Space Sunday: alien worlds, telescopes and lightsails

An artist’s impression of the Teegarden’s Star planetary system might look like when observing it from the “far side” relative to our own Sun (shown in the background and inset). Credit: University of Göttingen

Two Earth-sized planets have been found orbiting a star 12.5 light-years from our own, adding to the catalogue of exoplanets located in our own cosmic back yard.

The star in question is Teegarden’s Star, a M-type red dwarf, the most common type of star in our galaxy, and therefore the most frequent type found to have planets and planetary systems. However, Teegarden’s Star is a little different to other red dwarfs we’ve observed with or without planets. For a start, despite being only a short cosmic stone’s throw from Earth, it is incredibly dim – so dim that we didn’t even notice it until 2003. Not that that in itself is usual, it’s believed that the space around us for a distance of about 20 light years could have many dim red dwarf stars hiding within it, simply because this region of our galaxy seems to have a much lower density of such stars than we see elsewhere.

What makes Teegarden’s Star odd in this respect is that it wasn’t found as a result of a search for such nearby dim red dwarfs, but because astronomers tripped over it whilst reviewing data originally gathered in the 1990s by the Near-Earth Asteroid Tracking (NEAT) project. In fact, the star is actually named for the head of the review team, Bonnard J. Teegarden, an astrophysicist at NASA‘s Goddard Space Flight Centre. The star is also somewhat unusual in that it has a large proper motion (approximately 5 arcseconds per year), marking it as one of seven stars with such large proper motions currently known.

Observations of the star made in 2010 by the Red Optical Planet Survey (ROPS) suggested the star might have at least one planet orbiting it, but the data was insufficient to draw a definitive conclusion. However, in June 2019, and after three years of verifying their data, scientists conducting the CARMENES survey at the Calar Alto Observatory announced evidence of two Earth-mass exoplanets orbiting the star within its habitable zone.

A star and its planet moving around a common centre of mass. Credit: wikipedia / “Zhatt”

The planets were detected using the radial velocity method (aka Doppler spectroscopy), also informally referred to as the “wobble method”. Putting it simply, a star with planets doesn’t simply spin on its axis with the planets whizzing around it. Rather, the mass of the planet(s) works against the mass of the star, creating a common centre of mass which, although still inside the star, is sufficiently removed from its own centre to cause the star to effectively rolls around it (see the image on the right).

This means that when seen from Earth, there are times when the star can seem as if it is moving “away” from our telescopes, signified by its light shifting to the red end of the spectrum. Equally, there are other times when it appears to be moving “towards” us, signified by its light shifting to the blue end of the spectrum. It is by observing and measuring this visible Doppler shift that tells us there are planets present. In all, this method of stellar observation has accounted for almost one-third of all exoplanets found to date.

The key point with this method of observation is not only does it allow astronomers to locate planets orbiting other stars, it actually allows maths to be applied, allowing the number of potential planets to be discerned, their distance from their parent star and important factors such as their probable mass, which in turn allows their likely size and composition to be estimated.

In the case of Teegarden’s Star, the data indicates the two planets orbiting the star – called Teegarden’s b and Teegarden’s c respectively – have a mass of around 1.05 and 1.1 that of Earth each, suggesting they are probably around the same size as one another and comparable to Earth in size. Teegarden’s b, the innermost planet, orbits its parent every 4.9 terrestrial days, and Teegarden’s c every 11.4 terrestrial days.

An artist’s impression of the Teegarden’s Star system, as seen from “above”. Credit: University of Göttingen

The combined mass of these planets, coupled with the amount of Doppler shift exhibited by Teegarden’s Star has led to some speculation there may be other, larger planets orbiting much further out from the star. Such planets would be hard to locate because Teegarden’s Star is so dim when observed from Earth, astronomers cannot rely on the transit method  – where large planets passing in front of their parent star can cause regular dips in its apparent brightness – to identify their existence.

However, what is particularly interesting about Teegarden’s b and c is their location relative to their parent, and the nature of Teegarden’s Star itself. The latter is a particularly cool and low-mass red dwarf, with just one-tenth of the Sun’s mass and a surface temperature of 2,700°C (4890°F). This means that at their respective distances, both planets are within the star’s habitable zone – and may well have atmospheres.

The two planets resemble the inner planets of our solar system. They are only slightly heavier than Earth and are located in the so-called habitable zone, where water can be present in liquid form.

– Mathias Zechmeister, University of Göttingen, Teegarden planetary team lead

This latter point  – the existence of atmospheres around both planets – has yet to be proven. As noted previously in these articles, M-type stars are actually not nice places; when active (and Teegarden does seem to be well past its active stage) in their youth, they can be prone to violent irradiative outbursts which could both strip away the atmospheres of any planets orbiting them over time and irradiate the planets’ surfaces. And even if  the planets do have atmospheres, their close proximity to their parent likely means they are both tidally locked with their same face towards it. This is liable to make them pretty inhospitable places and potentially prone to extremes of weather.

But there is one other interesting point to note here. While Teegarden’s Star may well be dim to the point of being practically invisible when viewed from Earth, the same isn’t true the other way around: our Sun would be a bright star in the skies over Teegarden’s b and c. What’s more, the angle of our solar system to those worlds (practically edge-on) means that if we were to imagine one of them having an intelligent, scientific race, they could easily detect the planets orbiting our Sun using the transit method of observation, and could probably deduce up to three of the innermost planets might be capable of supporting life.

Spitzer Space Telescope to be Shut Down

NASA has announced that one of its Great Observatories, the Spitzer Space Telescope (SST) is to be shut down on January 20th, 2020.

Launched into a heliocentric orbit in 2003, Spitzer was expected to have a mission of up to five years, after which the on-board supplies of liquid helium required to keep the telescope operating at its required temperatures would be exhausted. While this happened in 2009, rendering most of the telescope’s instruments unusable, two have remained functional for the last ten years, allowing Spitzer to chalk up an impressive 16-year run, giving us a better understanding of how the universe works, as well as nebulas, galaxy clusters, star nurseries and much more.

An artist’s rendering of the Spitzer Space Telescope. Credit: NASA

The official reason for ending the mission, even though the two remaining IRAC instruments are still operational is down to increasing issue in managing communications with Earth. Spitzer occupies travels around the Sun “behind” the Earth, but is moving are a slower orbital speed, so the distance between the telescope and Earth is increasing. The problem here is that in order to communicate with Earth, the telescope must orient itself so its main antenna is directed to Earth, which moves its power-generating solar panels out of alignment with the Sun, leaving the telescope dependent on its batteries – and these periods on battery power are increasing. So the fear is that battery power will fail during one of these periods without solar power, killing the telescope.

However, some have suggested the reason for ending the mission that of cost. In 2017, NASA attempted to spin Spitzer’s mission off to academic institutions to finance, without success. Instead, provisions were made to continue to finance the telescope through until the launch of the James Webb Space Telescope (JWST), each at that time was expected to be in late 2019. However, JWST continues to face delays (it now won’t launch until 2021) and rising costs., so the claim is being made that SST is being ended to save NASA round $25 million in operating costs on a limited science mission in 2020/21.

When the end of the mission does occur, it will leave just two of the Great Observatories operational: the Hubble Space Telescope and the Chandra X-Ray Telescope.

Falcon Heavy Set for 3rd Flight

Monday, June 24th should see the world’s most powerful rocket, the Falcon Heavy, lift-off on its third – and most ambitious to date – flight.

A Falcon Heavy stack – 3 Falcon 9 core stages and an upper stage and payload -roles out of the SpaceX assembly facility adjacent to Pad 39-A at Kennedy Space Centre, en-route to the pad where it will be raised to the vertical position. Credit: SpaceX

Lift-off from Kennedy Space Centre’s Pad 39A is scheduled to be at 23:30 local time (03:30 on Tuesday, June 25th, UT). The rocket will be lifting 24 satellites into three distinct orbits around Earth, a flight that requires the rocket’s upper stage to make four engine burns, the most ever by a SpaceX launch vehicle.

The most notable of these satellites is the US Air Force Space Test Program-2, or STP-2, intended to confirm the Falcon Heavy’s suitability for military launches. The other payloads represent a mix of missions from the NOAA, NASA and academic institutions, representing a mix of missions from weather observation to technology demonstrations. In particular among the latter is the Planetary Society’s LightSail-2 mission. This is the non-profit organisation’s continuing investigations into using sunlight as a means of propulsion in space.

An artist’s impression of LightSail-2 being deployed by Prox-1. Credit: The Planetary Society

A staple of science fiction for decades, the idea sounds simple: deploy a large enough and lightweight enough “solar sail” so it catches the solar wind – the outward flow of particles from the Sun – to push payloads through space.

The Planetary Society has been investigating the potential of solar sails for decades, and in 2015, through public funding, flew their first test mission, LightSail-1. Due to issues with the vehicle, that mission was somewhat foreshortened, although classified a success. Prior to it, the Society attempted to launch the world’s first light sail in 2005, but that came to a sudden end when the launch vehicle was lost.

While small – roughly the size of the loaf of bread – LightSail-2 is an impressive technology demonstrator. It will be carried aloft inside another demonstrator vehicle, Prox-1, and will be deployed roughly a week after launch. It will then coast away from Prox-1 for a between 2 and 4 days while its systems are checked-out, before the order is sent to deploy a wafer-thin, 32 sq metre (344 sq ft) Mylar sail.

This sail is in four sections,  and once deployed, will be used to propel LightSail-2 up to an orbit of around 720 km (447 mi) – greater than that of the International Space Station. If successful, it will be first time sunlight has been used to raise the orbit of a spacecraft in Earth orbit – although it won’t be the first practical sunlight-propelled vehicle. That honour goes to Japan Aerospace Exploration Agency’s (JAXA) IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun) spacecraft, which successfully used a solar sail to perform a fly-by of Venus in 2010.

JAXA and The Planetary Society aren’t the only ones experimenting with solar sales. In 2020/21, NASA will be launching their Near Earth Asteroid (NEA) Scout mission. Designed to be a cubesat payload launched by one of NASA’s lunar missions, NEA Scout, as the name implies, is intended to encounter near earth asteroids and it will do so using a steerable solar sail as its propulsion system.

An artist’s impression of NASA’s solar sail propelled NEA Scout encountering a near-Earth asteroid. Credit: NASA

The force exerted by the Sun when using solar sails is not great, but it is cumulative; provide a big enough area of sail, and quite reasonable velocities could be achieved over time. It’s even been theorised that space-based lasers could be employed to provided focused beams of energy which could propel craft using solar sails from point to point around the solar system, allowing cargoes to be shunted around for a relatively low cost.

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