Kepler-1898 b is a small terrestrial exoplanet that would be easy to overlook in the ever-growing catalog of worlds beyond the Solar System. It is less than Earth in size, circles its star in only 4.8 days, and lies more than 1,000 light-years away. Yet the planet is scientifically interesting for a reason that goes beyond its modest dimensions: Kepler-1898 b appears to belong to an old, metal-poor planetary system, providing a glimpse into the kinds of terrestrial worlds that formed much earlier in the history of the Milky Way.
The planet orbits the star Kepler-1898, also known as KIC 3328080 and KOI-3075. The system is located in the constellation Cygnus at a distance of about 361 parsecs, or roughly 1,177 light-years, according to the current NASA Exoplanet Archive data. The host star has a radius of about 0.82 times that of the Sun and an effective temperature of approximately 5,575 kelvin, making it broadly comparable to the Sun in temperature, although its other properties differ significantly.
Kepler-1898 b was identified through the transit method. This technique looks for the tiny, periodic decrease in a star's brightness that occurs when a planet passes across the stellar disk as seen from Earth. In the case of Kepler-1898 b, the signal is exceptionally small: the transit depth is about 68 parts per million in the Kepler data. In other words, the planet blocks only around seven hundredths of one percent of its star's light. Detecting such a subtle signal repeatedly is precisely the kind of measurement for which the Kepler space telescope was designed.
The orbital period of Kepler-1898 b is approximately 4.7656 days. That means the planet completes more than 76 orbits around its star during the time Earth takes to make a single trip around the Sun. Its orbital distance is only about 0.05 astronomical units, roughly one-twentieth of the distance between Earth and the Sun. NASA currently lists an orbital radius of 0.05063 AU, while the Kepler catalog solutions give approximately 0.0522 AU. These small differences arise from different sets of stellar and planetary parameters used in the analyses rather than from any meaningful uncertainty about the planet's extremely close orbit.
In size, Kepler-1898 b is remarkably close to Earth. NASA gives it a radius of about 0.934 times Earth's radius. That makes it approximately 6.6 percent smaller in radius than our planet. Its estimated mass is about 0.76 Earth masses, placing it below Earth's mass as well. The NASA catalog consequently classifies it as a terrestrial planet.
The combination of a roughly Earth-sized radius and a sub-Earth mass is particularly interesting because the radius alone does not tell us what an exoplanet is made of. A planet with a radius close to Earth's could, in principle, have a substantially different internal structure depending on its iron content, rocky mantle, volatile inventory, and thermal history. For Kepler-1898 b, however, the available mass and radius estimates are not sufficiently precise to justify a detailed interior model comparable to what astronomers can construct for some of the best-characterized nearby exoplanets. It is therefore safer to describe it as a small terrestrial world than to claim that its composition is definitively Earth-like.
Its environment is very different from Earth's. Because Kepler-1898 b orbits only about 0.05 AU from its star, it receives vastly more stellar radiation than Earth does. Earlier Kepler-based solutions give an equilibrium temperature of roughly 1,170 K, although equilibrium temperature is a model-dependent quantity rather than a direct measurement of the planet's surface temperature. The high irradiation means that Kepler-1898 b is not considered an Earth-like habitable-zone planet. NASA's catalog classifies it simply as terrestrial rather than suggesting that it has conditions suitable for life.
The short orbital period also has important consequences for the planet's long-term evolution. At such a small star-planet separation, tidal interactions are expected to be much stronger than those experienced by Earth. Depending on the planet's internal structure, orbital history, and tidal dissipation, such interactions could have influenced its rotation and geological evolution. Some theoretical studies have consequently included Kepler-1898 b among small planets whose tidal environment may be relevant when considering possible geological activity, although these calculations should not be interpreted as observations of active volcanism or tectonics on the planet.
Perhaps the most intriguing characteristic of the system is not the planet itself but the apparent age and composition of its host star. One stellar solution associated with the California-Kepler Survey gives Kepler-1898 an estimated age of about 13.18 billion years, with an uncertainty of roughly +0.62 and −0.88 billion years. The same source gives a metallicity of about [Fe/H] = −0.45, meaning that the star contains substantially less iron relative to hydrogen than the Sun. The current NASA Exoplanet Archive also reports a metal-poor star, although the exact stellar parameters vary among the different solutions collected in the archive.
If the older age estimate is correct, Kepler-1898 b would be part of a planetary system that formed when the Milky Way itself was considerably younger. An age of around 13 billion years would place the system among the oldest known planetary systems, although the large uncertainty in the stellar age is important and should not be ignored. Stellar ages are notoriously difficult to determine precisely, especially for individual field stars, and different stellar models and input data can produce substantially different results.
The low metallicity is equally significant. In astronomy, "metallicity" refers to the abundance of elements heavier than hydrogen and helium. These heavier elements are essential ingredients for building rocky planets: iron, silicon, magnesium, oxygen and other elements contribute to planetary cores, mantles and crusts. A metal-poor star therefore formed from material containing fewer heavy elements than the material from which the Sun formed.
The existence of a small rocky planet around such a star illustrates an important point about planet formation. Rocky planets do not require a chemical environment identical to the Sun's to form. The discovery of Kepler-1898 b therefore contributes to the broader evidence that terrestrial planets can emerge in chemically diverse environments and at very different stages of Galactic history.
Kepler-1898 b's history as a discovery is also a useful illustration of how modern exoplanet science increasingly relies on statistical validation and machine learning. The object was originally identified in Kepler observations as KOI-3075.01, a candidate transit signal. It was subsequently included among the planets validated by ExoMiner, a deep-learning system developed to examine Kepler transit signals and their associated diagnostic information. The ExoMiner study reported the validation of 301 new exoplanets and was designed to reproduce aspects of the diagnostic reasoning used by human experts when distinguishing genuine planetary transits from astrophysical false positives.
This distinction matters because detecting a periodic dip in stellar brightness is not, by itself, enough to prove that a planet exists. Eclipsing binary stars, background stars, instrumental effects and other phenomena can produce signals that resemble planetary transits. Kepler-1898 b therefore passed through the broader process of candidate vetting before being incorporated into the confirmed-planet catalog. NASA's current Exoplanet Archive lists it as a confirmed planet and identifies Valizadegan et al. (2022) as the reference associated with its confirmation.
There is another reason to be cautious when interpreting its published parameters. The NASA Exoplanet Archive contains multiple historical solutions for both the star and the planet. For example, the stellar mass estimates range from roughly 0.76 to 0.98 solar masses depending on the source, while the stellar radius has also changed as improved measurements became available. The planetary radius has similarly evolved from earlier estimates with large uncertainties to the more precise value now displayed by NASA. This is normal in exoplanet astronomy: the apparent properties of a planet depend partly on how accurately the properties of its host star are known.
The planet's present catalog values should therefore be regarded as the best currently assembled description rather than immutable numbers. NASA lists a radius of 0.9335 Earth radii, a mass of 0.76 Earth masses, a period of approximately 4.8 days, an orbital radius of 0.05063 AU and zero orbital eccentricity in its catalog presentation. The NASA Exoplanet Archive gives the more precise period as 4.765620232 days in the Valizadegan et al. solution.
Despite its proximity to its star, Kepler-1898 b is not a gas giant or a mini-Neptune. Its radius is firmly in the terrestrial-planet regime, and its estimated mass is less than Earth's. The planet is consequently an example of a class of worlds that is particularly valuable for understanding how common small rocky planets may have been throughout Galactic history. Kepler's enormous survey demonstrated that planets of roughly Earth size are abundant, but individual systems such as Kepler-1898 help astronomers investigate whether the formation of small planets depends strongly on the age and chemical composition of their stellar environments.
There is also a striking contrast between its physical scale and its historical importance. Kepler-1898 b is only slightly smaller than Earth, yet its year lasts less than five Earth days. It receives hundreds of times more stellar energy than Earth according to the earlier Kepler-derived irradiation estimates, and it orbits a star whose chemical composition appears substantially poorer in heavy elements than the Sun's. If the oldest age estimate is approximately correct, the planet may have formed when the Milky Way was still in a very different phase of its evolution.
For now, however, Kepler-1898 b remains much better characterized in terms of its orbit and size than its atmosphere or surface. There is no direct observation establishing an atmosphere, oceans, continents, clouds or volcanic activity. Its small size and faint host star, combined with its distance of more than a thousand light-years, make detailed atmospheric characterization considerably more challenging than it is for the closest and brightest exoplanet systems. The planet's scientific value consequently comes primarily from what its existence tells us about planetary demographics, formation and evolution rather than from the possibility of directly studying its surface.
Kepler-1898 b is therefore not another "Earth 2.0," and describing it that way would obscure what makes it genuinely interesting. It is a hot, compact terrestrial world on an extremely short orbit, apparently associated with an unusually old and metal-poor star. Its discovery also demonstrates how the combination of the Kepler mission, improved stellar measurements and sophisticated statistical validation can transform a faint dip in a distant star's brightness into evidence for a new planetary system. In the expanding census of worlds beyond the Solar System, Kepler-1898 b stands as a small but revealing reminder that rocky planets were forming—and surviving—across the Milky Way long before our own Solar System came into existence.
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