145452 Ritona

Discovery

Ritona was discovered by astronomers Andrew Becker, Andrew Puckett and Jeremy Kubica on 10 September 2005, during observations for the Sloan Digital Sky Survey.^[1][15] The discovery observations were made using the 2.5-meter telescope at Apache Point Observatory in Sunspot, New Mexico.^[15] The discoverers further observed Ritona until November 2005 and found the object in precovery observations from dates as early as June 2001.^[15] The discovery of Ritona was announced by the Minor Planet Center on 23 July 2006.^[15] Since then, Ritona has been found in even earlier precovery observations dating back to June 1954.^[1]

The 2.5-meter telescope at Apache Point Observatory in Sunspot that was used to discover Ritona back in 2005

Ritona photographed in color by Apache Point Observatory's Sloan Digital Sky Survey on 18 and 19 October 2003, showing the object's slow movement across the sky

Name and number

The object is named after Ritona, the Celtic goddess of river fords.^{[16]: 24 } The naming of this object was announced by the International Astronomical Union's Working Group for Small Body Nomenclature on 21 July 2025.^{[16]: 24 } Before Ritona was officially named, it was known by its provisional designation 2005 RN₄₃,^[1] which indicates the year and half-month of the object's discovery date.^[17] Ritona's minor planet catalog number of 145452 was given by the Minor Planet Center on 5 December 2006.

Orbital characteristics

Ritona is a trans-Neptunian object orbiting the Sun at a semi-major axis or average distance of 41.5 astronomical units (AU).^[5][b] It follows a moderately inclined and nearly circular orbit,^{[7]: 2537 } with a low eccentricity of 0.02 and inclination of 19.3° with respect to the ecliptic.^[5] In its 267-year-long orbit, Ritona comes as close as 40.6 AU from the Sun at perihelion and as far as 42.5 AU from the Sun at aphelion.^[5] Ritona last passed perihelion in November 1760 and will make its next perihelion passage on 15 April 2029.^[19][6]

Classification

Ritona is located in the classical region of the Kuiper belt 39–48 AU from the Sun, and is thus classified as a classical Kuiper belt object (sometimes known as a "cubewano").^{[3]: 2–3 } The high orbital inclination of Ritona makes it a dynamically "hot" member of the classical Kuiper belt.^{[3]: 3 } The hot classical Kuiper belt objects are believed to have been scattered by Neptune's gravitational influence during the Solar System's early history.^{[20]: 230 }

Size

Ritona has a diameter of 679 km (422 mi) (full range 606 to 734 km or 377 to 456 mi when including uncertainties), according to thermal emission measurements by the infrared Herschel Space Observatory.^[3] Ritona is large enough that some astronomers consider it a possible dwarf planet.^{[21]: 178 [14]: 1 [22]: 397 }

However, based on its size of 606 to 734 km,^[3] it belongs to the proposed class of "mid-sized" TNOs between 400 and 1,000 km (250 and 620 mi) in diameter, which are believed to represent the transition between small, low-density TNOs and large, high-density dwarf planets.^{[23][24]: 1 } Planetary scientists have hypothesized that mid-sized TNOs should have highly porous and unheated interiors, because TNOs in this size range (namely Uni and Gǃkúnǁʼhòmdímà) have been found to have low densities around 1 g/cm³.^[23]

Surface

In visible light, the surface of Ritona appears dark and reddish in color,^[10][11] with a geometric albedo of about 0.11.^{[3]: 10 } Spectroscopic observations by the James Webb Space Telescope (JWST) in 2022 have shown that Ritona's surface is composed of water ice, carbon dioxide (CO
₂) ice, carbon monoxide (CO) ice, and various organic compounds (tholins).^{[25]: 2 } Ritona shares this composition with the CO
₂-type (aka "double-dip") trans-Neptunian objects, which are commonly found on dynamically excited orbits such as those in the hot classical Kuiper belt (where Ritona resides).^{[9]: 234, 236 [c]}

Analysis of JWST's spectroscopic observations has shown that Ritona's surface is more abundant in CO
₂ ice than water ice, which suggests that Ritona's surface is covered with a thin (a few micrometres thick) layer of fine, micron-sized CO
₂ ice particles.^{[25]: 1–2 } CO ice is also abundant in Ritona's surface, contrary to theoretical predictions that CO should sublimate and escape from Ritona's surface at its temperature and distance from the Sun.^{[25]: 1 } Planetary scientists Michael E. Brown and Wesley C. Fraser have hypothesized that the Sun's ultraviolet light produces CO in Ritona's surface by irradiating and breaking down CO
₂ molecules, and leaves the CO trapped within the surrounding CO
₂ ice.^{[25]: 1, 5 } A similar scenario has been hypothesized for (84522) 2002 TC302, another CO
₂-rich Kuiper belt object observed by JWST.^[25]

Rotation

As of 2018^[update], observations of Ritona's brightness over time indicate it has a rotation period of either 6.946 or 13.892 hours, depending on whether the object's brightness variability is caused by surface albedo variations or an elongated shape.^{[a][7]: 2537, 2542 } Studies from 2010 to 2018 have consistently shown that Ritona exhibits very little brightness variation (less than 0.06 magnitudes), which makes it difficult to accurately determine its rotation period.^{[7]: 2539 } The small brightness variations of Ritona can be explained if it has a spheroidal shape with small albedo variations across its surface.^{[21]: 177–178 }