Thursday, January 22, 2015

Lone Planet in Interstellar Space

There appears to be increasing evidence for the existence of isolated planetary-mass objects. These objects are either unbound (i.e. drifting alone in interstellar space) or orbit host stars in widely separated, distant orbits. Recently, Luhman (2014) reported the detection of a nearby brown dwarf with 3 to 10 times the mass of Jupiter and an estimated temperature of roughly 250K (-23°C). Such an object could have formed in-situ as stars do or it could be a planet that got ejected from its natal planetary system.

Using a novel technique, Freeman et al. (2014) reported the detection of an isolated planetary-mass object after re-analysis of observational data from the gravitational microlensing event MOA-2011-BLG-274. Gravitational microlensing occurs when the gravitational field of a foreground object (lens) bends and magnifies light from a background object (source). For this to happen, the observer, foreground object and background object must be in near perfect alignment.

MOA-2011-BLG-274 was previously reported by Choi et al. (2012). One indication the lens could be a planetary-mass object is the short duration of the gravitational microlensing event. At any given time, the observed amount of magnification and the time of peak magnification vary slightly from point to point on the Earth’s surface. This is known as the terrestrial parallax effect and it is particular pronounced for short duration gravitational microlensing events.

Because MOA-2011-BLG-274 was monitored from a number of separate locations on Earth, the terrestrial parallax effect could be measured by Freeman et al. (2014), making it possible to pin down the distance and hence the mass of the lens, identified in this case as MOA-2011-BLG-274L with the suffix “L”. The best fit to the data shows MOA-2011-BLG-274L is 0.8 ± 0.3 times the mass of Jupiter and located 2.6 ± 0.8 thousand light years away.

Also, the data excludes the presence of a host star out to ~40 AU (i.e. 40 times the distance of Earth from the Sun). MOA-2011-BLG-274L is either an isolated planetary-mass object drifting in the depths of interstellar space or a planet in a far-flung orbit around a host star, sufficiently far out that the host star has no effect on the gravitational microlensing light curve.

- Luhman (2014), “Discovery of a ~250 K Brown Dwarf at 2 pc from the Sun”, ApJ 786 L18
- Freeman et al. (2014), “Can the masses of isolated planetary-mass gravitational lenses be measured by terrestrial parallax?”, arXiv:1412.1546 [astro-ph.EP]
- Choi et al. (2012), “Characterizing lenses and lensed stars of high-magnification single-lens gravitational microlensing events with lenses passes over source stars”, arXiv:1111.4032 [astro-ph.SR]