J1407b is an astronomical object proposed to explain a series of eclipse-like dimming events observed in the star V1400 Centauri during 2007. These dimming events were recorded by automated telescopes that year but were not identified until 2010, when Mark Pecaut and Eric Mamajek analyzed the data. J1407b was initially thought to be a planet with a massive ring system orbiting V1400 Centauri, but no new eclipses have been observed, nor do historical photographic plates show evidence for eclipses in the last century, so this explanation is unlikely. Astronomers have alternatively proposed that J1407b might have been a free-floating substellar object, possibly a sub-brown dwarf, with a protoplanetary disk that coincidentally eclipsed V1400 Centauri, the most likely hypothesis to date.
J1407b's hypothetical disk spans a radius of about and consists of many rings and gaps which may indicate the formation of moons in orbit around the object. Follow-up observations have attempted to detect J1407b via high-resolution imaging, but found no object. The non-detection of J1407b places constraints on the nature of its disk, ruling out one dominated by grains with sizes over one millimeter.
2007 dimming and discovery
During 7 April to 4 June 2007, telescopes of the Super Wide Angle Search for Planets (SuperWASP) and All Sky Automated Survey (ASAS) projects automatically recorded V1400 Centauri undergoing a series of significant dimming events for 56 days. The pattern of these dimming events was complex yet nearly symmetrical, suggesting they were caused by an opaque, disk-like structure eclipsing the star. The light curve of V1400 Centauri during 2007 showed at least five major dimming events, including one long and very deep central eclipse bracketed by two pairs of shorter eclipses symmetrically occurring 12 days and 26 days before and after the middle of the deep eclipse. The deep eclipse lasted about 14 days and blocked out at least 95% of V1400 Centauri's light, causing it to dim by at least 3.3 magnitudes. The short eclipses before and after the deep eclipse blocked out at least 60% of the star's light, causing it to dim by at least 1 magnitude.
The event was not noticed until 3 December 2010, when Mark J. Pecaut, a graduate student of Eric E. Mamajek at the University of Rochester, discovered V1400 Centauri's 2007 dimming while investigating SuperWASP's public light curve database. Pecaut and Mamajek were originally intending to use the SuperWASP data to check for brightness variability in candidate low-mass stars of the Scorpius–Centaurus association, which they had been studying since 2009. Mamajek, Pecaut, and collaborators presented their discovery of V1400 Centauri's dimming in January 2012 at the 219th American Astronomical Society conference in Austin, Texas, and then formally published their results in The Astronomical Journal in March 2012.
Name The object was first dubbed "J1407b" in a paper published by Tim van Werkhoven, Matthew Kenworthy, and Eric Mamajek in 2014, which assumed the object was orbiting V1400 Centauri as an exoplanet. The name J1407b follows the exoplanet naming convention by adding the letter "b" after the host star's name. At the time of J1407b's discovery, V1400 Centauri was known as "J1407", which is the shortened form of the star's full SuperWASP catalogue designation 1SWASP J140747.93–394542.6. This designation shows the star's location in the sky in equatorial coordinates.
Disk and potential exomoons
The long duration and symmetrical nature of V1400 Centauri's dimming in 2007 led astronomers to suspect that it was caused by an object with a massive disk eclipsing the star, which became known as J1407b. This idea was first proposed by Mamajek's team in their 2012 paper describing their discovery of J1407b. The disk of J1407b may be considered a circumplanetary disk or a large ring system. J1407b has not been detected in radio waves, which indicates that its disk is not thick and is likely composed of particles smaller than 1 millimeter.
The rate of V1400 Centauri's 2007 dimming suggests that J1407b and its disk were moving at a transverse velocity of relative to the star, which indicates that J1407b's disk has a radius of 0.6 astronomical units (). This makes J1407b's disk roughly 200 times larger than that of Saturn's E Ring and between the orbital radii of Mercury () and Venus (). J1407b's circumplanetary disk or ring system has been frequently compared to that of Saturn's, which has led popular media outlets to dub it as a "Super Saturn" or a "Saturn on steroids".
The radius of the disk extends far beyond J1407b's Roche limit at , which allows exomoons (or exoplanets if J1407b is a brown dwarf) to form within the disk, as evidenced by gaps seen in J1407b's disk. J1407b's disk is tilted by 13° relative to the plane of J1407b's path and Earth's line of sight, which explains its nearly-symmetrical eclipse light curve and differing time durations between eclipse ingress and egress. Variations in V1400 Centauri's dimming rate during the eclipses suggest that J1407b's disk has a height-to-radius ratio of approximately 0.0015, which corresponds to a vertical disk thickness of .
The varying depths of J1407b's eclipses indicate that its disk consists of various concentric rings and gaps of different opacities. A 2015 analysis of J1407b's eclipse light curve by Kenworthy and Mamajek found that J1407b's disk comprises at least 37 distinct rings with radii ranging from . The innermost ring of J1407b's disk extends out to a radius of and is the most opaque region of the disk. Assuming the rings have a mass density proportional to their opacity, the total mass of J1407b's disk is roughly 100 lunar masses (1.23 Earth masses).
J1407b's disk has a -wide gap between radii , which is believed to have been created by a nearly-Earth-sized () exomoon orbiting within that gap and clearing out material, in a similar fashion to the shepherd moons of Saturn's rings. Another smaller, -wide gap in J1407b's disk between radii is also believed to have been created by an exomoon orbiting inside that gap. Other possible mechanisms for creating J1407b's disk gaps, such as orbital resonances between multiple exomoons, are deemed unlikely because they cannot produce other observed features of J1407b's disk. Altogether, the presence of rings and gaps outside J1407b's Roche limit combined with evidence of possible exomoons suggests that J1407b's disk is currently in the process of accreting into more exomoons, and will eventually become a satellite system (or a planetary system if J1407b is a brown dwarf) in less than a few billion years.
Bound companion hypothesis In their 2012 paper describing the discovery of J1407b, Mamajek's team proposed that J1407b could be a ringed substellar object or exoplanet orbiting V1400 Centauri, because the star is young enough that a dense disk of dust could theoretically exist around it and any massive companions orbiting it. Although it is now considered less likely, the hypothesis of J1407b being a ringed substellar companion or exoplanet of V1400 Centauri was popularized by Mamajek and Kenworthy in 2015, when they announced their research on J1407b in a press release published by their respective universities.
Proposed orbit
Following the assumption that J1407b is orbiting V1400 Centauri, its apparent transverse speed of during the 2007 eclipse should be the same as its orbital speed around the star. This orbital speed allows for a range of possible orbital periods depending on J1407b's orbital eccentricity: if J1407b has a circular orbit with a constant orbital speed, then it would have an orbital period around 200 days, whereas if J1407b's orbit is more elliptical with a varying orbital speed, then it could have longer orbital periods of up to several years.
Continuous observations of V1400 Centauri's brightness after 2007 did not show any signs of eclipse-like dimming, which rules out the possibility of near-circular and short-period orbits for J1407b. A more extensive analysis of V1400 Centauri's brightness in archival observations from 1890 to 1990 similarly found no signs of eclipses, ruling out 90% of possible orbital periods between 10 and 20 years for J1407b. Although these observations do not rule out the possibility of orbital periods longer than 25 years, such long orbital periods are considered unlikely because they require an extremely eccentric orbit for J1407b, which would destabilize J1407b's disk. Overall, these constraints suggest a probable orbital period range of 14–17 years (with the most probable orbital periods around 16.5–17 years) if J1407b orbits V1400 Centauri. For this orbital period range, J1407b's orbital eccentricity must be between 0.72 and 0.78.
Problems with the hypothesis A 2016 study by Steven Rieder and Matthew Kenworthy investigated the orbital dynamics of J1407b's postulated eccentric orbit and found that the disk of J1407b either fills a large fraction of or extends beyond J1407b's Hill radius (extent of J1407b's gravitational influence against V1400 Centauri) regardless of its mass, which meant that J1407b's disk could be easily destabilized by V1400 Centauri's gravitational influence whenever it makes its closest approach to the star at periapsis. To remedy the issue with J1407b's disk stability in an eccentric orbit, Rieder and Kenworthy proposed that J1407b must be a brown dwarf of at least 20 Jupiter masses () and its disk must orbit J1407b in retrograde motion, opposite to the direction J1407b orbits its host star. A retrograde-orbiting disk would survive longer against V1400 Centauri's gravitational influence, although it would still slowly shrink over timescales of 10,000 years. Rieder and Kenworthy suggested that the lifetime of a retrograde-orbiting disk could be prolonged by dust-producing processes such as tidal disruption of comets around J1407b. A 2018 study found that a lower mass of 5–20 Jupiter masses would be marginally consistent with the observations.
Despite the better stability of a retrograde-orbiting disk, it could not explain why J1407b's disk is flat and tilted relative to its postulated orbit around V1400 Centauri. The star's gravitational influence is strong enough to realign J1407b's disk to its orbital plane instead of J1407b's equator, which would result in significant warping of J1407b's disk. In addition to this issue, the origin of a retrograde-orbiting disk together with J1407b's postulated eccentric orbit could not be easily explained by current theories for planetary formation. If J1407b is a companion that formed in orbit around V1400 Centauri, then its disk is expected to be prograde, orbiting J1407b in the same direction as its orbit around the star.