MOA-2009-BLG-387Lb

Lore & Background

MOA-2009-BLG-387Lb is a gas giant with an estimated mass 2.6 times that of Jupiter and a radius of 1.75 times that of Jupiter. Its estimated mean distance from its host star is 1.8 AU, and it has an orbital period of approximately 1970 days. However, the confidence intervals for its mass and mean distance are very large, indicating substantial uncertainty, largely because the exact parameters of the host star are not known.

The host star, MOA-2009-BLG-387L, is an M-type dwarf star with a mass approximately 0.19 times that of the Sun. It is located at an estimated 5700 parsecs (18,591 light years) from Earth. The star is named for the Microlensing Observations in Astrophysics group, which observed it as a gravitational microlensing event in 2009.

The microlensing event MOA-2009-BLG-387 was detected on July 24, 2009. Over the next few days, two caustics were logged by the South African Astronomical Observatory, the Perth Observatory, and the Canopus Hill Observatory in Tasmania, with a separation of about seven days. On June 7, 2010, after the event had subsided, the science teams used the NACO adaptive optics facility at the Very Large Telescope in Chile to determine the star's actual apparent magnitude. A discrepancy was found that may have resulted from either error or a planetary body, and follow-up observations led to the planet's confirmation.

The Microlensing Detection and Confirmation

The story of MOA-2009-BLG-387Lb began not with a telescope pointed at a planet, but with a fleeting gravitational alignment. On July 24, 2009, the Microlensing Observations in Astrophysics collaboration flagged a microlensing event, a phenomenon in which the gravity of a foreground star bends and magnifies the light of a background star, producing distorted yet brighter images. Over the following days, observers at the South African Astronomical Observatory, the Perth Observatory, and the Canopus Hill Observatory in Tasmania recorded two caustic crossings separated by roughly seven days.

The real breakthrough came more than a year later, on June 7, 2010, when the science team turned the NACO adaptive optics system on the Very Large Telescope in Chile toward the lensing star. By measuring its true apparent magnitude and comparing it to the value recorded during the event, they uncovered a discrepancy that could not be dismissed as simple error. Follow-up observations ultimately confirmed the presence of a planetary companion, and the result was formally published in Astronomy and Astrophysics on February 21, 2011.

Physical Profile and Orbital Parameters

MOA-2009-BLG-387Lb is classified as a gas giant, with an estimated mass of roughly 2.6 times that of Jupiter and a radius approximately 1.75 times Jupiter's. It traces an orbit at a mean distance of about 1.8 astronomical units from its host star, which is roughly 80 percent larger than Earth's orbital radius around the Sun. One complete revolution takes approximately 1,970 days. Despite these seemingly concrete figures, the scientific community must treat them with considerable caution.

The confidence intervals surrounding both the mass and the orbital distance are exceptionally wide, a direct consequence of the host star's own parameters remaining poorly constrained. In other words, the planet's true size and orbital geometry could shift substantially once the red dwarf's mass is better pinned down. What can be stated with greater confidence is the mass ratio between the planet and its star, which the microlensing data constrains tightly even as the absolute masses remain uncertain.

The Red Dwarf and Its Distant Neighborhood

The sole known companion to MOA-2009-BLG-387L is a red dwarf of M spectral type, with a mass estimated at approximately 0.19 solar masses. This makes it a dim, low-mass star, far less luminous than our own Sun. The system resides at an estimated distance of 5,700 parsecs, or roughly 18,591 light-years, from Earth, placing it deep within the galactic disk. The star bears the name of the Microlensing Observations in Astrophysics group, the collaboration that first identified it during a gravitational microlensing event in 2009.

That initial detection was not made with the intention of finding a planet; rather, MOA's mandate is to catalogue brief, chance alignments between foreground and background stars. It was only after the team combed through the archived light curves that the planetary signature emerged. The star's imprecisely known mass is the single largest source of uncertainty in every derived property of the planet, from its absolute mass to its orbital radius.

Milestone in Microlensing Planet Detection

When the discovery was formally announced on February 21, 2011, MOA-2009-BLG-387Lb became the eleventh exoplanet ever identified through the technique of gravitational microlensing. The result appeared in the journal Astronomy and Astrophysics, published under the auspices of the European Southern Observatory. The milestone is notable not only for its place in the growing microlensing catalogue but also for what it reveals about the method's strengths and limitations.

The mass ratio between planet and host star is among the best-constrained quantities in the dataset, yet translating that ratio into an absolute planetary mass requires knowing the host star's mass, which in this case spans the full range of red dwarf masses. This fundamental ambiguity means that MOA-2009-BLG-387Lb remains a case study in how microlensing can reveal a world's existence with confidence while leaving its precise physical identity shrouded in wide error bars. It stands as both a triumph of the technique and a reminder of the observational challenges that persist.

Reader's Guide

MOA-2009-BLG-387Lb is notable as the eleventh exoplanet discovered using gravitational microlensing, a technique that exploits chance alignments of stars to detect planets. The discovery was published on February 21, 2011 in the journal Astronomy and Astrophysics by the European Southern Observatory. The planet's mass and orbit are poorly constrained because the host star's mass is known only within a large confidence interval spanning the mass of all red dwarf stars.

The ratio between the planet's mass and its host star's mass is well-constrained, but the uncertainty in the star's mass propagates into large uncertainties for the planet's properties. This highlights a key challenge of microlensing: while the technique can reveal planets, characterizing them often depends on knowledge of the host star, which can be difficult to obtain for distant, faint stars. The planet remains the only one known in the orbit of MOA-2009-BLG-387L.