Common Planets circle Uncommon Stars
The immense difficulty of reaching planets in other solar systems is something that is really hard to grasp. Voyager 1 essentially left our solar system back in 2012. By my reckoning, that was about 8 lifetimes ago. Despite all we have lived through, it is just a pixel in the timeline of this mission; a timeline that will span tens of thousands of years before we can expect it to see its first new star, a red dwarf by the name Gliese 445… and it will see it from a distance of more than a light year.
Small red dwarf stars like Gliese 445 are the most common out there. With a mass just 20–30% of our Sun’s, they are closer in size to Jupiter than the Sun. And while these stars can and do support entire solar systems, they don’t seem to be the hot bed of planet formation one might wish for. A new study appearing in the Astronomical Journal and led by Erik Diego Gillis (McMaster University), uses the NASA’s Transiting Exoplanet Survey Satellite or TESS to carefully sort planetary statistics. A quick glance at the number of stars in our galaxy — 100–400 billion — and the number of planets in our galaxy — 100 billion to maybe a trillion — would seem to indicate that on average every star probably has a planet.
But averages can be deceiving.
The largest stars typically don’t have planets because they blast away the stuff planets are made of. Metal poor stars, like those in globular clusters, haven’t been found to have planets, which makes sense — they just don’t have the materials needed to build planetary cores like we’re used to.
But around Sun-like stars… the most common kinds of planets are rocky worlds bigger than Earth and gasy worlds smaller than Neptune. These worlds are unimaginably named super earths and sub neptunes. The Kepler mission, which is responsible for nearly half of all confirmed planets, was particularly sensitive to warm stars like our Sun, and more than half these worlds were super Earths or sub Neptunes.
So… in our current catalogue, these are our common planets, and they are orbiting uncommon stars.
But this is a biased catalogue …
TESS, with its greater sensitivity to red dwarf stars is working toward a better planetary census, and when Gillis and his team members looked specifically at small stars, they find the super Earth’s we’re familiar with, but that’s where commonalities end. According to Gillis, “Around these stars, sub-Neptunes effectively vanish, which means the mechanisms shaping planets here are different.”
This isn’t surprising — with different heating, gravitational forces, and other physics, planets around small stars will form and evolve differently. How differently, I think surprised all of us. And as Gillis points out, “If we want to understand the origins of planets and the origins of life, we need a complete picture of how planets form and what they’re made of.”
The TESS mission and detailed analysis like this paper are providing us a first look that doesn’t ignore the small stuff… or at least the small stars… Small worlds… those we’re still working at detecting.
Sources
- McMaster University Press Release
- Gillis, Erik Diego, Ryan Cloutier, and Emily K. Pass. “TESS Planet Occurrence Rates Reveal the Disappearance of the Radius Valley around Mid-to-late M Dwarfs.” The Astronomical Journal 171.5 (2026): 317. DOI 10.3847/1538–3881/ae5810
This content was originally written for the EVSN podcast. Watch video episodes and check out additional content at EVSN.tv.
