Planets roaming space without a parent star may not be as lonely as thought.

They can sometimes harbor moons. And we can spot some of those moons with our current telescopes, researchers write in this paper.

Exomaniacs

In recent years, thousands of (candidate) planets have been discovered outside our solar system. Based on what we see in our own solar system, you would expect that a large part of these planets also have a moon. But detecting and confirming the existence of these so-called ‘exomaniacs’ has proved extremely difficult.

IPMOs

Reason enough for an international team of researchers to think about other ways to discover exomoons. And that brings them to so-called IPMOs: Isolated Planetary Mass Objects. These are planets that – unlike most exoplanets discovered to date – do not orbit a star, but roam solitary through space.

The detection of an exomoon

“It’s actually easier to detect exomaniacs around solitary exoplanets than it is to detect exomaniacs around ‘normal’ exoplanets,” said researcher Mary Anne Limbach. “Do you know why? Young gas giants are still warm shortly after they form. And that heat makes them bright (at infrared wavelengths, ed.) And thanks to that heat we are able to directly observe some young exoplanets.” A good example of this is HR8799, which can be seen in the image below. “But as you can also see in that image, there’s noise (that’s the wave-like features) and that noise is caused by the light from the star. That starlight makes it very difficult to measure exactly how bright the exoplanets around HR8799 are. But we are also aware of many (about 60) solitary planets that in many ways resemble the planets in the HR8799 system, but where there is no interfering starlight. This enables us to measure the brightness of these solitary planets very accurately. And because we can do that, we can also see very small changes in the brightness of those planets.” And those changes could indicate the presence of an exomoon. “When an exomoon moves in front of the planet, you see a small decrease in the brightness of the planet. And so, by monitoring the brightness of solitary planets, we can detect exomoons.”

HR8799, surrounded by three directly observed planets. Image:
NRC-HIA/C. Marois / W. M. Keck Observatory.

Candidate Exomaniac

In their study, the researchers go one step further; they not only present a method to discover exo-moons around solitary planets, but also directly apply the method. And that leads to the detection of a candidate exomane around the planet 2MASS J1119-1137 AB.

spitzer

To this end, the researchers examined images made by the Spitzer space telescope over time, which also feature 2MASS J1119-1137 AB. The images indicate that the planet’s brightness has changed over time. And those decreases in brightness could indicate the presence of an exomoon with a radius 1.7 times greater than Earth’s radius. Although the researchers cannot say with certainty that the changes in the brightness of 2MASS J1119-1137 AB can be traced back to an exomoon, the detection of those changes is nevertheless significant. “The characteristics of the data and the candidate signal suggest that it is possible to detect Earth-sized exomans around IPMOs with existing instruments.”

Bright planets, big moons

Not every solitary planet will give up its secrets just as easily. “Bright solitary planets are particularly suitable for the observation of exomoons,” says Limbach. “That means they have to be close and young (because they are still warm from their formation).” And then it also depends on the moons whether we can actually spot them. “Larger-radius moons block more light from the planet, making it easier to detect the diminution in brightness.” Thus, large moons are easier to spot than small ones. “In addition, the chance that a moon (seen from Earth, ed.) will pass in front of the planet is greater if that moon is closer to the planet.”

Important quest

So as observers, we have to be lucky, but what Limbach and colleagues are cautiously suggesting is that it should certainly be possible to spot exomoons around solitary planets. And there are plenty of reasons to start the hunt for these exomoons after this cautiously positive news. “We know very little about the exoman population at the moment,” says Limbach. By detecting more exomoons, we can map this population and, in the process, also better understand the moons in our own solar system. “Detecting exomoons also allows us to determine whether the moons orbiting the gas giants in our solar system are ‘normal’.” In addition, the exomoons may also be able to tell us something about our own planet. “Most solitary planets are young and the conditions on their moons may be similar to those on the young Earth. Being able to characterize these young moons will also give us a better idea of ​​what our planet used to look like.”

Extraterrestrial life

In addition, the search for exomoons may also have implications for our search for extraterrestrial life. After all, in our own solar system, it is moons such as Europa (Jupiter) and Enceladus (Saturn) on which the conditions for life appear to be favorable. And perhaps extraterrestrial life also has the greatest chance of success on exomoons. “Moons of solitary planets or exoplanets can harbor life if they are in the habitable zone. Estimating how many habitable exomoons there are helps us to get a more complete picture of the number of habitable worlds.”

Living around solitary planets?
One of the most important ingredients for life – as we know it – is liquid water. And that can only survive on a celestial body if it is not too hot (because then the water evaporates), but also not too cold (because then the water freezes). It means that on moons around solitary planets you also need a source of heat to keep water liquid and make life possible. Young planets that are still warm from their formation can be such a source of heat, Limbach says. “Over time, however, the planets cool down and then the moons are probably not so habitable anymore. Although tidal forces – just like you see on the moon Io – can also keep the moons warm.”

Creation of solitary planets

Finally, the exomoons around solitary planets may also reveal why these planets are not part of a ‘traditional’ planetary system. “We don’t know for sure whether solitary planets form around stars and then are thrown into space or whether they form in the same way as stars, namely from the collapse of a gas and dust cloud, which then has only enough mass to form a planet. instead of producing a star. If the solitary planets formed around a star and were ejected, we would expect that only the moons that were close to the planets survived this violent process. And so the discovery of moons around solitary planets can also give us more insight into how solitary planets form.”

It remains to be seen whether researchers with the candidate signal picked up around 2MASS J1119-1137 AB will be able to track down such a first significant exomoon. For the confirmation of the existence of the exomoon, we have the much more powerful James Webb Space Telescope required. That telescope will be launched later this year. Pending that launch, we can use existing telescopes to look for other moons that can measure themselves in size with the Earth and orbit around solitary planets. However, for the discovery of smaller moons – similar to Io (Jupiter) or Titan (Saturn) – we are again dependent on the James Webb Space Telescope. Will no doubt be continued.