Question: On what planet was a large rectangular outcrop that is composed mainly of sandstone first observed by an English astronomer born 1618?

Context paragraphs:
Gas giant: A gas giant is a giant planet composed mainly of hydrogen and helium.  Jupiter and Saturn are the gas giants of the Solar System.  The term "gas giant" was originally synonymous with "giant planet", but in the 1990s it became known that Uranus and Neptune are really a distinct class of giant planet, being composed mainly of heavier volatile substances (which are referred to as "ices").  For this reason, Uranus and Neptune are now often classified in the separate category of ice giants.

William Samuel Stratford: William Samuel Stratford (22 May 1789 – 29 March 1853) was an English astronomer born in Eltham, Surrey.

Jeremiah Horrocks: Jeremiah Horrocks (1618 – 3 January 1641), sometimes given as Jeremiah Horrox (the Latinised version that he used on the Emmanuel College register and in his Latin manuscripts), was an English astronomer.  He was the first person to demonstrate that the Moon moved around the Earth in an elliptical orbit; and he was the only person to predict the transit of Venus of 1639, an event which he and his friend William Crabtree were the only two people to observe and record.

Greencastle, County Down: Greencastle is a castle in County Down, Northern Ireland.  It is a royal castle built in the 13th century.  While it dates mainly from the 13th century, it had substantial 15th- and 16th-century alterations.  The large rectangular keep with three vaulted chambers at the ground floor is approached across a rock-cut ditch.  It was originally surrounded by outer rectangular walls with four corner towers (ruins of some remain).  It is likely to have been built by Hugh de Lacy, along with Carlingford Castle on the opposite side of Carlingford Lough, to guard the narrow entry channel to the Lough, and the ferry crossing between the two.

Horrocks Block: Horrocks Block ( ) is a large rectangular outcrop that is composed mainly of sandstone, lying on the north side of Venus Glacier, 2 nmi southwest of the Keystone Cliffs, on the east side of Alexander Island, Antarctica.  It was mapped by the Directorate of Overseas Surveys from satellite imagery supplied by the U.S. National Aeronautics and Space Administration in cooperation with the U.S. Geological Survey, and was named by the UK Antarctic Place-Names Committee from association with Venus Glacier after Jeremiah Horrocks, the British astronomer who predicted and first observed a transit of Venus, in 1639.

Ethynyl radical: The ethynyl radical (systematically named λ-ethyne and hydridodicarbon("C"—"C")) is an organic compound with the chemical formula C≡CH (also written [CCH] or C2H ).  It is a simple molecule that does not occur naturally on Earth but is abundant in the interstellar medium.  It was first observed by electron spin resonance isolated in a solid argon matrix at liquid helium temperatures in 1963 by Cochran and coworkers at the Johns Hopkins Applied Physics Laboratory.  It was first observed in the gas phase by Tucker and coworkers in November 1973 toward the Orion Nebula, using the NRAO 11m radio telescope.  It has since been detected in a large variety of interstellar environments, including dense molecular clouds, bok globules, star forming regions, the shells around carbon-rich evolved stars, and even in other galaxies.

Gliese 3470 b: Gliese 3470 b, abbreviated as GJ3470b, is an exoplanet orbiting the star Gliese 3470, both of which are located in the constellation Cancer.  With a mass of just under 14 Earth-masses and a radius approximately 4.3 times that of Earth's, it is likely something akin to a mini-Neptune despite the initially strong belief that the planet was not covered in clouds like the gas giants we are familiar with in our solar system.  The exoplanet’s atmosphere was first observed by researchers Akihiko Fukui, Norio Narita and Kenji Kuroda at the University of Tokyo in 2013, and afterwards, Fukui commented, "Suppose the atmosphere consists of hydrogen and helium, the mass of the atmosphere would be 5-20% of the total mass of the planet.  Comparing that to the fact that the mass of Earth's atmosphere is about one ten-thousandth of a percent (0.0001%) of the total mass of the Earth, this planet has a considerably thick atmosphere."  Recently in 2015, a team using the Las Cumbres Observatory Global Telescope (LCOGT) network reported their detection of Rayleigh scattering in the atmosphere of this planet.  In the Las Cumbres researchers' paper published in The Astrophysical Journal, they conclude that the most plausible explanation for the scattering effect to be an atmosphere made predominantly of hydrogen and helium, causing the exoplanet to be veiled by dense clouds and hazes.  It is thought that the planet would appear blue to the human eye due to this scattering.  As of now, GJ3470b is the only exoplanet orbiting the Gliese 3470 star that has been observed and analyzed by astronomers and astrophysicists.

Triton (moon): Triton is the largest natural satellite of the planet Neptune, and the first Neptunian moon to be discovered.  It was discovered on October 10, 1846, by English astronomer William Lassell.  It is the only large moon in the Solar System with a retrograde orbit, an orbit in the opposite direction to its planet's rotation.  At 2700 km in diameter, it is the seventh-largest moon in the Solar System.  Because of its retrograde orbit and composition similar to Pluto's, Triton is thought to have been a dwarf planet captured from the Kuiper belt.  Triton has a surface of mostly frozen nitrogen, a mostly water-ice crust, an icy mantle and a substantial core of rock and metal.  The core makes up two-thirds of its total mass.  Triton has a mean density of 2.061 g/cm and is composed of approximately 15–35% water ice.

NGC 7: NGC 7 is a spiral galaxy located in the Sculptor constellation.  It was discovered by English astronomer John Herschel in 1834, who was using an 18.7 inch reflector telescope at the time.  Astronomer Steve Gottlieb described the galaxy as faint, albeit large, and edge-on from the perspective of the Milky Way; he also noted how the galaxy could only be observed clearly with the peripheral vision, not by looking directly at it.

El Capitan (Mars): El Capitan is a layered rock outcrop found within the Margaritifer Sinus quadrangle (MC-19) region of the planet Mars, this geological outcrop was first discovered and observed by the Mars Exploration Rover "Opportunity" in February 2004.  The rock outcrop was named for El Capitan, a topographical mountain lying within the state, Texas.

Supporting paragraphs: Horrocks Block Jeremiah Horrocks
Answer: Venus