1SWASP J1407 b

In 2007, J1407 was observed to be eclipsed and orbited by at least one major body, 1SWASP J1407b (J1407b), thought to be either a large gas giant planet or a brown dwarf, with an immense ring system. Subsequent observations have not successfully detected J1407b, suggesting that it is on a highly eccentric orbit around the star.

By 2021, it was discovered the parent star is strongly variable, with a 5.4-year long magnetic activity cycle, and no evidence was detected of additional planets or a repeat of the deep eclipses attributed to a transiting ring system.

## Physical properties

### Other designations

V1400 Centauri b, 1SWASP J140747.93-394542.6 b, Mamajek's Object b

## Stellar properties

### Other designations (stellar)

1SWASP J140747.93-394542.6, 1SWASP J1407, J1407, Mamajek's Object

## Variability

V1400 Centauri is the variable star designation of this star, assigned in 2015. It is known to be a young star, not yet on the main sequence, and is classified as a T Tauri variable which shows variability as it rotates due to surface spots. It also underwent a series of deep eclipse-like fadings in 2007 that have been ascribed to a ring system around a sub-stellar companion. These have not been repeated and the existence of a companion is still uncertain, pending a repeat of the eclipse.

The rotation of the V1400 Centauri every 3.2 days causes regular brightness changes with an amplitude of about 0.1 magnitudes. The star is also intrinsically variable, as is common for pre-main-sequence stars, with a slow decrease in brightness being shown over the periods 2001 to 2007 and 2012 to 2020. The 2007 eclipse was up to three magnitudes in depth.

## Suspected substellar companion

The discovery of the J1407 system and its unusual eclipses were first reported by a team led by University of Rochester astronomer Eric Mamajek in 2012. The existence and parameters of the ring system around the substellar companion J1407b were deduced from the observation of a very long and complex eclipse of the parent star lasting 56 days during April and May 2007. The low-mass companion J1407b has been referred to as a "Saturn on steroids" or "Super Saturn" due to its massive system of circumplanetary rings with a radius of approximately 90 million km (0.6 AU). The orbital period of J1407b is estimated to be around a decade (3.5 to 13.8 years or 3825 days), and its most probable mass is approximately 13 to 26 Jupiter masses which would make the companion a brown dwarf, but with considerable uncertainty. The ringed body can be ruled out as being a star with mass of over 80 Jupiter masses at greater than 99% confidence. The ring system has an estimated mass similar to that of the Earth. A major gap in the rings at about 61 million km (0.4 AU) from its center is considered to be indirect evidence of the existence of a shepherd exomoon with mass up to 0.8 Earth masses.

J1407b is the first exoplanet or brown dwarf discovered with a ring system by the transit method. A sequence of occultations (eclipses) of the star occurred over a 56-day period in 2007. The pattern was consistent with that expected for the transit of a large array of multiple rings, indicating the substellar companion dubbed "J1407b". The J1407b ring system has an outer radius of approximately 90 million km (about 640 times the extent of Saturn's rings). Cleared gaps in the rings indicate satellites ("exomoons") have accreted from denser rings. The young age of the stellar system (about 16 million years) and the high mass of the ring system (roughly an Earth mass) are more consistent with it being an early (proto-)exomoon or moons, rather than a long-term stable ring system in an evolved planetary system (such as Saturn's rings).

J1407b has not been observed since its transit in 2007, suggesting that it is on a highly eccentric orbit around the star. Such an orbit could disrupt the ring system of J1407b. Dynamical simulations run by astronomers Steven Rieder and Matthew Kenworthy indicate that in order for J1407b's ring system to be stable, the rings must orbit J1407b in a retrograde motion, opposite to the direction J1407b orbits its host star. This retrograde solution for the ring system of J1407b allows for longer ring lifetimes as well as further constraints to the age of the ring system. The rings may be replenished over timescales as a result of processes that produce additional debris around J1407b, such as the tidal disruption of comets.

Alternatively, J1407b may also not be bound to J1407. The previous explanation involving J1407b orbiting J1407 would have J1407b as a brown dwarf companion with a large, Hill sphere filling ring system. However, issues with the stability of any rings combined with the lack of detection of another eclipse, suggests that J1407b may not be bound to J1407. No other deep eclipses has been found in the data spanning from 1890 to 1990, nor in recent time-series photometry from 2012-2018. A significant proportion of orbital periods for J1407b from 5 to 20 years can be disregarded, therefore if there is an actual orbital period it is likely outside of this range.

From Earth's point of view, the ring system of J1407b would have an angular diameter of about 3.7 milliarcseconds across. For comparison, if Saturn were as many light years away, its rings in full breadth would be 0.006 milliarcseconds across, and the diameter of Pluto from earth varies between 60 and 110 milliarcseconds across.

## See also

* Saturn
* ROXs 42Bb

-Wikipedia