Kojima-1Lb
A microlensing Neptune around the second-brightest host star.
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Kojima-1Lb (TCP J050742+244755 b) is a microlensing exoplanet discovered orbiting the red dwarf star Kojima-1L. At the time of its discovery it was the planet around the brightest microlensing host star and consequently around a nearby star, unlike most microlensing planets which are found around distant and inaccessible host stars. It is a mildly cold Neptune with a mass of about 20 Earth masses.
Lore & Background
The microlensing event caused by Kojima-1L moving in front of a background star was first observed by amateur astronomer Tadashi Kojima from Gunma prefecture in Japan on 2 and 25 October 2017 using a Canon EOS 6D with a 135 mm f3.2 lens. ASASSN confirmed it as a microlensing event but described it as a single-lens event. Nucita et al. used photometry by AAVSO and the R.P. Feynman Observatory to first establish that the event was a binary lens with a hint to a new planetary system, and later announced the discovery of the planet. The lensed background star is a single late F-type main-sequence star with a temperature of about 6,400 K and is about 800 parsecs from Earth.
The planet has a projected separation of 0.8 or 0.9 astronomical units from its host star, translating into a semi-major axis of about 1.1 astronomical units, which was inside and near the ice line at the younger age of the system. The planet might have first formed while the snow line was at a distance larger than the orbit; as the snow line decreased it might have crossed the orbit at around 2.2 Myrs after the system formed. During planet formation the planet might have experienced a gas-rich but ice-poor environment before becoming more ice-rich. It is difficult to form planets as massive as Kojima-1Lb if the ice-rich period was during a gas-poor period; it is more likely that some gas remained in the ice-rich period, allowing the planet to grow fast by accreting solid material and then the remaining gas.
The Amateur Who Found a World
Tadashi Kojima, an amateur astronomer based in Gunma prefecture, Japan, made the initial detection of what would become one of the most remarkable microlensing planets known. Using a Canon EOS 6D paired with a 135 mm f/3.2 lens, he recorded the microlensing event on 2 and 25 October 2017 and reported it on the CBAT platform under the designation TCP J05074264+2447555. The ASASSN survey later confirmed the event as a microlensing signal, though initially classified it as a single-lens phenomenon.
It was not until Nucita and colleagues in 2017, drawing on photometric data from the AAVSO and the R.P. Feynman Observatory, that the event was reinterpreted as a binary lens with a tantalizing hint of a planetary companion. The following year, the same group formally announced the discovery of the planet. In keeping with microlensing naming conventions, the planet bears the name of the event's discoverer rather than the team that identified the planetary signal; the discovery group themselves had affectionately dubbed the event "Feynman-01" after the observatory that contributed the decisive data.
A Rare Bright Red Dwarf
At roughly 0.5 solar masses, Kojima-1L is a red dwarf whose K-band magnitude of 13.7 places it as the second-brightest known microlensing host star, trailing only Gaia22dkvL (V≈14) as of late 2023. This brightness is extraordinary in the microlensing community, where host stars are typically faint, distant, and practically unreachable by conventional follow-up. Kojima-1L sits about 429±21 parsecs from the Solar System and exhibits a proper motion of 25.55±0.36 milliarcseconds per year, indicating a relatively nearby and dynamically active star.
Crucially, it lies outside the crowded central bulge of the Milky Way. Together with Gaia22dkvL, it forms a small but growing collection of microlensing planets found in less-congested galactic regions, offering a valuable contrast to the vast majority of microlensing detections that cluster in the dense stellar fields of the bulge. In SIMBAD, the star is catalogued simply as Kojima-1, a quiet nod to the amateur who first noticed its gravitational dance with a background star.
A Cold Neptune Born at the Ice Line
Kojima-1Lb is a mildly cold Neptune with a mass of approximately 20 Earth masses, orbiting its red dwarf at a semi-major axis of roughly 1.1 astronomical units. Its projected separation from the host star is estimated at 0.8 to 0.9 AU. What makes this orbit particularly intriguing is its relationship to the system's ice line. At the younger age of the system, the planet's orbit sat inside or very near the snow line.
Models suggest the planet may have initially formed while the snow line lay at a greater distance, and that the retreating snow line crossed the planet's orbit around 2.2 million years after the system's birth. Because circumstellar disks around low-mass stars persist for only a few tens of millions of years, the planet likely experienced a gas-rich but ice-poor phase before transitioning into a colder, ice-rich environment. For a body of this mass to assemble, some gas must have remained available during that ice-rich window, allowing Kojima-1Lb to accrete solid material rapidly and then sweep up the residual gas.
A Window for Future Characterization
Unlike most microlensing planets, which vanish behind the glare of their host stars or are simply too distant for direct study, Kojima-1Lb offers a rare path toward independent characterization. The lensed background star—a single late F-type main-sequence star with a temperature near 6,400 K and a radius of 1.49±0.25 solar radii, located roughly 800 parsecs from Earth—sits close enough on the sky that current adaptive-optics instruments were predicted to resolve it separately from the lens star as early as 2021.
Once separated, a spectrum of Kojima-1L itself could be obtained, providing a direct measurement of its properties independent of the microlensing model. Even more ambitiously, the VLT's ESPRESSO spectrograph might detect the planet's 1.3-year orbital period through the radial-velocity method, while a follow-up campaign with Subaru's IRD instrument could potentially reveal additional inner or more massive planets in the system. For a field where most discoveries remain statistical, Kojima-1Lb stands out as a system that may yet yield a full physical portrait.
Reader's Guide
Kojima-1Lb is notable as the planet around the brightest microlensing host star at the time of its discovery, and consequently around a nearby star, contrasting with most microlensing planets which are found around distant and inaccessible hosts. The host star Kojima-1L is the second-brightest microlensing host star with Ks=13.7 mag, after Gaia22dkvL. Together with Gaia22dkvL, Kojima-1L is located outside the bulge of the Milky Way, representing a small growing sample of microlensing planets discovered in less-crowded regions. The discovery was initiated by an amateur astronomer, Tadashi Kojima, and the planetary feature was identified by the Osservatorio Astrofisico R.P. Feynman, which nicknamed the event Feynman-01.
The planet's formation history suggests it may have formed near the ice line, with a period of ice-poor then ice-rich accretion, requiring some gas to remain during the ice-rich phase to reach its mass. Future observations may resolve the host and background star, enabling independent characterization of the host star via spectroscopy, and radial velocity instruments like VLT/Espresso might detect the planet's 1.3-year orbital period. Follow-up with instruments like Subaru/IRD could detect additional inner or massive planets in the system.
Frequently Asked Questions
How far is Kojima-1Lb from its host star?
The projected star–planet separation is estimated at about 0.8 to 0.9 astronomical units, with a semi-major axis near 1.1 AU. This places the planet in a mildly cold orbital zone around its red dwarf.
What are the key physical properties of Kojima-1Lb and its host star?
Kojima-1Lb is a mildly cold Neptune at approximately 20 Earth masses, and its host, Kojima-1L, is a red dwarf with a mass of about 0.5 solar masses. Together they form one of the brighter and more nearby microlensing planet systems identified to date.
More in Exoplanets Found by Imaging and Microlensing
Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Kojima-1Lb (CC BY-SA 4.0).
- Word definitions: the Codexery glossary, each quoted from its Wikipedia article.
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