Discovery of the Least Massive Quadruple System

2M0441+2301 AabBab is a quadruple system comprised of two pairs of binary objects with a projected separation of about 1,800 AU between them. The more massive binary system, 2M0441+2301 Aab, consists of a low-mass star (the "Aa" component) and a brown dwarf companion (the "Ab" component) 33 AU away. The less massive binary system, 2M0441+2301 Bab, consists of a brown dwarf (the "Ba" component) and a planetary-mass companion (the "Bb" component) 15 AU away. Mass estimates indicate Aa has 200 times Jupiter's mass, Ab has 35 times Jupiter's mass, Ba has 19 times Jupiter's mass and Bb has 9.8 times Jupiter's mass. 2M0441+2301 AabBab is currently the least massive quadruple system known.

Figure 1: Artist’s impression of a multiple system.

2M0441+2301 AabBab is a relatively young system, estimated to be between 1 and 3 million years old. All four objects show spectroscopic signs of low surface gravity, indicating that they are young and bloated, and still in the process of contracting down. Additionally, Aa and Ab both show weak emissions indicating the ongoing accretion of material. If 1,800 AU is the true separation between Aab and Bab, then both binaries would have completed less than 20 orbits around each other in the last 3 million years. All four objects of this young quadruple system are believed to have formed in the same way stars do.

Figure 2: Comparison of 2M0441+2301 AabBab with theoretical isochrones from two models. From top to bottom, the 5 dotted lines on each chart are the 1 Myr, 5 Myr, 10 Myr, 100 Myr and 1 Gyr isochrones (Myr - million years; Gyr - billion years). Bowler & Hillenbrand (2015).

Reference:
Bowler & Hillenbrand (2015), "Near-Infrared Spectroscopy of 2M0441+2301 AabBab: A Quadruple System Spanning the Stellar to Planetary Mass Regimes", arXiv:1509.01658 [astro-ph.EP]

Red Dwarf Star with a Cold Neptune-Mass Planet

Figure 1: Artist’s impression of a cold Neptune-mass planet.

Combining observations by the Spitzer space telescope and observations by ground-based telescopes, Street et al. (2015) present the discovery of a cold Neptune-mass planet in orbit around a red dwarf star. The planet was detected using a technique known as gravitational microlensing, whereby the gravitational field of a foreground object, in this case, the newfound planet and its host star, acts as a lens, magnifying the light of a more distant background star.

As the planet and its host star crossed the line of sight to the background star, it caused the brightness of the background star to change with time. The change in brightness was observed in the form of a light curve. The gravitational microlensing event is designated as OGLE-2015-BLG-0966, and the planet is identified as OGLE-2015-BLG-0966b, with the lowercase suffix “b” indicating its planetary nature.

By analysing the light curve, the planet is estimated to have 21 ± 2 times the mass of Earth and it orbits a relatively small star with 38 ± 4 percent the Sun’s mass. Both the planet and its host star are estimated to lie 8,000 to 11,000 light years away, in the direction towards the center of the Milky Way galaxy. The projected separation of OGLE-2015-BLG-0966b from its host star is between 2.1 to 2.7 AU, where one AU is the Earth-Sun distance. At such a distance from its host star, OGLE-2015-BLG-0966b receives a relatively low amount of stellar insolation, making it a cold Neptune-mass planet.

Figure 2: Light curve of the gravitational microlensing event OGLE-2015-BLG-0966. Street et al. (2015)

Reference:
Street et al. (2015), “Spitzer Parallax of OGLE-2015-BLG-0966: A Cold Neptune in the Galactic Disk”, arXiv:1508.07027 [astro-ph.EP]