taking data about the system’s planets into account, Professor Kane ran multiple computer simulations to understand the effect of these four giants.
He wanted specifically to look at the habitable zone in this star system and see if an Earth could remain in a stable orbit there.
“The answer is yes, but it’s very unlikely. There are only a select few areas where the giants’ gravitational pull would not knock a rocky planet out of its orbit and send it flying right out of the zone,” Professor Kane said.
While the first paper shows giant planets outside the habitable zone destroying the chances for life, the second paper, by Professor Kane and his colleague, University of California, Riverside’s Dr. Tara Fetherolf, shows how one big planet in the middle of the zone would have a similar effect.
The authors examined the Gliese 357 system (also known as GJ 357), which is located only 30 light-years away in the constellation of Hydra.
The parent star harbors three exoplanets — Gliese 357b, c and d — with orbital periods of 4, 9, and 56 days.
Earlier studies found that Gliese 357d resides in the system’s habitable zone and has been measured at about 6 times the mass of the Earth. However, the new study shows the mass is likely much bigger.
“It’s possible Gliese 357d is as much as 10 Earth masses, which means it’s probably not terrestrial, so you couldn’t have life on it. Or at least, it would not be able to host life as we know it,” Professor Kane said.
Professor Kane and Dr. Fetherolf demonstrated that if the planet is much larger than previously believed, it is certain to prevent more Earth-like planets from residing in the habitable zone alongside it.
Though there are also a select few locations in the habitable zone of this system where an Earth could potentially reside, their orbits would be highly elliptical around the star.
“In other words, the orbits would produce crazy climates on those planets,” Professor Kane said.
“Our paper is really a warning, when we find planets in the habitable zone, not to assume they are automatically capable of hosting life.”
Ultimately, the pair of papers shows how uncommon it is to find the right set of circumstances to host life elsewhere in the Universe.
“Our work gives us more reasons to be very grateful for the particular planetary configuration we have in our Solar System,” Professor Kane said.
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Stephen R. Kane. 2023. Surrounded by Giants: Habitable Zone Stability Within the HD 141399 System. AJ 166, 187; doi: 10.3847/1538-3881/acfb01
Stephen R. Kane & Tara Fetherolf. 2023. GJ 357 d: Potentially Habitable World or Agent of Chaos? AJ 166, 205; doi: 10.3847/1538-3881/acff5a
