Jupiter is often described as the “big brother” planet of our solar system that made the formation and evolution of Earth possible.
In the early days of the solar system, massive Jupiter helped the planet grow rapidly while serving as a gravity well that shielded the planet from the most violent planetesimal, asteroid and debris impacts. Being such a massive target, it cleared the field enough for Earth to grow.
This history raises the inevitable question of whether other solar systems need a Jupiter-size planet to allow smaller rocky planets to emerge and develop like our own planet?
An answer is by no means at hand, but new research that will appear in the The Astronomical Journal, does point to a complicating factor for the formation of Earth-sized planets: The solar systems of red dwarf stars, the small, “cool” stars that are by far the most common in the galaxy, are largely devoid of Jupiters.
Taking statistical uncertainties into account, the researchers 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 similar to 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,” said study co-author David Charbonneau, a professor at Harvard University and a member of the Center for Astrophysics | Harvard & Smithsonian. “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.”
And what happens in a solar system if there’s no Jupiter-equivalent?
An answer requires a look into Jupiter’s history.
Formed in the far reaches of the solar system when our system was in its infancy 4.5 billion years ago, scientists theorize that it moved towards the Sun in the first few hundred million years after formation. In the process, hefty Jupiter’s gravity scattered a vast number 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 crashed on our young planet, they are theorized to have delivered substantial amounts of water, potentially along with organic (carbon-containing) molecules, the chemical building blocks of life.
The waters pooled on our world’s surface and created 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.
Without Jupiter, these conditions might not have ever developed, and the journey to life might never have gotten underway.
Adding to its central role in Earth’s history, enormous Jupiter and its super-powerful gravitational pull, appears to have allowed some rocky planets to evolve while pulling other objects, rocky and gaseous, into its vicinity and then adding them to its heft.
So if habitable planets are at all present around red dwarf stars, they would have been formed in ways very different from what happened on Earth.
The new findings are especially important because many red dwarf stars 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,” said Charbonneau in a release.
“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.”
For instance, the Trappist-1 collection of seven rocky planets just 40 light-years from us orbit a red dwarf star. The system is very actively studied by the JWST and other observatories now for possible habitability. But it has no Jupiter-sized planet in its system.
The lead author for the upcoming red dwarf study is Emily Pass, a researcher at the CfA.
To gauge the frequency of Jupiter planets, Pass and colleagues examined an unusually 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 search for 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 actually did not detect a single Jupiter-equivalent planet. Although the study size was substantial, the researchers allowed for a 2 percent statistical chance of finding a Jupiter in a low-mass red dwarf planetary systems.
“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,” Pass said in a release.
But she also said that the absence of Jupiters does not necessarily mean the pathway to habitable planets is foreclosed in red dwarf systems.
“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,” she said.
“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,” Pass said. “We’re excited to see what exactly that means as we forge ahead in remotely exploring the planets in our cosmic neighborhood.”
While many red dwarf stars are relatively close, relatively easy to observe and often orbited by rocky exoplanet, they pose some significant problems for habitability beyond their lack of Jupiters.
Most of the planets orbiting red dwarf stars are tidally locked, meaning that only one side ever faces the star. Astrobiologists acknowledge this can be an obstacle to habitability, but they also have proposed mechanisms allowing for tidal locking to still allow for necessary global thermal conditions.
Additionally, red dwarf stars are known to begin their existence with millions of years of intense flaring before calming down. Especially for close-in exoplanets, this is seen as a reality that could forever sterilize planets that might otherwise develop in promising ways.
Red dwarfs make up about 75 percent of the stars in the Milky Way. Given the neighborhoods that they create, they may substantially limit a large percentage of the galaxy’s stars from supporting habitable planets. While essential for learning about the dynamics and compositions of exoplanets, the long-term role of red dwarfs in the search for life beyond Earth may be quite constrained.
But if they are unlikely to have planets that can support life, there are many other stars out there that just might. The Milky Way has in the range of 100 billion stars, and that means there are an awful lot of stars, including those like our Sun, that are not red dwarfs and do not present the red dwarf problems for habitability.
Many Worlds 