Lens Effect reveals little vagabonds

The duration of a microlens event and the distortion of the background star enable conclusions to be drawn about the mass of a passing lonely planet. (Image: Jan Skowron, Astronomical Observatory / University of Warsaw)

Not bound to any star, it wanders through space: Using a gravitational lens effect, researchers have tracked down the smallest solitary planet known to date. The celestial body is slightly smaller than the earth and wanders freely through the Milky Way, according to the data. The planet betrayed itself by the so-called microlens effect of its gravity as it passed in front of a star. The researchers suspect that there could be many such little vagabonds in space.

Astronomers now have thousands of exoplanets online. These include a few unusual specimens – but they usually have one thing in common: They orbit stars like the earth or the sun. But there are exceptions, as earlier studies have already shown: There are planets that are not gravitationally bound to a star. Models show that certain constellations and gravitational effects can maneuver planets out of their star systems so that they can then move freely through space. So far, however, astronomers have actually only been able to detect very few of these cosmic loners.

Micro lens effect brings light into the dark

The reason: The lonely celestial bodies are not illuminated and they also do not make themselves noticeable by influencing their parent star, like other exoplanets. Therefore, they cannot be traced using the traditional methods of astrophysical detection. But there is one possibility that has already been used successfully: the scientists working with Przemek Mroz from the University of Warsaw use the so-called microlens effect to reveal the dark fellows. It is a special form of the gravitational lens effect. It is based on the effect of masses on light: Similar to optical lenses, gravity can deflect radiation and lead to amazing effects. In its November 2020 issue, bild der Wissenschaft reports on the history of the discovery of the gravitational lens effect and its versatile use as a tool in astrophysics.

In the case of the loner planets, the gravity of these celestial bodies causes a lens effect, which affects the light of a star behind them. Gravitation deflects the light from this source and focuses it. “This makes the distant star appear to be brighter for a short time,” explains Mroz. To determine this, however, the star, the loner planet and the earth must be exactly on one line. In order to record such rare micro-lens events, the astronomers cast a “broad view” of millions of stars in the center of the Milky Way. This enables them to detect specimens that change their brightness.

And so the special catch succeeded in June 2016: The 1.3-meter telescope at the Las Campanas Observatory in Chile recorded the glow of a star that normally shines evenly. The researchers then examined the resulting light curve in order to track down the cause of the effect. The features enabled them to rule out that this temporary increase in brightness was due to the star itself. However, it became apparent that they had actually recorded a microlensing event. It was given the designation OGLE-2016-BLG-1928.

Smallest loner so far

The special thing about it was that the microlens event was very short. As the researchers explain, the duration depends on the mass of the lens object – the less massive, the shorter the light effect. Most microlens events are caused by stars and typically last several days. With loner planets it is a few hours and in the case of OGLE-2016-BLG-1928 the duration reached a previous minimum of only 42 minutes. “So it was clear that it was caused by a very small object,” says Poleski. According to the calculations, the lens object must have been less massive than Earth – it was probably a celestial body about the size of Mars, the scientists say.

As they explain, their data also shows that it is not a planet of a star system, but a lonely vagabond. “We would see in the light curve of the event if the lens object were bound to a star,” says Poleski. “We can at least rule out the possibility that this planet orbits a star at a distance of up to eight astronomical units,” emphasizes the astronomer.

As he and his colleagues conclude, planet formation theories predict that planets ejected from star systems should typically be smaller than Earth. In this context, the current results suggest that many loner planets like OGLE-2016-BLG-1928 could still be waiting to be discovered. Poleski and his colleagues will definitely keep an eye out for them.


Source: University of Warsaw, specialist article: Astrophysical Journal Letters, doi: 10.3847 / 2041-8213 / abbfad

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