The G2 cloud in our Galaxy's core has survived an encounter with the central black hole and failed to trigger a major flare-up in the black hole's activity. A promising theory endeavours to explain the cloud's nature.
The centre of the Milky Way hosts a supermassive black hole (SMBH) weighing approximately 3.6 million solar masses. In 2012 a faint, dusty object of cloudy appearance was discovered1, accelerating towards the SMBH on an eccentric orbit with a predicted closest approach (periapse) of only about 200 times the mean Earth–Sun separation (Fig. 1). During its encounter with the SMBH, the object, known as G2, avoided complete tidal disruption by the black hole's gravity, implying that it is gravitationally bound (held together by gravity). But what is the nature of G2? Various early searches failed to find an ordinary or an evolved giant star lurking in the cloud. Writing in the Astrophysical Journal, Mapelli and Ripamonti2 propose that the central object might be a planetary embryo that was dynamically ejected from its parent system and is nearly as large as the distance between Earth and the Sun.
The central few light years of the Milky Way encompass a dense stellar field containing more than 10 million low-mass stars (Fig. 2); a comparable volume near the Sun typically only contains one star. Among these dim, ancient stars, there is a swarm of luminous massive stars formed in the past 10 million years3, surrounded by a ring of molecular gas containing the equivalent mass of tens of thousands of Suns. The young hot stars emit intense ultraviolet light that ionizes a region of hot plasma, which is heated by shock waves driven into it by powerful stellar winds. Right in the middle of this region lurks the SMBH known as Sagittarius A* (ref. 4).
The discovery of the G2 cloud's headlong plunge towards the central black hole caused much excitement among astronomers5. Models of G2's orbit predicted that the cloud would pass close to the black hole sometime during the spring of 2014. Even by late 2013, there was tentative evidence that parts of a tidal gas stream escaping from G2 had passed behind the SMBH and were emerging on the other side. Would the black hole swallow enough gas to ignite a burst of emissions activity? Would our Galaxy's centre suddenly brighten across the spectrum from X-rays to radio wavelengths and briefly resemble the powerful active galactic nuclei (quasars) associated with SMBHs in some galaxies? Dozens of papers emerged with predictions. Numerical models6 of clouds on plunging orbits predicted that a considerable amount of gas would spiral into the clutches of Sagittarius A*, igniting fireworks.
A year after the plunge, we can conclusively state that there was no major mass-accretion event onto the SMBH. Periapse of G2 occurred in May 2014 and there was no subsequent flare-up of Sagittarius A*, which continued its dim flickering in X-ray and near-infrared wavelengths, and remained completely oblivious to the object zipping by at thousands of kilometres per second.
What created the gas and dust seen as G2? An obvious candidate was the swollen atmosphere of a red giant or supergiant star. But such stars are luminous, and no such star could be detected associated with G2. It was proposed7,8 that G2 was a shock front caused by the interaction between a wind from a low-mass star embedded in the cloud and the dense plasma found in the vicinity of the Galactic Centre. It might also have been a protoplanetary disk; or a star ripped apart by tidal forces following previous close encounters with the black hole; or even a merger of two stars.
Armed with hindsight and more data, a few conclusions can now be drawn. The fact that G2 survived the plunge towards the SMBH and emerged intact9,10 means that it must be associated with a compact, gravitationally bound object. Its dim infrared emission implies a luminosity less than 30 times that of the Sun. And because it is best detected in the wavelength region between 2 and 5 micrometres, its emission is dominated by that of dust at a temperature of 400 to 600 kelvin. A plausible explanation has been that the G2 cloud contains a young, low-mass star surrounded by a dusty disk that is losing mass, or that it has a wind similar to those found escaping from young low- to intermediate-mass stars near the Sun7,10,11.
Young stars that exhibit excess infrared emission, which is an indicator of extreme youth, have been found in the Galactic Centre before12. Circumstellar disks irradiated by ultraviolet radiation, such as those found in the nearby Orion Nebula13, or those that are probably associated with young stars in the Galactic Centre, lose mass in a process called photo-ablation. Protoplanets, planetary embryos or planets in dense star clusters can be gravitationally stripped away from their parent stars following encounters with other stars to become free-floating, sub-stellar-mass objects14.
The occasional encounters of young, low-mass objects with the Milky Way's SMBH may cause them6 to brighten dramatically as tidal forces pull them apart. Therefore, events such as the G2 encounter might provide a new way of detecting such otherwise invisible, low-mass objects. Mapelli and Ripamonti model the effects of tidal disruption and photo-ablation on G2 and demonstrate that young planetary embryos with a mass of about 10 to 100 times that of Jupiter, or protoplanets with diameters comparable to the Earth–Sun separation, may become detectable as objects resembling G2. This would occur as their outer layers are stripped away by the SMBH, heat up, and become ionized by the ambient radiation field.
If further analyses show that G2 does not contain a star, the authors' proposal that G2 is a planetary embryo bound together by its relatively weak self-gravity could be a plausible explanation of this mysterious object. Continued monitoring of the Galactic Centre by facilities such as the Atacama Large Millimeter/submillimeter Array in Chile and the Jansky Very Large Array in New Mexico, could complement infrared observations and tell us about the formation15 of low-mass stars, and even planetary systems, in extreme environments such as our Galaxy's centre.