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Astronomers have confirmed thousands of planets outside our solar system, but not a single exomoon. New research reinforces the case of an alleged exomoon detected for the first time last year, but scientists say the evidence is still not definitive.
To date, astronomers have confirmed the existence of 3,851 exoplanets, a surprisingly high figure given that the first exoplanet was discovered just 26 years ago. Yet during all this time and after all these amazing planetary discoveries, astronomers have not been able to prove the existence of a single exomoon, that is, a moon orbiting around a planet from another stellar system.
This result is surprising given that exomoons are guaranteed to be ubiquitous in the cosmos. Take our own solar system, for example. Our eight planets harbor 185 known natural satellites. Of these, 79 belong to Jupiter and 62 to Saturn – an embarrassment of riches that shows the potential of gas giants to house vast collections of natural satellites.
In addition, many of the exomoons may be liveable, as a research paper published earlier this year argues.
But we have not found one yet, despite these predictions.
Indeed, exomoons are very difficult to detect and for obvious reasons. Unlike giant gas giants and super-Earths, moons tend to be rather small.
Not only that, they revolve around an object that also rotates around an object; after a few observations, astronomers can predict when an exoplanet will pass in front of its host star, but knowing when the exonambule of an exoplanet could appear is another challenge.
It is therefore logical that astronomers are struggling to make a definitive detection.
Some of you may remember from last year, astronomers using the Kepler Space Telescope detected a slight drop in the brightness of a star called Kepler-1625, which was interpreted as a potential sign of an exomoon.
The data was not definitive, but the observation allowed astronomers to obtain 40 hours of access to the Hubble Space Telescope. Thanks to Hubble, astronomers Alex Teachey and David Kipping of Columbia University were able to collect data four times more accurate than Kepler could acquire.
The results of this second survey, published this week in Progress of science, argues in favor of the existence of this exomoon, although, as before, the data are still not definitive.
"Our initial report was based solely on Kepler data, which in our opinion was not enough to claim a discovery," said Teaching. Gizmodo.
The new data has essentially two characteristics suggesting the presence of a moon.
The first of these features is an apparent decrease in the luminosity of the host star as a result of the transit of the planet, which astronomers attribute to the exomooon. The alleged exomoon is in orbit around a known exoplanet called Kepler-1625b, a giant gas giant several times bigger than Jupiter. It takes about 19 hours for Kepler-1625b to cover the face of its star host, from our point of view here on Earth.
At 8000 light-years away from the planet, astronomers can not really see the planet, but they can detect a drop in the brightness of the host star as it passes by. it is this eclipse effect, called transit, that allows scientists to detect distant exoplanets.
About 3.5 hours after the Kepler-1625b pass, Hubble detected a second, significantly lower attenuation of the star's brightness. In a statement, Kipping said it was consistent with "a moon tracking the planet like a dog following his owner on a leash". Unfortunately, however, Hubble's scheduled observations ended before the complete transit of the supposed exonçon could be measured.
The second observation was the presence of transit timing effects, which indicate the presence of something in the system that was pulling gravity onto the planet.
"Basically, when a planet has a moon, it naturally pulls on the moon by gravitation, but the moon also pulls on the planet and they revolve around their common center of mass – a point in the space along the line between the centers of the two objects, "explained the teaching.
"So, because the planet revolves around this point – called the barycenter – it wobbles in its orbit around the star. What it does is, it affects the timings of the transits. Sometimes the planet transits a little earlier, sometimes a little later. These are the transit timing variations, or TTV. "
According to the teaching, the explanation of the moon is superior to other explanations, such as the gravitational influence of an unexplored exoplanet in the Kepler-1625 system. Exomoon's explanation also has the added advantage of being a unique explanation of the two observed phenomena.
But, as Teaching and Kipping acknowledge, the data is still incomplete and an unknown element can affect the observations.
"We would just like to point out that this remains a candidate for exoamon," said Teaching. Gizmodo. "This is an enticing result, certainly, but it has yet to be confirmed by more observations and, hopefully, by other teams that will replicate our results."
Anyway, an image of this exoplanet and this moon begins to emerge. If it's an exomoon, it's actually quite bulky – about the size of Neptune. For comparison, Jupiter's Ganymede – the largest moon in our solar system – is 5268 km in diameter, compared to Neptune at 50,000 km in diameter.
This exomoon represents about 1.5% of the mass of its host planet, which, interestingly, corresponds to the same mass ratio between the Earth and the Moon, but on a much less grandiose scale. According to the standards of our solar system, it is a moon of gigantic proportions – an attribute which, according to the researchers, is largely responsible for its discovery.
Natalie Hinkel, an astronomer at the Southwest Research Institute in San Antonio who did not participate in the new study, was impressed by the way in which the authors verified their data and analyzes.
"They envisioned a wide variety of angles, warnings and pathways that could make their discoveries null and void – but all of them bring back the likely presence of an exomoon," Hinkel said. Gizmodo. "However, as no such observations have been found, I agree with their view that it is the" unknown unknowns "who can or can not make this discovery."
Astronomer Rob Wittenmyer of the University of Southern Queensland stated that the document was complete and that the authors had not exaggerated their claim.
"The result is not surprising, as evidenced by the abundance of moons in our solar system," Wittenmyer said. Gizmodo. "As moons, this one is big enough, and therefore detectable. I suspect that, like exoplanets and cockroaches, exomoons are ubiquitous. When you see one, you missed a lot of others. As techniques are refined, we will surely find more exomoons in the coming decades. "
David Bennett, scientist at NASA's Goddard Space Flight Center, agrees that additional observations will be needed to qualify this discovery as a definitive discovery of an exomoon.
"In addition, the planet is 10 times more massive than Jupiter and the predicted mass of the moon is close to that of Neptune. So, if we saw these objects in isolation, we would both consider them planets, "said Bennett. Gizmodo.
"If the observed companion of the planet is confirmed, there is a good chance that it is considered a binary planet instead of a moon." It could be the action of a Neptune mass planet captured by a super-Jupiter planet during the formation of the planetary system. "
Which is a good point. How is it possible that this particular moon-planet architecture exists? If it is confirmed that Kepler-1625b hosts such a large moon, astronomers will have the added challenge of understanding how this star system has taken shape.
Another interesting fact about Kepler-1625b is that its orbit resides in the habitable zone of its host star. Now, it is very unlikely that this gas giant and its exomoon just as gaseous as Neptune can shelter life. But what happens if this so-called exomoon has its own moons and if these moons are terrestrial in nature? Could life really emerge on the moon of a moon?
"There's really an exoonie around Kepler-1625b, a whole new sub-field could open." Scientists, as well as science fiction writers, have only speculated about the discovery and possible livability of exomoons, "Hinkel said.
"Essentially, it could be like the TV show Firefly where people live on central worlds as well as on peripheral moons. "
It will take some time before we can begin to answer these kinds of questions, but this last observation means that we can at least start asking the questions.
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