Dark new horizons shed light on an old Earth

My last postcard was about context. In that postcard, I explained how MESSENGER's exploration of Mercury has helped us learn more about the planetary neighbourhood in which our Earth sits. And for this latest offering, I want to follow a similar theme. But first, we need to take a pretty huge leap (about 5.85 billion km) across the Solar System. Where we will find ourselves in the vicinity of Pluto.

Pluto—once famous for being the ninth and most distant planet from the Sun—is now more famous for being the planet that isn't a planet. Following its discovery in 1930, Pluto—which has a diameter of about 2,300 km—enjoyed more than 75 years at the planet level of the Solar System hierarchy. But in 2006, members of the International Astronomical Union (IAU) decided to demote Pluto, and assign it a new status as a dwarf planet. This decision was prompted when it became clear that Pluto is just one of many large, Sun-orbiting icy bodies in the outer Solar System. The astronomers therefore decided to officially define the term planet, specifically so that Pluto (and other bodies like it) would be excluded from this class.

Photographic plates used for the discovery of Pluto. The arrows mark Pluto's position. Pluto clearly moved against the background of stars in the six days between the two observations, which were made by Clyde Tombaugh in 1930. Credit: Lowell Observatory Archives

Under the IAU's new formal definition, a planet must meet three requirements:

1. The celestial body must orbit the Sun.
2. The body must have a large enough mass to give it a nearly round shape. 
3. The body must have cleared the neighbourhood (of other material) of its own orbit.

Unfortunately—for Pluto at least—the former ninth planet could not meet this third requirement. And a global public outcry—which continues today—followed.

The International Astronomical Union's decision to reclassify Pluto and strip it of its planet status hit the headlines in 2006 and caused a huge public outcry.

But the question of Pluto's planethood is currently being pushed aside, as the level of excitement surrounding NASA's New Horizons mission rapidly grows, prior to the spacecraft's Pluto fly-by. New Horizons—first launched in 2006—is the first spacecraft to visit Pluto and its system of five known moons (Charon, Styx, Nix, Kerberos, and Hydra). The probe will not go into orbit around Pluto, but will instead zoom by a week from now, on 14 July 2015. The fly-by will only last about eight or 10 hours, but at its closest approach the spacecraft will be about 12,500 km from the surface of Pluto. 

Photograph of Pluto and its five moons taken with the Hubble Space Telescope in 2012. Credit: NASA, ESA, and L. Frattare (STScI)

The scientific payload of the spacecraft consists of seven instruments that were chosen so that the geology, surface composition and temperature, and atmospheric characteristics of Pluto and its moons could be investigated. The bulk of the scientific data will be obtained during a period of about 24 hours around the time of the fly-by. The best pictures should reveal features as small as 60 metres across on Pluto's surface.

Map of Pluto released by the New Horizons team on 7 July 2015. The map was created from images obtained with the spacecraft's Long Range Reconnaissance Imager (LORRI) instrument, which were combined with low-resolution colour data obtained with the Ralph instrument. The map clearly shows an intriguing pattern of bright and dark markings on Pluto's surface. The brightest region may contain fresh deposits of methane, nitrogen, and/or carbon monoxide frost. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

This newest installment in the history of human exploration of the Solar System is all very nice, but how is it relevant to my original brief? How can learning about this far-distant world help us convey the uniqueness of Earth to a hypothetical alien planetary geologist? Well, if all goes to plan, then the fly-by of Pluto will not be the end of the New Horizons mission. It  should just be the end of the beginning. Pending approval from NASA for an extended mission, New Horizons will be sent on an onwards journey to study another Kuiper belt object.

The Kuiper belt is a region that extends outwards from the orbit of Neptune for about 20 AU (astronomical unit, equal to about 150 million km). It is similar to the asteroid belt (which lies between the orbits of Mars and Jupiter), as it contains many—relatively small—bodies that are remnants from the formation of the Solar System. Most Kuiper belt objects are icy bodies, composed mainly of substances such as methane, ammonia, and water. Pluto is the largest known object in the Kuiper belt, but about 100,000 objects (with diameters of more than 100 km) are expected to exist in this region, and more than one thousand have been discovered since 1992.

The path of the New Horizons spacecraft (yellow line) through the outer Solar System and the Kuiper belt. The orbits of the planets are shown in blue. The largest Kuiper belt objects are labelled. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker

By studying Pluto and its Kuiper belt companions, we can potentially learn about two aspects of Earth's earliest history. As remnants of the Solar System's formation, the Kuiper belt objects are seen as akin to planetary embryos or protoplanets. Planetary accretion is believed to begin with the condensation of solids from the gas cloud that surrounds a star. Accretion of gas and dust then produces bodies that have diameters of 1–10 km, which are known as planetismals. The Kuiper belt objects can help us understand this early accretionary stage and thus what processes went into building our Earth. Violent impacts that occurred during the stage of runaway growth allowed the many planetismals to coallesce and form the large planets we know today.

In addition, the Kuiper belt is thought to be the region from which most short-period comets (i.e., those with orbits of less than 200 years) originate. As the European Space Agency's current Rosetta mission has wonderfully shown, comets are intriguing bodies in our Solar System. Indeed, many scientists believe that comets may have contributed a significant proportion of Earth's water inventory. In a previous postcard, I discussed how a Jupiter-family comet—which probably originated in the Kuiper belt—has a water signature that is a good match for that of Earth. 

New Horizons therefore provides us with a great opportunity to get to know the Kuiper belt better and to potentially understand the building blocks of Earth just a little bit more. But furthermore, the Kuiper belt may provide a big clue to any alien astronomers of our Earth's existence. Neptune, as a giant gas planet, exerts a great gravitational force on the cloud of dust that surrounds it in the Solar System (which includes the Kuiper belt). The gravity tugs on this cloud of dust and creates a distinctive ring structure. Computer simulations show that this ring contains a gap where Neptune itself resides. So even if the alien astronomers cannot directly image the planets of our Solar System from afar, they might be able to detect Neptune's presence. As such, they would know that our Sun possesses a planetary system, and we on Earth are here to be found.

Computer simulations show what the Solar System might look like to an alien astronomer. The gravity of Neptune creates this distinctive ring structure in the dust cloud. The planet itself resides in the gap that can be seen as the dark area in the right of the image. Credit: NASA/Goddard/Marc Kuchner and Christopher Stark