When is a planet not a planet – or more precisely, when should what is not regarded as a planet be a planet?
Right now, according to the International Astronomical Union (IAU), our solar system comprises eight formally recognised planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. That’s been the case since 2006, when the IAU opted to classify bodies orbiting the Sun in three ways:
- As planets – defined as a) celestial bodies that (a) are in orbit around the sun; b) have sufficient mass for their self-gravity to overcome rigid body forces so they assume a hydrostatic equilibrium (nearly round) shape; c) have cleared the neighbourhood around their orbit of other objects
- As Dwarf planets – defined as celestial bodies which a) orbit the sun; b) have sufficient mass for their self-gravity to overcome rigid body forces to assume hydrostatic equilibrium (aka “is nearly round” in shape); c) have not cleared the neighbourhood around their orbit; and d) is not a natural satellite
- As Small Solar System bodies: all other objects except satellites orbiting the Sun.
Thus, since 2006, Pluto has been a dwarf planet. However, moves are afoot to get things changed – and not just for Pluto.
In a paper authored by planetary scientists involved in the New Horizons mission which zipped through the Pluto system in July 2015, there is a call for the term “planet” to be redefined; if not by the IAU then at least in popular use. Should it happen, it could see the number of planets in the solar system leap from 8 to over 100.
The scientists argue that the IAU definition of “planet” focuses only on the intrinsic qualities of the body itself, rather than external factors such as its orbit or other objects around it. In fact, under the IAU’s definition, Earth, Mars, Jupiter and Neptune don’t actually qualify as “planets” as none meet the third criteria (c) – Earth, for example, has regular “close encounters” with asteroids which cross its orbit. Instead, the team offer a simpler definition:
A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has enough gravitation to be round due to hydrostatic equilibrium regardless of its orbital parameters.
Such a definition would mean that Pluto could regain its planetary status – as would the proto-planet (or small solar system body) Ceres, the dwarf planets of 136199 Eris (discovered in 2005, and the trigger-point for Pluto’s “downgrading”) , 136472 Makemake, and 136108 Haumea, together with (possibly) 50000 Quaoar, 90377 Sedna, 90482 Orcus and a host of trans-Neptunian objects tumbling around the Sun. Nor is that all; the new definition would also mean that the likes of Jupiter’s Galilean moons, Saturn’s Titan and Enceladus, Neptune’s Triton and many other bodies we regard as “moons” would be lifted to planetary status – including our own Moon.
The paper proposing the change will be presented at the Lunar and Planetary Science Conference on March 20th to 24th, 2017 in Texas. And it has already come in for some criticism.
Mike Brown is the scientist largely behind Pluto’s demotion. Currently engaged in the search for the elusive “Planet Nine”, he (somewhat harshly) sees the efforts of the New Horizons team to get Pluto reclassified as being about them wanting the prestige of having run a planetary mission, more than anything else.
However, there are valid reasons for seeking some kind of change, even if it is only informal. One is as basic as gaining more public interest in efforts to explore and understand the many environments found on planets and moons alike within our solar system.
“Every time I talk about this [the science and data gathered about Pluto by New Horizons] to the general public, the very next thing people say is ‘Pluto is not a planet any more’,” said Kirby Runyon, the lead author of the paper. “People’s interest in a body and exploring it seems tied to whether or not it has the name ‘planet’ labelled on it.”
There are scientific reasons for the definition to be broadened as well. Places like Pluto, Ceres, Europa, Io, Ganyemede, Callisto and Triton all evidence geophysical, hydrothermal, atmospheric and other characteristics very much in keeping with bodies such as Earth, Mars, and Venus. They are thus of exceptional interest to planetary scientists the world over. In fact, many of them (like Pluto) are completely re-writing our understanding of “planetary bodies”.
Ultimately, the team behind the paper aren’t going to put their proposal before the IAU for a change in the “official” definition of “planet”. “As a geophysical definition, this does not fall under the domain of the IAU, Runyon notes, “[It] is an alternate and parallel definition that can be used by different scientists. It is “official” without IAU approval, partly via usage.”
Would You Like to Fly (Into Space) In My Beautiful Balloon
We’re increasingly familiar with the growing ways people are investing in getting payloads (and other people) into space more efficiently and cost-effectively.
SpaceX and Blue Origin are developing the means to recover and re-use rocket stages as well as capsules; Virgin Galactic is looking at the use of aircraft to fly people and payloads to high altitude from whence they can continue into space / or obit via space plane or air-launched rocket. Britain’s Reaction Engines are developing a revolutionary vehicle called Skylon which could take off from a conventional runway anywhere in the world and fly into space. Even balloons are being considered as potential launch platforms, and a company called Zero 2 Infinity is well on the way to showing it can be done.
On March 1st, 2017, the Barcelona-based company successfully tested the deployment and launch of a system they call Bloostar, one of three balloon-based systems they are developing for space and near-space activities. Intended to launch micro satellites in the 75 kg (169 lb) range. It uses a helium filled balloon to carry a launch vehicle to an altitude of about 40 km (25 mi), where it can be released and use its rocket motors to reach a speed sufficient enough to place a payload in orbit.
By using a balloon to lift the launch vehicle to an attitude twice that of a commercial airliner, the Bloostar greatly reduces both the size of the vehicle, and the amount of propellants it needs to reach orbit, increasing its cost-effectiveness. The vehicle itself comprises three “stages”, nested as concentric rings. Once used, all three stages return to Earth under parachute to be recovered and reused. The balloon is also recovered and reused, having deflated itself and returned to Earth once the launch vehicle has been deployed.
The March 1st flight saw the deployment of a test balloon and scaled version of the payload launcher deployed from a ship off the Spanish coast. The helium-filled balloon rose to an altitude of 25km (15.6 mi), around half the operational altitude for launches, where the payload vehicle was released to fire its motors.
The aim was not to fly the vehicle into space, but rather to test the release mechanism on the tether connecting it to the balloon, and monitor the vehicle’s ability to ignite its motors, fly a stable trajectory and then deploy its parachute systems for recovery. It marked the first step in a series of test flights the company will conduct with Bloostar.
Zero 2 Infinity is not the first company to consider a balloon / rocket launcher combination – America’s United Launch Alliance (among others) also looked at the idea, specifically with their upcoming Vulcan rocket. However, Zero 2 Infinity are the first to successfully reach this point in developing such a capability. If all goes according to plan, they aim to start commercial operations in 2019, and state they already have letters of intent worth €250 million ($266.4 million) from interested customers.
Trump Budget Cuts ARM, Europa Lander and Earth Sciences
The first federal budget mandated by America’s Trump administration has caused some upset for a number of reasons. However, NASA has come off exceptionally well in the proposed budget, with just a 0.8% reduction for 2018 when compared to 2017. However, this still means a number of programmes now face the axe. These include:
- The Asteroid Redirect Mission (ARM) mission. Always controversial, this would have captured a chunk of an asteroid and place it in Lunar orbit where it would later be visited by astronauts in a mission somewhat dubiously pitched as “testing technologies” for a mission to Mars
- The Deep Space Climate Observatory (DSCOVR) mission, which plays a critical role in real-time solar wind monitoring capabilities crucial to the accuracy and lead time of space weather alerts and forecasts used to limit the risk of widespread disruption to major public infrastructure systems including power grids, telecommunications, aviation and GPS
- Three more Earth resources missions: the Climate Absolute Radiance and Refractivity Observatory (CLARREO) – a mission vital to understanding the Earth’s radiative budget & the potential of climate change; the Orbiting Carbon Observatory (OCO) science suite intended for the International Space Station, and the Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) satellite
- RESTORE-L, a satellite servicing programme due to refurbish the LANDSAT-7 mission, which plays a key role in observing global resource usage and pollution, with consumables in 2020
- NASA’s Office of Education.
Countering this, the Orion space vehicle and its Space Launch System booster remain funded as required, receiving US $3.7 billion between them. NASA’s planetary science programme also gets a boost from US $1.63 billion to US $1.9 billion, with both the Mars 2020 rover and the Europa Clipper missions being specifically funded. However, the Europa lander mission, which had strong support in Congress, looks slated for cancellation.
The Office of Education is seen as “duplicative” of other NASA outreach programmes, hence it being cut. Acting Administrator Robert Lightfoot is keen to stress NASA’s overall education outreach will be unaffected, but it seems likely that a number of education programmes will be hit, including the National Space Grant College and Fellowship Programme and the Experimental Programme to Stimulate Competitive Research and Minority University Research Education Project.
RESTORE-L is also seen as a “duplicative” programme, as the Defence Advanced Research Projects Agency (DARPA) is developing a similar capability. However, the DARPA project is intended to support satellites in equatorial geosynchronous orbits, not those occupying lower, polar orbits like Landsat-7.
NASA is thought to have been spared much deeper cuts as space exploration – notably a return to the Moon – are thought to be of major interest to the new administration.