Do exoplanets shrink with age?
We may be closer to knowing why super-Earths are so scarce
As we all know, planets come in many different sizes, and you only have to look at our Solar System to see how varied they can be.
Just take Mercury as the smallest, for example, and Jupiter as the most humongous. If you had enough Mercurys at your disposal, you could take 24,462 of them and pack them tightly into Jupiter. For comparison, you could do the same with 1,300 Earths.
Interestingly, though, there is a size gap. Try as you may, you’re not going to find too many planets anywhere in the universe that are between 1.5 and two times the size of Earth.
You certainly won’t find them in our Solar
System and, so far at least, there are very few in planetary systems elsewhere, suggesting there’s no middle ground between so-called rocky superEarths and larger gas-shrouded mini-Neptunes. Quite why, however, has been a mystery.
Such a gap was not apparent at first, certainly not when scientists began discovering planets outside of our Solar System from 1992. Back then the first extrasolar planet, or exoplanet, was found when astronomers Aleksander Wolszczan and Dale Frail provided evidence of two planets orbiting a pulsar some 2,300 light years away.
But they were large and easily spotted. It’s only when technology grew more sophisticated that smaller exoplanets began being discovered, and the gap became rather stark.
To underline just how recent a mystery this is, a planet 1.4 times the size of Earth, Kepler-10b, was only discovered by the now-deactivated Kepler space telescope in January 2011. Even so, it took until 2017 for the gap’s existence to be reported as more and more exoplanets were confirmed – there have been more than 4,400 discovered to date. That was the year a study headed by Benjamin J. Fulton emerged, which is why the observation is often referred to as the Fulton gap.
Since this gap became apparent, there has been a great deal of academic detective work to try and discover what could be the best explanation for why planets orbiting close to their star are either small or large, with very few in between. Further findings by the Transiting Exoplanet Survey Satellite (TESS), which launched in 2018, have only added further weight, encouraging scientists to press on.
“The gap in planet sizes has prompted research simply because it was unexpected and because it is still unexplained,” says Trevor David, a research fellow at the Flatiron Institute’s Center for Computational Astrophysics in New York City. “The gap is statistically significant, and it’s not the result of any observational bias, so the exoplanet community generally believes this is a real feature in the data that requires an explanation.
“For many natural processes, there often exists a spectrum of possible outcomes, so when we see something that is so clearly bimodal – almost binary outcomes in the sense that a planet either resides above or below this gap – it immediately catches the interest of scientists. Why are so few planets found in the gap? And are planets below or above the gap different in ways other than their sizes?”
To help resolve such questions, David and his team selected 700 exoplanets that were less than ten times the size of Earth. These were taken from the California-Kepler Survey, which measures thee precise properties of planets and their host stars using data from NASA’s Kepler mission.
“The size of a transiting planet is measured relative to its host star’s size,” David tells All About
Space. “Once the stellar sizes were determined with sufficient precision, it became clear that the planet size distribution was multimodal – that it had many peaks. Researchers had previously detected the planet signals and measured the planet-to-star size ratios. But the radius gap was essentially ‘blurred’ out due to inaccurate and/or imprecise stellar radii.”
The ‘twist’ in the research carried out by David and his team was that they looked to determine the age of the exoplanets they selected. They wanted to see if the ageing of planets had any bearing on whether or not the radius gap changed by making those rare sizes more common, and their study became the first to show that the precise location of the gap shifts with the age of the planetary systems being studied.
But how did they do this? First of all, ageing the exoplanets can be done using a combination of their chemical composition, brightness, colour and distance. Based on the premise that planets form around the same time as their host stars, the researchers were then able to place the exoplanets into two distinct categories: those that were older than 2 billion years, and those that were aged at less than 2 billion years.
“I was primarily motivated by the observation that the youngest transiting planets appear to be unusually large,” David explains. “This motivated me to understand how long this phase of apparent inflation may last, and whether planet sizes continue to evolve over longer timescales.”
The study suggested that the least common planet radii from the younger set was 1.6 times Earth’s radius and that this was smaller, on average, than the least common radii from the older set – some 1.8 times Earth’s radius.
“Just a few months prior to our research, there were two other studies that appeared to show that the overall distribution of planet sizes evolves over billions of years,” says David. “Those studies looked at a single metric: the number of superEarth detections relative to the number of miniNeptune detections. It appears that super-Earths are more common, relative to their mini-Neptune counterparts, at older ages. One way to explain this observation is if some mini-Neptunes are converted into super-Earths over time, where that timescale happens to be billions of years.”
In other words, the larger mini-Neptunes appear to be shrinking down to their rocky cores as they
get older – an example of atmospheric loss which has previously been put forward as an explanation for the gap. Crucially, given there were very few mid-sized planets in the younger group and more in the older one, it suggested rare sizes of planets become more common over time.
By shrinking, the study found that miniNeptunes could be leaping over the planet radius gap to become super-Earths. Over time this will involve ever-larger mini-Neptunes making the leap as they transform into larger super-Earths.
“For some, and perhaps most mini-Neptunes that conversion may occur quickly [within the first billion years], but for other planets it appears that their evolution proceeds slowly,” says David. “What I chose to focus on was not how the relative number of super-Earths and mini-Neptunes changed over time, but rather whether the gap in the planet size distribution shifts with time or remains fixed at all ages.”
One of the questions which could emerge around a study such as this is why – if indeed miniNeptunes reduce in size because their atmospheres are leaking away to leave behind a solid core – this only seems to happen rapidly with the smaller ones. The explanation is that mini-Neptunes with enough mass can retain their atmospheres thanks to gravity, but the smaller mini-Neptunes can’t hold on to their gas, meaning they’ll shed quickly. The astronomers say the gap is the “chasm between the largest size super-Earths and the smallest size mini-Neptunes”. And that gap evolves over billions of years.
Indeed, as David explains: “Theoretical models predict that the super-Earth population fills in from the ‘bottom up’.” If super-Earths are formed from atmospheric loss, then we expect the smallest and least massive cores to lose atmospheres first. “Over time, larger and larger planetary cores lose their atmospheres and fill out the super-Earth population we observe today,” David says.
But what are they shrinking from and to, in terms of size? It’s hard to pinpoint exactly because planets form with a range of core masses, atmospheric masses and separations from their host stars. Some planets may never lose their atmospheres entirely, while others can become completely stripped, David explains.
“The largest super-Earths are around 1.8 Earth radii, but the average super-Earth size is closer to 1.3 Earth radii,” he continues. “The average mini-Neptune size is about 2.4 Earth radii, so if a super-Earth started off with a sizable atmosphere, it could have been anywhere between two and ten Earth radii in the past, depending on its specific evolutionary history and how far back in time you went.
“We don’t know very well how large planets are at the beginning of their lives, but the few examples of extremely young transiting planets that we do know of are unusually large. Some of those young planets are between five and ten times the size of Earth, but finding planets of those sizes at older ages is a rare occurrence.”
But what is causing the planets to lose their atmospheres? As you may expect, this isn’t clear, but there are a number of theories. One is that heat left over from planetary formation will transfer energy into the planet’s atmosphere. The gas then escapes into space, and the exoplanet shrinks.
“We believe that planets are assembled from collisions of ever-larger rocky bodies,” says David. These collisions deposit a large amount of energy into the growing planet’s core, so much so that the core may be totally molten early on. “Over time the core cools and contracts, and the energy radiated away from the core must go somewhere. That energy is deposited into the atmosphere, and if the energy transferred becomes comparable to the binding energy of the atmosphere, then an outflow will launch atmospheric gas away from the planet.”
Another theory has much the same effect, again allowing gas to escape. “It says high-energy radiation [specifically X-ray and ultraviolet radiation] from
“The gap in planet sizes has prompted research simply because it was unexpected”
Trevor David
the host star heats the upper layers of a planet’s atmosphere. The heat provided by the highenergy stellar radiation can be enough to drive an ‘outflow’ of gas from the planet. Depending on how substantial and how prolonged this mass loss is, a planet can become partially or totally stripped of its atmosphere,” David says.
If planets are losing their atmospheres and becoming smaller over time, however, why don’t many more stick around in the gap? “In some analyses the gap appears to be completely empty – totally devoid of planets. In other analyses of generally less precise data, the gap appears to be sparsely populated, but not completely empty.
“In my view the community hasn’t conclusively answered whether planets can exist in this gap because there are so many factors that can cause an observer to inaccurately infer a planet’s size, even with very precise data in hand,” David answers. “Some studies have invoked exotic compositions such as water worlds to explain the apparent presence of planets in the gap. But many observations of the exoplanets found with Kepler can be explained with a single population of exoplanets that are born with rocky cores and gaseous envelopes dominated by hydrogen and helium. Some of these planets retain their primordial atmospheres, while others lose them. This would be the simplest explanation for many observations made regarding Kepler-type planets.”
There’s clearly more work to be done, and research will be ongoing, yet we’re getting a clearer picture of how planets evolve. “Our study suggests that, for some planets, evolution in something as basic as a planet’s size may continue for billions of years,” David says. “I didn’t expect to see such slow evolution, nor was I expecting the radius gap to move as a function of age in the data we were looking at.”