Category: Space Sunday

I missed my usual Space Sunday slot due to Christmas activities taking up much of my time, so thought I’d round out the year of astronomy / spaceflight reporting with a last look at a subject that has dominated space news this year: exoplanets.

Back in February, it was confirmed that a red dwarf star had no fewer than seven planets in orbit around it, all of them roughly Earth-sized, and three of them within the star’s habitable zone (see Space update special: the 7-exoplanet system for more). At the time it was the largest number of planets thus far found to be orbiting a star – in this case, TRAPPIST-1, as it is informally called – named for the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) system that discovered it.

At the time, the discovery meant TRAPPIST-1 tied with Kepler-90 for having the most exoplanets discovered to date orbiting it. However, as announced earlier in December, Kepler 90 has now regained the title, thanks to the work of a researcher from Google AI, and an astronomer from the Harvard-Smithsonian Center of Astrophysics (CfA), with the discovery of an eighth planet orbiting the star designated Kepler-90. However, what is particularly interesting about this discovery is both the way in which it was made.

Located about 2,545 light-years (780 parsecs) from Earth in the constellation of Draco, Kepler-90, unlike TRAPPIST-1 and the majority of other planet-bearing stars, in not a M-class red dwarf star. Rather, it is a G-class main sequence star, with approximately 120% the mass and radius of the Sun. It is thought to be around 2 billion years old and it has a surface temperature of 6080 Kelvin – compared to the Sun’s 4.6 billion years of age and 5778 Kelvin surface temperature. Thus, the star and its planetary system has certain key similarities to our own solar system in terms of Kepler-90’s nature, the number of major planets now known to be orbiting it, and their distribution – the smaller rocky planets being closer to their parent than the system’s gas giants.

The Kepler designation for the star indicates it was a subject of study for the Kepler Space Telescope. Prior to that, the star was designated 2MASS J18574403+4918185 in the Two Micron All-Sky Survey catalogue, compiled following the 1997-2001 whole sky astronomical survey of the heavens visible from Earth. At that time, transit data gathered from earth-based observations suggested it may have a planet orbiting it, so it was made a target for observation by Kepler, and re-designated Kepler Object of Interest 351 (KOI-351). In 2013, thanks to Kepler’s observations, it was confirmed the star had six or possibly seven planets orbiting it (the outermost remained a subject of doubt for a while after it was initially identified).

All seven of the initial discoveries were made using the transit method (Transit Photometry) to discern the presence of planets around brighter stars. This consists of observing stars for periodic dips in brightness, which are an indication that a planet is passing in front of the star (i.e. transiting) relative to the observer. Kepler’s data revealed the seven planets orbiting the star over a period of two months, with the planets being designated as follows (in order of distance from their parent star):

Kepler-90 b	Kepler-90 c	Kepler-90 d	Kepler-90 e	Kepler-90 f	Kepler-90 g	Kepler-90 h
Radius: 1.31 Earth	Radius: 1.19 Earth	Radius: 2.9 Earth	Radius: 2.7 Earth	Radius: 2.9 Earth	Radius: 8.1 Earth	Radius: 11.3 Earth
“Super Earth”	“Super Earth”	“Mini Neptune”	“Mini Neptune”	“Mini Neptune”	“Saturn size”	“Jupiter size”
Orbital period: 7 days*	Orbital period: 8.7 days*	Orbital period: 59.7 days*	Orbital period: 92 days*	Orbital period: 125 days*	Orbital period: 210 days*	Orbital period: 311 days*

*=terrestrial days

However, while the system does have similarities to our own, all of the planets within it orbit much closer to their parent star than do the planets of the solar system. So much so that the largest and outermost of those discovered, the Jupiter-sized Kepler-90 h, is the only one to orbit within the star’s habitable zone – the point at which liquid water and other essentials for life might exist in the right combinations. And while it may well sit on the inner edge of the star’s habitable zone, given that Kepler-90 h is a gas giant world somewhat equitable with Jupiter in size and mass, it is highly unlikely it is a suitable environment in which life might arise – but there is the intriguing question that should it have a sufficiently large moon orbiting it – say one the size of Titan or Ganymede – which has a good magnetic field protecting it, life might arise there.

The inner planets of the system, while more Earth-like in their size, are unlikely to support life, even if the three “mini Neptunes” were to prove to be solid bodies with atmospheres. Kepler 90 b through Kepler 90 e all orbit within or at about the same distance Mercury orbits the Sun, meaning they all experience similar or hotter surface temperatures the innermost planet of the solar system experiences. Kepler-90 f orbits at approximately the same distance as Venus does from the Sun, which likely means that if it is a mini-Neptune and, it could well be like Venus it terms of the conditions within any atmosphere it might have.

Continue reading “Space update special: the 8-exoplanet system and AI” →

K2-18 is a red dwarf star system located about 111 light-years from Earth in the constellation Leo. It has been of interest to astronomers because it is home to an exoplanet – K2-18b, also referred to as EPIC 201912552 b, discovered in 2015 by the Kepler Space Observatory.

At the time of its discovery, K2-18b was placed within its parent star’s habitable zone, and was believed to be receiving around the same about of radiation as Earth does from the Sun. However, at the time of its discovery, it was unclear if the planet was a rocky super-Earth or a mini-Neptune gas planet. Because of this, an international team of scientists have been studying the planet using the High Accuracy Radial Velocity Planet Searcher (HARPS) instrument at the European Southern Observatory.

They had been intending to more accurately characterise K2-18b’s mass, the first step in determining it’s atmospheric properties and bulk composition. And they actually succeeded, determining that K2-18b has a mass of about 8.0 ± 1.9 Earth masses and a bulk density of 3.3 ± 1.2 g/cm³. This is consistent with a terrestrial (aka. rocky) planet with a significant gaseous envelope and a water mass fraction that is equal to or less than 50%. This makes K2-18b is either a super-Earth with a gases atmosphere, or it is a “water world” with a surface layer of thick ice.

However, the team also found something that had not been expected: a second planet orbiting K2-18.

Now referenced as K2-18c, this planet is much closer to its parent star than K2-18b, orbiting its parent once every nine terrestrial days. The team responsible for the discovery believe the planet is 7.5 ± 1.3 Earth masses, making it a “warm super-Earth”. It is far too close to its parent star to be within the habitable zone, making it an unlikely candidate to support life. It was most likely “missed” by Kepler both because of its proximity to the star, and because its orbit does not lie in the same plane.

The discovery of K2-18c was actually made in October 2017. But because it had been missed by Kepler, those detecting it were initially cautious with their findings and sought to further verify them before announcing the find. As the study’s lead, Ryan Cloutier of the University of Toronto said:

When we first threw the data on the table we were trying to figure out what it was. You have to ensure the signal isn’t just noise, and you need to do careful analysis to verify it, but seeing that initial signal was a good indication there was another planet… It wasn’t a eureka moment because we still had to go through a check list of things to do in order to verify the data. Once all the boxes were checked it sunk in that, wow, this actually is a planet.

However, now it has been discovered, it will be the subject of further investigation – as will K2-18b.

In fact, given the findings of the study, K2-18b is now considered as having a reasonable chance that it might have conditions suitable for life. Thus, it is now likely to be a candidate for study by the James Webb Space Telescope (JWST) when it starts operations in 2019.  JWST will be able to probe the planet’s atmosphere and determine how extensive it is, its composition, and what lies beneath it – be is a planet of an ice-covered ocean or a dry, rocky world – or something between the two.

In addition, the K2-18 system further underlines M-class red dwarf stars as the home of multi-planet systems, while the relatively proximity of K2-18b make it a prime target to further our understanding of the atmospheres around Earth-type exoplanets.

Icy Worlds Might Offer More Chances for Life and Rocky Planets

That K2-18b might be an icy water world fits with the findings of a new study form the  Harvard Smithsonian Centee for Astrophysics, which suggests such planets might be far more prevalent in the galaxy than rocky Earth-type planets.

When we discuss exoplanets, there is a tendency to focus on those within the so-called habitable zone around a star, because this is the most likely region where conditions – based on our own solar system – where life is to arise.

However, as the new study notes, there are actually two other planets within the Sun’s habitable zone where conditions are such that life either never got started or didn’t last that long (Venus) and another where life, if it got started, would have encountered environmental conditions which may have limited it or again, destroyed it. However, there are at least five worlds outside of the Sun’s habitable zone  – Europa, Ganymede, Enceladus, Dione and Titan – which all have the potential to support life. Thus, the so-called “habitable zone” around a star need not necessarily be the only place where conditions for life to arise might exist.

Using the solar system as a basis for modelling, the researchers widened their consideration of habitability to include worlds that could have subsurface biospheres. Such environments go beyond icy moons such as Europa and Enceladus and could include many other types deep subterranean environments.

They then went about assessing the likelihood that such bodies are habitable, what advantages and challenges life will have to deal with in these environments, and the likelihood of such worlds existing beyond our Solar System (compared to potentially habitable terrestrial planets).

There are several advantages to “water world” when it comes to harbouring life. They tended to be internally heated (keeping the ocean liquid), may suffer of tectonic activity (as is now thought to be the case with Europa), which could pump living-forming energy and minerals into their oceans, while their icy crusts could offer shielding from harsher UV radiation and cosmic rays (energetic particles). The latter could be a major consideration considering the propensity for re dwarf stars to form planetary systems, and the fact they tend to be quite violently active.

Overall, the researchers determined that a wide range of worlds with ice shells of moderate thickness may exist in a wide range of habitats throughout the cosmos. Based on how statistically likely such worlds are, they concluded that “water worlds” like Europa, Enceladus, and others like them are about 1000 times more common than rocky planets that exist within the habitable zones of their parent stars.

However, while such worlds might be more common, there are negative aspects to the findings. Ice covered ocean worlds would lack sunlight as a source of energy, limiting the available energy supply to localised sources – ocean bottom fumeroles, etc., which in turn limit the size of available biospheres where life might survive – and tectonics could lead to these energy sources shifting or even dying. Also, nutrients needed to support life would likely be available in lower concentrations. That these worlds are ice-covered also makes identify whether the do in fact support life nest to impossible.

Thus, the finding could indicate that basic life might be far more prevalent in the galaxy – but also potentially much harder to detect.

 

On October 30th, 2017 I wrote about the extra-solar body which had crossed the orbit of Earth after swinging around the Sun during a rapid flight into and back out of the solar system. The object, originally designated A/2017 U1 and then as 1I/2017 U1 (the “1I” indicating it is the first positively identified interstellar object we’ve observed in 2017), 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.

Since that time, the object has been under intense study, as has been reported in the media, and is proving to be most unusual. Now dubbed `Oumuamua, roughly translated as “scout” (ou being Hawaiian for “reach out for” and mua meaning “first, in advance of” – which is repeated for emphasis). At first thought to be a comet on account of initial observations, it was reclassified as an asteroid following more details observations.

In particular, observations made using the Very Large Telescope (VLT), operated by the European Southern Observatory (ESO) at the Paranal Observatory in Chile revealed the object to be cigar-shaped, rather than being a more rounded shape, as had been expected. Overall, it is estimated to be around 400 metres (1312 ft) in length, and approximately 40-50 metres (130-162.5 ft) in height and width. It is tumbling .

Using the VLT, ESO were able to accurately measure the brightness, colour and orbit of the asteroid and refine measurements of its trajectory as it leaves the solar system at a stunning 95,000 km/h (59,000 mph). These have revealed that `Oumuamua varies dramatically in terms of brightness (by a factor of ten) as it spins on its axis every 7.3 hours. As Karen Meech of the Institute for Astronomy in Hawaii explained in an ESO press release, this was both surprising and highly significant:

This unusually large variation in brightness means that the object is highly elongated: about ten times as long as it is wide, with a complex, convoluted shape. We also found that it has a dark red colour, similar to objects in the outer Solar System, and confirmed that it is completely inert, without the faintest hint of dust around it.

These observations also allowed Dr. Meech and her team to constrain `Oumuamua’s composition and basic properties. Essentially, the asteroid is now believed to be a dense and rocky asteroid with a high metal content and little in the way of water ice. It’s dark and reddened surface is also an indication of tholins, which are the result of organic molecules (like methane) being irradiated by cosmic rays for millions of years.

The measurements confirmed that the asteroid came to us from the general vicinity of Vega  in the Constellation of Lyra, and has taken around 300,000 years to reach the solar system, which it has been passing through for the last 20,000. However, whether it originated around Vega is still being debated. Some of those observing the object believe it could have been wandering the interstellar void for 45 million years, having originally been ejected from a stellar system in the Carina–Columba association, which had once been far more aligned with the constellation of Lyra, relative to the solar system.

Passing through most of the solar system at a speed of around 80.0oo km/h (58,000 mph), the asteroid gradually accelerated under the Sun’s gravity so that it reached a velocity of 315,700 km/h (196,000 mph) at perihelion – the point closest to the Sun, which it reached on September 17th, 2017. Since then, the object has been heading away from the Sun and decelerating, again under the influence of gravity, passing the orbit of Earth in October. It will pass Jupiter’s orbit in May 2018, Saturn’s orbit in January 2019, and Neptune’s orbit in 2022, passing onwards through the solar system. It will be another 20,000 years before the object re-enters the interstellar medium.

Even it is of extra-solar origin, `Oumuamua is seen as being of significant import for our understanding of the formation of other solar systems. If nothing else, a study of the asteroid as it continues onward and outward from the Sun could potentially teach us a lot about its origins and the likely conditions within the system where it was born.

To this end, there have been numerous calls for the development of one or more missions to investigate the asteroid, some of which, such as Project Lyra, are already being mapped out.  However, planning such a mission is one thing – actually pulling it off is quite another. `Oumuamua is currently travelling at 95,000 km/h (59,375 mph) – a velocity it will now more-or-less maintain.That is equivalent to 5.5 AU (Astronomical Units – the average distance from Earth to the Sun) per year, or 26 metres (84.5 ft) per second – what is technically referred to as its hyperbolic excess velocity.

No space vehicle launched from Earth has been able to attain that kind of velocity – even the fastest human-made objects in space, Voyager 1, and the fastest space probe at launch, New Horizons, are both only managing around two-thirds of that velocity. So just getting to a point where we can launch a vehicle capable on eventually matching the speed of the asteroid is a major challenge  – without the worry of getting it to a speed where it might eventually catch with `Oumuamua at a speed which would allow it sufficient time to gather data on the rock as it flies by, rather than shooting right on past it at such a speed, it has next to no time to gather data of significant value. Nevertheless, the proponents of Project Lyra are going so far as to suggest a mission might rendezvous with  `Oumuamua and gather samples for on-board analysis.

Of course, the asteroid will be travelling through the outer solar system – and by that I mean the Kuiper Belt outwards to, and through, the Oort cloud – for thousands of years; it’s not just going to vanish in a decade or so. So this does give some leeway. An encounter with  `Oumuamua within the Kuiper Belt for example (say, 50-200 AU from Earth) wouldn’t need to be launched for another 5-10 years. This could potentially allow for the use of an upcoming launch vehicle, such as NASA’s Space Launch System rocket or even SpaceX’s gigantic Interplanetary Transport System launcher, the BFR.

However, looking towards an encounter that far from earth still means that the probe would have to achieve a hyperbolic excess velocity of up to 76 metres (247 ft) per second – or half as much again as the asteroid’s velocity – again calling into question the effectiveness of a mission in gathering and returning data. Certainly, at those kinds of speeds, an actual rendezvous with `Oumuamua to gather a sample would be out of the question.

An alternative approach might be more “slow and steady” approach using solar sail technology – such as that being developed with projects such as the Breakthrough Initiatives’ Starshot. This might allow a vehicle propelled by an earth-based array of lasers to eventually catch the asteroid, and with a rate of steady acceleration, overhaul it at a rate at which data can be gathered in earnest. However, such technology is in its infancy; thus the chances of such a mission being used for catching `Oumuamua are perhaps slim. However, development of the technology and a mission for intercepting an extra-solar object in the future a distinct possibility – particularly as it is now estimated at least one extra-solar object passes through the solar system a year.

Whether intended to study `Oumuamua or one of these other interstellar wanderers, any such mission – using rockets, ion drive propulsion, solar sail technologies -, if pursued, could led to technological breakthroughs as well as scientific rewards. As the project authors note:

As 1I/‘Oumuamua is the nearest macroscopic sample of interstellar material, likely with an isotopic signature distinct from any other object in our solar system, the scientific returns from sampling the object are hard to understate. Detailed study of interstellar materials at interstellar distances are likely decades away, even if Breakthrough Initiatives’ Project Starshot, for example, is vigorously pursued. Hence, an interesting question is if there is a way to exploit this unique opportunity by sending a spacecraft to 1I/‘Oumuamua to make observations at close range.

[A] mission to the object will stretch the boundary of what is technologically possible today. A mission using conventional chemical propulsion system would be feasible using a Jupiter flyby to gravity-assist into a close encounter with the Sun. Given the right materials, solar sail technology or laser sails could be used… Future work within Project Lyra will focus on analysing the different mission concepts and technology options in more detail and to down select 2 – 3 promising concepts for further development.