New study: There are lots of icy super-Earths

"Microlensing" lets us find planets at much greater distances from their star.

What does the "typical" exosolar system look like? We know it's not likely to look like our own Solar System, given that our familiar planets don't include entire classes of planets (Hot Jupiters! Mini-Neptunes!) that we've found elsewhere. And our discovery methods have been heavily biased toward planets that orbit close to their host star, so we don't really have a strong sense of what might be lurking in more distant orbits.

A new study released on Thursday describes a search for what are called "microlensing" events, where a planet acts as a gravitational lens that magnifies the star it's orbiting, causing it to brighten briefly. These events are difficult to capture, but can potentially indicate the presence of planets in more distant orbits. The researchers behind the new work find indications that there's a significant population of rocky super-Earths that are traveling in orbits similar to that of Jupiter and Saturn.

Lenses go micro

The two primary methods we've used to discover exoplanets are called transit and radial velocity. In the transit method, we simply watch the star for dips in the light it sends to Earth, which can be an indication of a planet orbiting in a way that it eclipses a small fraction of the star. For radial velocity, we look for red- or blue-shifts in the light received from the star, caused by a planet tugging the star in different directions as it orbits.

Obviously, a planet's gravitational influence is stronger when it's closer to the host star. And stars can temporarily dim for all sorts of reasons, so we've generally set a standard for discovery that involves observing multiple transits. That, in turn, means a shorter orbital period, and so also biases us toward discovering planets that are close to their host star. As a result, most of what we know about exosolar systems comes from planets that are far closer to their host star than Earth is to the Sun. Even the most distant object discovered by the Kepler mission orbits is only about as distant as Mars.

Looking at this another way, if we'd known of a star with a planet that took as long as Jupiter to orbit, and started observations back in the mid-1990s, when the first exoplanets were discovered, there's a good chance we'd only have observed three transits so far. For something out in the neighborhood of Neptune, the odds are that we'd not have seen any.

Microlensing can be thought of as a bit like the inverse of a transit event, in that gravitational lensing will cause a star to appear brighter. These are difficult to detect partly because the magnitude of the brightening is relatively small, and because it can last for as little as a few hours. If a microlensing event happens during daytime or on a cloudy day, you miss it if you're not observing from space.

The other challenge with microlensing is that it doesn't tell you much about the planet itself. Transit methods give us a sense of the size of the planet, while radial velocity sets limits on the planet's mass. Microlensing only tells us the ratio of the mass of the planet to the mass of the star. Unless we can get a good picture of the star's mass, it's not especially informative.

Earth-like planet, Saturn-like orbit

The team behind the new publication relied on the Korea Microlensing Telescope Network, which has access to widely spaced telescopes spread around the globe. This reduces the chance of missing an event because of bad timing or weather. The new paper is both the description of a one of the microlensing events it captured, as well as an attempt to understand the big picture using all of the potential planetary discoveries the network has made so far.

The microlensing event described here, OGLE-2016-BLG-0007, was first reported by another similar effort (the Optical Gravitational Lensing Experiment, or OGLE), but was also picked up by the Korean network. It was identified as part of a longer microlensing event where one star was creating a lens that brightened a second star. Amid that gradual, multi-month brightening, there was a small bump in the light. There are several ways to potentially explain that smaller bump (a third star, a very large planet in a very close orbit), but most of them are highly improbable. The only thing that makes sense is a planet orbiting at a considerable distance from its host star.

From there, we get into the issue of figuring out what that planet might look like. The ratio of the masses of the planet to its host star is roughly twice that of the Earth to the Sun. But there is no good imaging of the host star available, so we don't know how massive it is. Based on the fact that the typical star in the Milky Way is considerably smaller than the Sun, the researchers assume a red dwarf, which produces a planet with a mass about 1.3 times that of Earth. Given those numbers, the best fit for microlensing data is an orbit about 10 times wider than the Earth's.

That means a super-Earth with an orbital distance roughly that of Saturn's.

Not alone

To get a better sense of how typical this is, the researchers run through all the data obtained with the Korean telescope network, which has identified a bit over 60 likely exoplanets so far. Their analysis of the planet:star mass ratios suggests that there are likely to be a lot of planets similar to this one in orbits that keep them distant from their host stars. Separately, there seems to be a second population of planets that are considerably larger, assuming the stars they orbit are typical of the Milky Way's population.

These two populations are consistent with what we currently view as the typical planet formation process. In this view, rocky planets can grow up to a certain point, after which they become large enough to rapidly pull in gas and other materials nearby, quickly growing to gas giants. The two populations found here would be separated by the gap between the largest planets that failed to start a runaway gas accretion, and those that did begin the process and grew into gas giants.

If that's correct, then the microlensing data also implies that there's a large population of rocky planets, including many super-Earths, in orbits similar to Jupiter's and beyond, which would ensure they're perpetually icy. That's something that's completely absent in our own Solar System, where the rocky planets end with Mars.

It's important to be cautious about this. The total number of planets discovered through microlensing remains small, and there are significant uncertainties in what we can learn about planetary masses using it. At the moment, this method accounts for most of the planets in more distant orbits. Still, if this pattern holds up as we gradually increase our knowledge of more distant planets, then it will be one more bit of evidence that we live in a rather unusual solar system.

Science, 2025. DOI: 10.1126/science.adn6088  (About DOIs).

Source

Hope you enjoyed this news post.

Thank you for appreciating my time and effort posting news every day for many years.

News posts... 2023: 5,800+ | 2024: 5,700+ | 2025 (till end of March): 1,357

RIP Matrix | Farewell my friend