The Case of the Missing Jupiters: Gas Giant Planets are a No-Show around Small Red Stars

Cambridge, Mass. – Astronomers have revealed that the smallest and most common kinds of stars in the universe, called red dwarfs, very rarely host big, Jupiter-like planets. This absence of Jupiter analogs could have major impacts on the development of Earth-like planets around red dwarfs and in the search for worlds capable of supporting alien life.

Befitting its distinction as the locally largest planet, Jupiter has played a dominant role in the evolution of our Solar System. Scientists think Jupiter ultimately set the stage for Earth becoming habitable, influencing our world's formation, size, and composition. Thus, the lack of hulking gas giants in red dwarf planetary systems suggests that any resident rocky worlds may not have evolved into particularly Earthly, life-friendly places.

"We have shown that the least massive stars don't have Jupiters, meaning Jupiter-mass planets that receive similar amounts of starlight as Jupiter receives from our Sun," says Emily Pass, a researcher at the Center for Astrophysics | Harvard & Smithsonian (CfA) and lead author of a new study to be published in The Astronomical Journal conveying the results. "While this discovery suggests truly Earth-like planets might be in short supply around red dwarfs, there still is so much we don't yet know about these systems, so we must keep our minds open."

The findings have additional importance because many red dwarfs are among our nearest cosmic neighbors. That proximity, coupled with the fact that cool, dim red dwarfs do not overwhelm their planets in glare, has established them as the most amenable targets for investigating the atmospheres of exoplanets—a key research priority now and for the next few decades.

"The pipsqueak red dwarf stars that we looked at for this study are our most immediate cosmic neighbors, which means their planets are ideal candidates for detailed examination by the James Webb Space Telescope," says study co-author David Charbonneau, a professor at Harvard University and a member of the Center for Astrophysics | Harvard & Smithsonian. "But now that we have very strong evidence of cold gas giants like Jupiter and Saturn being exceedingly rare around these stars, the temperate rocky planets we end up studying could diverge greatly from our terrestrial expectations."

To gauge the frequency of Jupiter planets, Pass and colleagues examined an unprecedently large population of 200 small red dwarfs, each only 10% to 30% of the mass of the Sun. Such tiny red dwarfs are the cosmic norm, vastly outnumbering Sun-sized stars in our galaxy. The observations were gathered between 2016 and 2022 primarily from the Fred Lawrence Whipple Observatory, located in Arizona, as well as the Cerro Tololo Inter-American Observatory in Chile.

The researchers relied on the radial-velocity technique to suss out any large exoplanets in their stellar dataset. As planets orbit their host stars, the bodies' interacting gravities cause the stars to "wobble" ever so slightly, an effect discernible in detailed starlight readouts.

Across the entire sample of stars, the researchers did not detect a single Jupiter-equivalent planet. Based on inherent statistical uncertainties, the researchers can safely say that Jupiters occur in less than 2% of low-mass red dwarf planetary systems.

The findings starkly contrast with similar surveys of mid-sized stars like our Sun, which commonly sport massive planets at Jupiter-like distances. The tremendous masses of these worlds—Jupiter alone contains more mass than all the other planets put together—translates to tremendous gravity, and tremendous gravity translates to far-reaching influence on other celestial bodies.

"In the Solar System, Jupiter is the bully," says Charbonneau. "A lot of what makes Earth the way it is traces back to what Jupiter was doing in the early phases of the Solar System's history."

Among the most significant events is Jupiter's migration in the first few hundred million years of the Solar System's existence. After formation in the far reaches of the Solar System, Jupiter, along with the other outer planets, is theorized to have moved inward toward the Sun. In the process, hefty Jupiter's gravity scattered loads of ice-rich cometary bodies onto collision courses with the four rocky worlds in the inner Solar System.

As a large number of those icy bodies impacted on our young planet, they delivered copious amounts of water, potentially along with organic (carbon-containing) molecules. The waters pooled on our world's surface, creating the oceans, within which organic molecules are thought to have gone on to mix together for millions of years. Eventually, the molecules evolved complexity and began self-replicating, having transitioned to what we refer to as life.

Sans Jupiter, these conditions might not have come to be, and the journey to life might never have gotten underway.

Although the new findings do suggest that the circumstances that led to at least one world in our Solar System becoming habitable are not likely to be matched in solar systems hosted by tiny red stars, the door is far from closed when it comes to extraterrestrial life in these systems.

"We don't think that the absence of Jupiters necessarily means rocky planets around red dwarfs are uninhabitable," says Charbonneau.

The conspicuous absence of Jupiter-esque mega-planets means more raw material should be available for building up smaller, rocky bodies, because this material wasn't incorporated into Jupiter-like worlds. Indeed, other studies have shown that red dwarfs' solid worlds tend to be correspondingly larger in size than those around Sunlike stars.

Relatedly, rocky planets seem to form in greater numbers around red dwarfs versus Sunlike stars. For instance, the famous TRAPPIST-1 planet system packs seven rocky worlds into orbits much closer to the host red dwarf star than Mercury is to our Sun.

In a word, red dwarf planetary systems are just different from ours. And that difference could perhaps lead to rich habitability possibilities we have not yet realized.

"Our work implies that rocky worlds with masses similar to Earth and orbiting red dwarfs were born and raised in a very different environment from that of our own planet," says Pass. "We're excited to see what exactly that means as we forge ahead in remotely exploring the planets in our cosmic neighborhood."

Other members of the research team include Jennifer Winters (CfA and Williams College), Jonathan Irwin (CfA and the University of Cambridge, UK), and David Latham, Perry Berlind, Michael Calkins, Gilbert Esquerdo, and Jessica Mink (CfA).

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