Here on La Palma, one of the volcanic Canary Islands located about a 1000 km off the Northwest coast of Africa, some 250 researchers have gathered for the latest scientific meeting of the European Astrobiology Institute (EAI). Astrobiology —- the quest to understand how life arose here on Earth and potentially elsewhere —- is the glue that binds everyone here. And one of the more intriguing presentations this week posited how a given galactic environment affects exoplanet properties and habitability.

Our findings indicate that habitable worlds could be preferentially found in low-phase-space density environments, Scarlett Royle, a doctoral student at Liverpool John Moores University in the U.K., writes in a paper given at the conference. In broad terms, low-phase space density means the stars are born in a galactically less dense environment, whereas high-phase space density is a more galactically dense environment. This corresponds to areas of ripples and streams in the galactic disk.

To date, according to the NASA Exoplanet Archive, there are more than 5200 extrasolar planets confirmed to be detected with some third of all stars in our Milky Way Galaxy estimated to host planetary systems of some stripe. Yet planetary systems that reside in phase-space overdensities have been shown to have lower planetary multiplicity, much shorter planetary orbits, the team notes. They also tellingly exhibit an excess of hot Jupiters in comparison to systems in underdense regions like our own, the team notes.

The team’s research included data from both the European Space Agency’s (ESA) Gaia spacecraft as well as NASA’s archive, among other sources. Royle and colleagues restricted their research to solar type stars no larger than twice the mass of our Sun and with ages known ranging from 1.2 to 3.5 billion years all of which lie within some 120 lights years of Earth. They then characterized the stars as either low phase-space density or high phase-space density, determined by a combination of either the star’s velocity or momentum.

The current paradigm of planet formation is to treat planetary systems in isolation, Royle, who was awarded the Bell Burnell Graduate Scholarship in 2021, told me here at the conference. But we’ve shown that properties of the planetary system are affected by the outside galactic environment, she says. We find that in under dense regions, planets tend to have wider orbits around their parents stars which are more in line with our habitable zone, says Royle.

Planetary systems, says Royle, can be influenced by gravitational perturbations of our Milky Way’s central bar; its spiral arms; or, from passing dwarf galaxies. Or even from a wholescale merger of giant spiral galaxies, such as is predicted to happen when our grand spiral neighbor, Andromeda (M31) eventually merges with our Milky Way several billion years from now.

As for our own solar system?

We’re in what I would call an under dense stellar environment where the stars that are spatially nearest to us are quite low, Royle says.

Stars begin their lives in giant molecular clouds and naturally cluster in their birth environments in star forming regions throughout the galaxy. Our own yellow dwarf star formed from one of these clouds some 4.6 billion years ago in what is known as an open cluster of stars in the plane of our Milky Way Galaxy.

Although we had stellar siblings, they have long since dispersed and our star is now thought to lie within the mid-plane of one of the Milky Way’s spiral arms. Over hundreds of millions of years, we are co-moving as our Milky Way rotates but we are also moving up and down through the mid-plane of the galaxy, somewhat akin to a fishing cork bobbing in and out of water on a small pond.

They are usually naturally clustered when they’re born; they’re a co-moving group traveling at the same speed and in roughly the same direction, says Royle. But then they start to disperse spatially, she says.

Jupiter-sized planets are typically thought to be formed at between 5 and 10 astronomical units (AU) from their parent stars. For example, our own Jupiter orbits at 5 Earth-Sun distances. But exoplanets that harbor hot Jupiters (so named because they orbit on shockingly close orbits to their parent stars) were likely formed farther out in the planetary disk then migrated inwards. But the question to date has been: What causes such migration?

Galactic dynamical perturbations have somehow affected the planetary system and potentially sent those hot Jupiters inwards, says Royle. We also find that planets in overdense regions tend to have more single planet systems than those in underdense regions, she says.

Once the planets are fully formed, we think that the system could be affected by the outside environment, sending planets inwards, potentially ejecting them from the system altogether, says Royle.

I also expect planetary systems where there’s been a lot of galactic perturbation to be less likely to be habitable because they’ve not had stable conditions long enough for life to form, she says.

As for how rare our own solar system might be?

“My feeling is very rare,” said Royle.

That’s even more reason to protect the planet we have.

Royle points to what she sees as a “dangerous amount of miscommunication in the media about exoplanets.” When we’ve got people that say we should find a new place to live due to climate change, that is very unrealistic and a dangerous attitude towards this planet, she says.

“This planet is amazing; we’ve got like over 10,000 species of grass and over 10,000 species of birds and even if we discovered algae on an extrasolar planet, we’d be very, very excited,” said Royle.

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