Smallest lonely planet ever discovered in our Milky Way


The planet – which is therefore not accompanied by a parent star – would be about as massive as Mars.

More than 4,000 exoplanets have already been discovered in our Milky Way. Nearly all of these exoplanets orbit a parent star, and most of them were also discovered thanks to that star (see box below). But judging from what we know about the – sometimes downright messy – evolution of planetary systems, researchers have actually assumed for years that there are also many planets that do not orbit a parent star, but move lonely through space. And yes; for several years we have known that such ‘lonely planets’ exist.

And today, Polish researchers are announcing that they have once again managed to discover such a lonely planet. And what makes this discovery extra special is that the planet in question will go down in the books for now as the smallest lonely planet discovered in the Milky Way to date.

The astronomers discovered the planet with the help of it Optical Gravitational Lensing Experiment (OGLE for short), a Polish research project using a telescope in Chile to hunt for planets and variable stars, among other things. “Our discovery shows that lone small-mass planets can also be detected and characterized using Earth-based telescopes,” says researcher Andrzej Udalski.

Most of the exoplanets known to date have been discovered using the transit method. Telescopes stare at stars for a long time, hoping to witness that the brightness of those stars decreases regularly. Such a decrease in brightness may indicate the presence of a planet orbiting the star and occasionally – if it is between the telescope and the star – blocking part of the starlight. A smaller portion of the planets have been discovered because the parent star was witnessed to wobble by the gravitational pull of the planet orbiting around it. In either case, planets are discovered thanks to the stars they orbit. It gets a lot more difficult if a planet is not accompanied by a star …

Hundreds of millions of stars
The small lonely planet was discovered using gravitational microlensing. If a heavy object (a star or a planet) moves between an observer based on Earth (in this case the telescope in Chile) and a distant star, the gravity of this object will act as a lens and the light from that one behind deflect and amplify the star, making that star brighter for a short time (see video below). It sounds quite simple, but it is not, explains researcher Przemek Mróz. “The chance that you spot a micro lens is very small, because all three objects – the light source, the lens and observer – have to be aligned. If you focused on just one star, you would have to wait nearly a million years to witness the light being deflected from that star. ” Mróz and colleagues do not have that much patience, so they study hundreds of millions of stars within the OGLE project, hoping to witness the sudden brightness of one of these stars increase.

A lonely planet deflects the light of a more distant star. Image: Jan Skowron / Astronomical Observatory, University of Warsaw.

How long such an increase in brightness lasts depends on the mass of the ‘lens’. The brighter the object that deflects and amplifies the light from the star behind, the shorter it will last. Most gravitational microlensing cases are caused by stars and last for several days. Planets are much less massive and as a lens often only affect the brightness of the star behind for a few hours. The small lonely planet now discovered in this way only deflected the light from the star behind it for 42 minutes. This microlensing event therefore goes down in the books as the shortest ever.

Because the duration of the micro mixing event is dictated by the mass of the object, researchers can also say more about the mass of the object by looking at the duration of the event. And that is why they can also make a fairly accurate estimate of its mass. It is estimated that the planet is less massive than Earth and roughly as massive as Mars. Also, the researchers are quite sure that this planet is lonely and not accompanied by a star. “If the lens orbited a star, we would have seen that star’s presence in the light curve,” says researcher Radoslaw Poleski.

How a planet becomes lonely
Lonely planets are not born lonely, but arise – just like ‘normal’ planets – around a mother star. Such a young star is surrounded by a disc of dust and gas from which planets form. But the evolution of such a planetary system can be a bit messy in the initial phase. Newborn planets pull and push together with their gravitational pull and that can lead to planets being swept out of their system, leaving them lonely.

By studying lonely planets, researchers hope to gain more insight into the origin and evolution of planetary systems. The more lonely planets we can track down, the more complete our picture becomes. That is why the Polish researchers enthusiastically continue their search. And reinforcements are on the way. In a few years, NASA hopes to release the Nancy Grace Roman Space Telescope to launch; a telescope that is ideal for detecting planetary outcasts.

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