MOA-2009-BLG-387L is a

red dwarf A red dwarf is the smallest kind of star on the main sequence. Red dwarfs are by far the most common type of fusing star in the Milky Way, at least in the neighborhood of the Sun. However, due to their low luminosity, individual red dwarfs are ...

in the Sagittarius constellation that is host to the planet

MOA-2009-BLG-387Lb MOA-2009-BLG-387Lb is an exoplanet in the orbit of the red dwarf MOA-2009-BLG-387L. Its discovery was announced on February 21, 2011, making it the eleventh planet discovered using gravitational microlensing. The planet is thought to be over twi ...

. The star is estimated to be nearly 20,000 light years away and approximately one fifth the mass of the Sun, although large confidence intervals exist, reflecting the uncertainties in both the mass and distance. The star drew the attention of astronomers when it became the

lens A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'') ...

gravitational microlensing Gravitational microlensing is an astronomical phenomenon caused by the gravitational lens effect. It can be used to detect objects that range from the mass of a planet to the mass of a star, regardless of the light they emit. Typically, astronom ...

event MOA-2009-BLG-387L, in which it eclipsed a background star and created distorted caustics, an envelope of reflected or refracted light rays. Analysis of the caustic events and of follow-up observational data led to the planet's discovery, which was reported in February 2011.

Observational history

Aerial View of the VLTI with Tunnels Superimposed

On July 24, 2009, the

Microlensing Observations in Astrophysics Microlensing Observations in Astrophysics (MOA) is a collaborative project between researchers in New Zealand and Japan, led by Professor Yasushi Muraki of Nagoya University. They use gravitational microlensing, microlensing to observe dark mat ...

collaboration (MOA) detected the star MOA-2009-BLG-387L eclipsing a background star in a microlensing event that was named MOA-2009-BLG-387. In a process called

, the star MOA-2009-BLG-387L became a

that created two distorted

caustic Caustic most commonly refers to: * Causticity, the property of being able to corrode organic tissue ** Sodium hydroxide, sometimes called ''caustic soda'' ** Potassium hydroxide, sometimes called ''caustic potash'' ** Calcium oxide, sometimes cal ...

images. In the case of the microlensing event MOA-2009-BLG-387, these caustics produced a series of small "resonating" diffractions; such resonant-caustic events are valued because they tend to yield more information about an orbiting planet. The first caustic event was detected by the South African Astronomical Observatory (SAAO) on July 24, 2009. An alert was issued, which attracted many to cover the caustic event; as such, the end of the first caustic event was well-documented. The microlensing event's second caustic event was seven days later, an unusually long middle period for planetary microlensing events. An alert brought three different telescopes at SAAO and telescopes at ten different observatories to focus on the event. Follow-up observations on the star MOA-2009-BLG-387L using the NACO imager at the

Very Large Telescope The Very Large Telescope (VLT) is an astronomical facility operated since 1998 by the European Southern Observatory, located on Cerro Paranal in the Atacama Desert of northern Chile. It consists of four individual telescopes, each equipped with ...

array successfully distinguished the star's mass. The collected data from VLT and from observations during the microlensing event was run through a series of models and analyzed. An orbiting planetary body larger than Jupiter (or of a similar size, given uncertainties) was discovered. The discovery of the planet was reported on February 21, 2011 in the journal ''

Astronomy and Astrophysics ''Astronomy & Astrophysics (A&A)'' is a monthly peer-reviewed scientific journal covering theoretical, observational, and instrumental astronomy and astrophysics. It is operated by an editorial team under the supervision of a board of directors re ...

''.

Characteristics

MOA-2009-BLG-387L is an M-type

in the Sagittarius constellation. It is estimated to be 5700

parsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (AU), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, and ...

s (18,591

light year A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distance, astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by t ...

s) away, although uncertainty has led the discovering team to place the confidence intervals at ± 2200 parsecs (7,176 light years); in other words, although the distance of MOA-2009-BLG-387L from Earth is best placed at 5700 parsecs, astronomers can only be 90% certain that it is somewhere between 3,500 and 7,900 parsecs away. Likewise, although MOA-2009-BLG-387L's mass has been estimated at 0.19 times that of the Sun, the confidence intervals remain large (), as uncertainty in the mass of the planet places its true mass between 0.07 and 0.49 times the mass of the Sun. This covers the entirety of the range of masses known in red dwarfs. The ratio between planet MOA-2009-BLG-387Lb's mass to that of its host star has been found with accuracy. However, because the host star's characteristics are not as well-constrained, the planet's characteristics are not well-constrained either. The inability to constrain many of MOA-2009-BLG-387L's characteristics is a consequence of the fact that the star acted as the

in the microlensing event, which compromised the ability to collect most of the star's stellar parameters.

Planetary system

MOA-2009-BLG-387Lb is the only known exoplanet in the orbit of host star MOA-2009-BLG-387L. The planet is estimated to be 2.6 times the

mass of Jupiter The Jupiter mass, also called Jovian mass, is the units of mass, unit of mass equal to the total mass of the planet Jupiter. This value may refer to the mass of the planet alone, or the mass of the entire Jovian system to include the moons of Jup ...

. However, because knowledge of the exact parameters of the planet are tied to the host star's parameters, and the host star's parameters are not well-constrained, uncertainty places MOA-2009-BLG-387Lb's mass between 1.0 and 6.7 times that of Jupiter. The planet is estimated to orbit its host star every 1970 days at a distance of 1.8 AU, some 1.8 times the mean distance between Earth and the Sun. Uncertainty broadens the mean distance to between 1.1 and 2.7 AU.

References

{{DEFAULTSORT:MOA-2009-BLG-387L Planetary systems with one confirmed planet Gravitational lensing M-type main-sequence stars Sagittarius (constellation)