Artist's conception of a rocky planet in a binary, low-mass star system that resembles the star/brown dwarf/planet system discovered by the OGLE team. Image credit: Cheongho Han, Chungbuk National University.

Jul 17, 2015 Microlensing exoplanet could be missing link to planet moon formation

Sen—One of the overarching goals of extrasolar planet detection is to find out if other planetary systems look like our own Solar System, complete with small rocky planets, large gas giant planets, and hundreds of small moons. Since the discovery of the first exoplanet in the early '90s scientists have detected and confirmed nearly 2,000 exoplanets of all sizes and dozens of protoplanetary systems that are still in the process of forming planets themselves. From these discoveries we have refined our theories of planet formation and determined that planets form in stellar accretion disks from leftover star-forming material. Likewise, we think that moons must form in planetary accretion disks from leftover planet-forming material. While we have ample evidence of planet formation and have even caught it in the act, as yet we have no firm detections of moon formation.

Last week, Andrzej Udalski and collaborators announced their discovery of OGLE-2013-BLG-0723LA,B,Bb, a planetary system that may shed light on the issue of exomoon formation. This system contains a Venus-massed planet in orbit around a brown dwarf which is also in orbit around a low-mass star. The Optical Gravitational Lensing Experiment (OGLE) team first detected this system in May, 2013 at Las Campanas Observatory in Chile via the gravitational microlensing method. It was monitored for nearly a year through collaborative efforts by multiple observatories around the world. This system can be looked at as either a small binary star/planet system or a scaled-up version of a star/planet/moon system. After a great deal of analysis the OGLE team finally concluded that the system must have formed in a similar way to how a star/planet/moon system must form and could serve as a "missing link" between what we know about planet formation and what we hope to learn about moon formation.

Microlensing is the unsung hero of exoplanet detection because microlensing events are impossible to predict and impossible to replicate, but can detect the types of planets that no other method is yet able to find. Microlensing is an indirect planet detection method since we are not measuring the planet itself, but rather the planet's effect on another object. The transit method, the radial velocity method, and the transit timing variation method are also indirect detection methods.

A microlensing detection works like this: while you are closely observing a star (the "source") it appears to brighten considerably over a short period of time before going back to normal—typically over a few weeks or months. The brightening occurs because a second star (the "lens") physically crosses between your telescope and the source and magnifies the source light through general relativity. Instead of blocking the source light, the gravity of the lens star splits the light from the source and curves the light paths around the lens. This creates an Einstein ring composed of two separately magnified images of the same source star. The size of the Einstein ring—measured by how the split images move as the lens crosses the source—tells us the mass of the lens.

Because the microlensing event depends on one star moving across the sky relative to another, microlensing measurements can only be taken at that one time when the two stars align and cannot be repeated. We are unable to predict when most microlensing events will occur simply because we lack good measurements of the three-dimensional motion of most stars and so we can't predict when one star will cross in front of another.

A microlensing event with a single lens star is a relatively simple event compared to microlensing with a two-object lens, like a binary star system or a star/planet system. If the lens system is aligned so that both objects can magnify the source, there are two periods of brightening/dimming which mark first one then the other object passing in front of the source. This makes a distinctive two-spiked magnification curve. The second object can only lens the source if one of the split and magnified images passes near it, a rare configuration. Three-object lensing systems in the right alignment are even more rare, and things get even more complicated from there.

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The magnification of a source (open red circle) by a lens star (yellow star) that hosts a planet (purple dot). In this simulation the lens is kept stationary while the source moves relative to the lens (the opposite of what is physically happening). The green circle represents the size of the Einstein ring and the blue ovals represent the split source images as they move across their paths (shaded regions). If the planet lies in one of the shaded regions is will lens the source. The lens star causes the broad magnification and the planet causes the sharp spike. Simulation credit: B. Scott Gaudi, Ohio State University.

If a star/planet system lenses a background source, we can get a very accurate measurement of the lensing star and lensing planet's masses, distance, and physical separation from each other. Even low-mass objects like planets can lens a source significantly, and so we can probe down into the very low-mass planet regime with this method and detect planets much smaller than current radial velocity or transit capabilities allow. Additionally, microlensing is the only method capable of detecting free-floating planets that no longer have a host star. While rare and hard to predict, when microlensing planets are detected they often provide insight into previously unexplored regimes of the exoplanet population.

Such is the case with the system OGLE-2013-BLG-0723LA,B,Bb. The primary component, "A", is a low-mass star roughly one tenth the mass of the Sun, making it an M dwarf star that shines mostly in near-infrared wavelengths. The secondary component, "B", is a brown dwarf—a star-like object that is too small to fuse hydrogen in its core—that is about three per cent the mass of the Sun, pretty large for a brown dwarf. The planetary component, "Bb" is likely a rocky or icy planet about the mass of Venus (about 70 per cent the mass of Earth) in orbit around "B". The brown dwarf/planet system is, in turn, orbiting around "A", forming a three-object system around 1,600 lightyears away from Earth. Neither the transit method nor the radial velocity method are able to detect such a small planet so far away, which shows how incredible microlensing planets can be.

The original microlensing event was detected in May, 2013 and was observed by OGLE around once per hour for many months. This primary microlensing signal is from "A", and appeared to be a regular, single star lensing event. However, during this time they noticed a small additional lensing event that suggested there was a planet in the system. This event was caused by "Bb" lensing the source on one part of its orbit.

Once the OGLE team realized this would be a planet detection, they contacted the Microlensing Follow Up Network, microFUN, to see if someone could continue the observations and detect the planet again while it was daytime at Las Campanas, and therefore unobservable. microFUN is a global network of telescopes around the world dedicated to measuring rapidly-evolving microlensing events, and together they are capable of following an event like this with nearly 24-hours of continuous coverage. When observers could not see the event in Chile because it was daytime, observers at the Wise Observatory in Mitzpe Ramon, Israel picked it up and detected the second planetary lens event. Just before the second planetary lens event the OGLE team detected unexpected lensing by brown dwarf "B", and their models confirmed that "A", "B", and "Bb" are almost definitely in the same, coplanar system.

Because of microFUN, this discovery represents a real-time, multi-national collaboration on a single scientific project. While multi-national projects are not uncommon, they are usually the result of months-long planning processes to coordinate between the various institutions. Participants in microFUN sign up to help when needed in a global effort to track these rapidly evolving events. All told, this groundbreaking discovery has contributions from six institutions representing five different countries around the world.

Planet "Bb" is also one of the lowest-mass planets ever detected. Only two confirmed planets have smaller measured masses, one of which was also a microlensing planet discovery. Because of its very low mass, we can be relatively certain that "Bb" is a rocky/icy planet as opposed to a gaseous planet or a water world. We also learned from this that even not-quite-stars as small as "B" can have planets, which gives hope that our efforts to improve our other planet-detecting instruments will bear fruit like "Bb".

The fact that all three objects in the system lensed the source star tells us that the three objects must all orbit in the same plane, just like the planets and moons in our Solar System do (for the most part). The mass ratios of "A"/"B" and "B"/"Bb" are similar to those of the Sun/Neptune and Neptune/Triton, so it is not a far stretch to claim that "A"/"B"/"Bb" resembles a scaled up star/planet/moon system and could have formed in a similar way. This system could indeed be our first piece of evidence to show how stars, planets, and moons all form together and provide that "missing link" to transition between binary stars with planets to stars with planets with moons.

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The magnification curve of OGLE-2013-BLG-0723 over an eight month period starting from May, 2013. The broad magnification across the image is from "A", the double horned peaks are from "B", and the two spikes shown in the insets are from "Bb". The grey and blue points were observed by OGLE and the green points were observed by Wise Observatory as part of microFUN. Image credit: Udalski et al. 2015.

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