Space Sunday: ancient oceans, comets from beyond, and exoplanet hunting

Dawn mission patch. NASA/JPL

Studies of Ceres, the largest dwarf planet within the orbit of Neptune, and the focus of the joint NASA / ESA Dawn mission for the last 30 months, are beginning to be published at a high rate of knots. In my previous Space Sunday I covered the report that the water ice discovered around Ernutet crater was likely of local origin. Now, two further studies point to Ceres once having a liquid water ocean.

The first study used gravity measurements to characterise Ceres’ interior, the second sought to determine its interior structure by studying its topography. Both came to similar conclusions.

The NASA team conducting the gravity measurements used data gathered by the spacecraft, together with an analysis of gravity-induced variations in the vehicle’s orbit around the dwarf planet as tracked by NASA’s Deep Space Network (DSN) and an analysis of the gravity anomalies associated with four of Ceres’ most notable surface features: the craters Occator (famous for having bright deposits in its basin which caused excitement in the early months of the spacecraft’s time at Ceres), Kerwan and Yalode, and Ceres one significant mountain, Ahuna Mons. This allowed them to draw a number of conclusions, the most notable being Ceres was once very geologically active, and that its surface crust has an overall density closer to that of ice than rock.

The second study focused on investigating the strength and composition of Ceres’ crust and deeper interior by studying the dwarf planet’s topography. By modelling Ceres’ crustal flow, the researchers determined that it is a mixture of ice, salts, rock, and clathrate hydrates, crystalline water-based solids resembling water ice but with up to 1,000 time its strength.

Diagram showing a possible internal structure for Ceres. Credit: NASA/ESA/STScI

The researchers further determined this high-strength crust probably rests on a softer layer that contains some liquid, allowing Ceres’ topography to deform over time, smoothing down features that were once more pronounced and producing the surface environment we see today.

Taken together, these studies suggest that Ceres once had a sub-surface ocean, likely kept liquid by internal heating (which has been suggested by other studies). This ocean may have been similar to the liquid water oceans thought to exist under the surfaces of Europa and Enceladus today. However, in the case of Ceres, much of it has long since frozen out into the dwarf planet’s crust. Most, but not all. The studies, together with the visual evidence of cryovolcanism on Ceres suggest that beneath the frozen crust there is a “soft” layer, possibly a slushy, semi-frozen layer of liquid.

It’s not clear how liquid this residual ocean might be, but as Julie Castillo-Rogez, the Dawn project scientist at JPL and a co-author on both studies, explained, “More and more, we are learning that Ceres is a complex, dynamic world that may have hosted a lot of liquid water in the past, and may still have some underground.” It is also further evidence that many of the smaller bodies in the solar system from Pluto to the asteroid belt, have histories every bit as complex as the major planets in the solar system.

Have We Just Witnessed an Extra-Solar Visitor?

We’re familiar with the concept of comets. They generally originate from one of two points in the outer solar system. The Kuiper Belt,  extending from the orbit of Neptune (at 30 AU) to approximately 50 AU from the Sun, gives rise to what we call “short period” comets which follow a predictable orbit that swings them past the Sun on a regular basis. Halley’s Comet, with its 76-year period, is perhaps the most famous of these.

Then there is the Oort Cloud, predominantly comprising icy planetesimals believed to surround the Sun to as far as somewhere between 50,000 and 200,000 AU (0.8 and 3.2 light years), and thought to be the origin for “long period” comets with orbits around the Sun measured in the hundreds of years.

However, some astronomers believe the solar system might currently be being visited by an altogether rarer type of comet: one that originated in another star system.

A fast-moving object, designated A/2017 U1, was initially spotted on October 18th in Hawaii by the Pan-STARRS 1 telescope. Since then it has been closely tracked by astronomer around the world.  What is particularly interesting about it is that Sun-orbiting eccentricity of between 0 (a circular orbit), and 1 (a parabolic orbit). Anything above 1 would tend to point to an object being entirely extra-solar in origin. A/2017 U1 has an orbital eccentricity of 1.2.

Because of this high eccentricity, the Minor Planets Centre put out a call for more observations on the object in attempts to confirm it is likely extra-solar in nature. It passed around the Sun on September 9th, and was detected as it crossed back over Earth’s orbit on its way back out into space. At the time it was spotted, the comet was about 30 million km (19 million mi) from Earth, and travelling at a velocity of 26 km/s (16 mi/s) –  much faster than the velocity required to escape the Sun, but within ~5 km/s of other stars within the Sun’s stellar neighbourhood, further indicating an interstellar origin.

A simulation of A/2017 U1’s flight through the solar system. At the centre is the Sun and the inner planets, including Earth. The purple item in Jupiter, and the yellow object just beyond it is Saturn. The three pale green items are comets originating within the solar system, and the outermost bright green item represents the orbit of Uranus. A/2017 U1 is indicated by the yellow high inclination parabola, which has swung the object around the Sun. Travelling at 26 km/s, it will escape the Sun’s influence and head back out into interstellar space. Credit: Tony Dunn.

The object’s trajectory is also unusual, approaching the Sun from high above the plane of the ecliptic, and observations made from the Pan-STARRS 1 telescope in Hawaii, the William Herschel Telescope in the Canary Islands and the Very Large Telescope in Chile suggest the comet is a rocky / ice object roughly 160 metres along at least one of its axes.

Tony Dunn, an undergraduate physics and astronomy teacher at San Francisco State University has been running a series of computer simulations using tracking data on the comet, which he has been publishing on his Twitter feed. These suggest the comet may have originated as a body orbiting the star Vega, some 25 light years from the Sun; however, the likely point of origin is still being hotly debated and may never be accurately known.

Another simulation of the object’s passage through the inner solar system. Credit: NASA/JPL

If the object did originate in another star system, then it would suggest the other stars have rings or clouds or material surrounding them at great distances in a manner similar to the Oort cloud. It would also be confirmation of the idea that other stars passing within a few light-years of the Sun disturb the Oort cloud, causing objects there to be disrupted in their orbits, some of which fall towards the Sun and become long-period comets. Presumably, the Sun and other stars can influence rocky clouds around their neighbours in the same way – and that as well as falling towards their local star as comets, the disturbed objects can be kicked out of their local system to become interstellar wanderers.

“We have been waiting for this day for decades,” said Paul Chodas, responsible for NASA’s Centre for Near-Earth Object Studies (CNEOS), which has also been observing the object. “It’s long been theorised that such objects exist — asteroids or comets moving around between the stars and occasionally passing through our solar system — but this is the first such detection.”

“We have long suspected that these objects should exist, because during the process of planet formation a lot of material should be ejected from planetary systems,” Karen Meech, an astronomer at the Institute for Astronomy, Hawaii which operates the Pan-STARRs telescope, added. “What’s most surprising is that we’ve never seen interstellar objects pass through before.”

Crowdfunding Planet Hunting

Crowdfunding has been a popular way to get all kinds of things kick-started with seed money. Now, a consortium of scientists, universities and institutions is using Indiegogo to finance the development of a space telescope that will start looking for exoplanets in the Alpha Centauri system by 2021.

Project Blue is seeking a flexible goal of US $175,000 to help seed the telescope’s funding. The consortium involved in the exercise the BoldlyGo Institute, Mission Centaur, the SETI Institute, the University of Massachusetts Lowell, Yale University, University of Arizona, Arizona State University, Penn State University, and the University of Victoria, Canada. It is steered by a Science & Technology Advisory Committee (STAC) composed of science and technology experts who are dedicated to space exploration and the search for life in our Universe.

The telescope will be roughly the size of a large washing machine, and have a 45-50 cm aperture capable of directly imaging the nearest Sun-like stars to Earth: Alpha Centauri A and B in the hope of imaging “Earth-like” planets (roughly 0.5 to 1.5 times of the size of Earth) orbiting within either star’s habitable zone and possessing an atmosphere that could allow liquid water to exist on its surface.

Project Blue is an attempt to directly image earth-sized planets orbiting either Alpha Centauri A or B, and the first part of the project is using an Indiegogo campaign for financing. Credit: Project Blue

The detection of extra-solar planets – exoplanets – has so far relied on indirect means. The most common of these is Transit Photometery, as used by NASA’s Kepler observatory. It requires monitoring stars for periodic dips in their brightness due to a planet transiting in front of the star.  With Kepler, this has resulted in the cataloguing to date of some 2,470 exoplanets out of a total of 5,017 potential candidates. Another indirect technique is the Radial Velocity (or Doppler) Method, measuring changes in a star’s position relative to the observer to determine how massive its system of planets might be.

Direct observation of possible planets is a lot harder, largely because of the cancelling effect stars have on optical instruments. Simply put, the light given off by a star can be as much as a billion times brighter than any light reflected by a planet orbiting it, making the direct detection of the latter next to impossible.

However, Project Blue plans to get around this by effectively suppressing the light from each of the two stars of Alpha Centauri  through a three-fold approach:

  • Including an instrument called a coronagraph within the telescope which will effectively block starlight. Corongraphs are generally used to reveal the Sun’s corona; however, a new class of stellar coronagraphs is being developed which are capable of directly imaging exoplanets – although thus far they have only been successful in  imaging gas giants the size of Jupiter or larger.
  • Using a deformable mirror within the telescope together with special software to manipulate incoming light, allowing light from the stars to be isolated and “removed”
  • Employing a post-processing method called Orbital Differential Imaging (ODI), to enhance image contrast for light reflected by any planetary bodies which might be orbiting either star

US $175,000 might not sound like enough to finance the development and launch of an orbiting telescope, and it isn’t. Rather, it is intended to finance the initial science requirements, the advanced mission design process, and the development of and on-line mission simulator. It is hoped that these will be sufficient for the project to go forward and obtain additional financing / backing from industry.

Should the  Indiegogo campaign  exceed the initial US $175,000 funding, a number of stretch goals had also been identified. However, with just eight days of the campaign left to run and around 25% of the initial funding target yet to be met, it is doubtful whether any of these will be achieved.

It’s not clear what the total cost of the mission will be. However, Project Blue aims to reduce overheads by using an extremely compact telescope design and a single-goal mission with a defined lifespan (six years from initial design through to mission end, with two years of orbital observations of the Alpha Centauri system). The compact size of the telescope should help in areas of development, construction and launch costs, while the defined mission span also  means total mission development and operational costs can be properly defined.

So, if you feel like contributing to the first phase of the project, hop over to the Indiegogo page and project website.

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