Space Sunday 30 years with HST; planets in lockstep

Hubble’s 30th anniversary image: a colour-enhanced view “Cosmic Reef” showing two nebulae – star forming regions – the blue NGC2020 (actually material ejected from a single, massive star 200,000 times brighter than our own), and the red NGC 2014. Both are part of the Large Magellanic Cloud, a satellite galaxy to ours, 163,000 light-years away. The clarity of the image reveals the star-forming region of HGC 2014 in stunning detail, and offers testament to the deep field imaging capabilities of the Hubble Space Telescope. Credit: NASA / ESA.

On April 24th, 1990, the Space Shuttle Discovery thundered into a spring Florida sky on one of the most important missions of the entire space shuttle programme: the launch of the Hubble Space Telescope (HST), one of the four great orbital observatories placed in orbit in the closing years of the 20th century.

At the time of its launch, the telescope probably didn’t surface to any great degree in the broader public consciousness, although in the 30 years it has been in operation it has become if not a household name, then certainly one most people will recognise, even when abbreviated down to just “Hubble”.

April 24th, 1990: space shuttle Discovery rises from Lunch Complex 39B at Kennedy Space Centre, carrying aloft the Hubble Space Telescope. In the foreground can be seen the external tank and a solid rocket booster attached to the shuttle Columbia, sitting on pad 39A, as it awaits its own launch date. Credit: NASA

As I noted when marking 25 years of HST operations, Hubble’s roots go well back in history  –  to 1946, in fact;  while the whole idea of putting a telescope above the distorting effects of the Earth’s atmosphere can be traced back as far as the early 1920s. A joint NASA / European Space Agency operation, HST faced many challenges along the road to commencing operations: it’s low Earth orbit – vital for it to be within reach of servicing astronauts – meant it had to face bot extremes of temperature as it orbited the Earth, passing in and out of sunlight, and it would also have to contend with a slow but inexorable atmospheric draft, so would have to be periodically boosted in its altitude.

However, these issues paled into insignificance after HST was launched, when the commissioning process revealed something was badly wrong with the telescope’s optics, resulting in badly blurred images being returned to Earth. The problem was traced back to an error in the production of the 2.4m primary mirror – one side of which has been ground an etra 2.2 nanometres (a nanometre being one billionth of a metre) compared to the other, leaving it “out of shape”. Small as the error was, it was enough to prevent Hubble focusing correctly, leading to the blurred images  –  and the entire programme being seen as a huge white elephant around the world, despite HST completing some excellent science between 1990 and 1993.

Before and after: on the left, an image of the spiral galaxy M100 taken on on November 27th, 1993, without the corrective optics and camera system. On the right, M100 imaged by Hubble on December 31st, 1993, after the installation of the corrective optics and camera system

Again, as I reported five years ago, the optical error lead to a “Hubble rescue mission” in 1993, when the crew of the space shuttle Endeavour arrived to give HST corrective optics called COSTAR and an updated imaging system, the Wide Field and Planetary Camera (WF/PC). Together these effectively gave HST a corrective set of glasses that overcame the flaw in the primary mirror. In doing so, they assured Hubble’s place in history, as they allowed the telescope to exceed all expectations in its imaging capabilities, turning into into perhaps the most successful astronomical / science instrument of modern times.

When launched, HST could see both in the visible light and in the ultraviolet (the region in which it saw outstanding results even before the operation to correct its “eyesight”). In 1997, during another servicing mission which saw the Discovery return to the telescope it had launched and deployed, HST was given a set of infra-red eyes as well. These allowed it to see farther into space (and thus further back in time) than we’d been able to do previously, and they allowed Hubble to peer into the the dusty regions of the galaxy where stars are born, opening their secrets.

A HST image released on April 6th 2020 showing the barred spiral galaxy NGC 2273, some 95 million light years from our own. It is unusual because it comprises two arms extending from a central bar made up of densely-packed stars, gas, and dust, and which conceal a second set of spiral arms within them, giving the galaxy two pairs of curved arms. Credit: NASA / ESA

Together, Hubble’s various eyes and its science instruments – and the men and women supporting HST operations here on Earth – have given us the ability to look back towards the very faintest – and earliest – light in the cosmos, study star clusters, look for planetary systems around other stars, increase our understanding of our own galaxy, look upon and study our galactic neighbours, help to verify Einstein’s theories of the universe, and do so much more.

Before Hubble, we knew essentially nothing about galaxies in the first half of the life of the universe. That’s the first 7 billion years of the universe’s 13.8-billion-year life. Now Hubble, through remarkable surveys like HXDF [Hubble Extreme Deep Field] capability, has probed into the era of the first galaxies. Through this type of work, Hubble has discovered galaxies like GN-z11, the most distant discovered by Hubble. Just 400 million years after the Big Bang, Hubble is looking back through 97% of all time to see it, far outstripping what can be done with the biggest telescopes on the ground.

– Garth Illingworth, HST project scientist

The Hubble Space Telescope with the aperture door open to allow light into the optics, as seen from the space shuttle Columbia during the 2002 servicing mission

Hubble is a truly unique platform in this regard. Despite issues over the years such as with its various flywheels (the gyroscopes designed to hold it in place whilst it is capturing images), it can remain rock-steady for extended periods with no more than 0.007 arcseconds of deviation. To put this into context, that’s the equivalent to someone standing at the top of The Shard in London and keeping the beam of a laser pointer focused on a penny taped to the side of the Eiffel Tower in Paris, for 24 hours without wavering.

HST’s science mission is so broad, it occupies the working days of literally thousands of people around the globe. Dedicated teams manage the programme for both NASA and ESA, with the Space Telescope Science Institute (STScI) located at the Johns Hopkins University Homewood Campus in Baltimore being the primary operations centre, supported by the European Space Astronomy Centre (ESAC), Spain, both of which will manage operations with the James Web Space Telescope when it is launched. Beyond these teams, scientists and astronomers around the globe can request time using HST and its instruments for their projects and observations, all of which makes the telescope one of the most used globally.

A visible light image of Jupiter captured by Hubble composited with an ultraviolet image of the planet’s northern aurora. Credit: NASA / ESA

Many of those currently working with Hubble share a unique link to it: they have either grown up with it as a part of their lives, learning about it at school and through astronomy and science lessons, or they been with Hubble since its launch, and have lived their entire careers with it.

Hubble has changed the landscape of astronomy and astrophysics,. It has far exceeded its early goals — no other science facility has ever made such a range of fundamental discoveries. It’s been a privilege to be associated with this effort that has become embedded in the culture of our time.

– Colin Norman, HST manger and senior manager, STScI (1990-2020)

In that time, Hubble has been responsible for 1.4 million observations of over 47,000 celestial objects. It has produced over 164 terabytes of data and leading to over 17,000 scientific papers that include many exciting new discoveries, such as the supermassive black holes lurking in the centres of galaxies. It has also been directly responsible for the awarding of one Nobel Prize when in 2011, Hubble scientist Adam Riess of the Space Telescope Science Institute (STScI) was awarded the prize for his part in observations that the universe’s expansion is accelerating, suggesting the presence of mysterious dark energy.

Part of Hubble’s longevity is due to the five servicing and altitude-boosting visits it received from the space shuttle. The last of these came more than a decade ago, in 2009. That mission was actually initially cancelled in the wake of the 2003 loss of the shuttle Columbia, a mission to HST being viewed as being too risky by the now risk-adverse NASA management on account of the difficulties involved in mounting any rescue mission should the shuttle leading the mission ran into difficulties.

However, and in contrast to the lambasting of NASA over the original imaging issues that marred Hubble’s initial time in space, there was a massive public outcry over the decision – and in 2005, the newly-appointed NASA Administrator, Michael Griffith reversed the decision, agreeing with the public and the science community that a last servicing mission – one that could leave HST in a near-perfect operating status and boosted to a higher altitude as could be achieved – was more than worth the risk.

As it is, Hubble remains operational and is seeing huge demand for time with its instruments. Thanks to that 2009 servicing mission and careful planning by both STScl and ESAC, and barring unforeseen failures, the telescope should be able to continue its observations well into the 2030. Which is just as well, really, given much of its work was supposed to have been handed over to the James Webb Space Telescope by now – and that probably will not launch much before late 2021, if not later than that.

So, happy anniversary, Hubble, and here’s to many more!

Six Planets Dancing in Lockstep

To date, astronomers have confirmed the existence of 4,152 extrasolar planets in 3,077 star systems. While the majority of these discoveries involved a single planet, several hundred have been found to be multi-planetary, although systems that contain six planets or more seem to be rare, with less than a dozen discovered to date, including TRAPPIST-1 (see: The 7-exoplanet system) and Kepler 90 (see: The 8-exoplanet system and AI).

Another of these systems, with six planets, lies some 88 light years from Earth, around a star designated HD 158259, which has been the subject of extensive study for seven years using the ground-based SOPHIE spectrograph. Now that data has been combined with that gathered on the system by NASA’s Transiting Exoplanet Space Satellite (TESS). It has confirmed the presence of the six planets, and the fact they have some very unique properties.

An artist’s impression of the HD 158259 system. This six planets orbit their primary in an almost perfect 3:2 orbit resonance, one to the next. Credit: unknown

The system comprises one Earth-type solid planet, roughly twice as big as Earth, and five “mini-Neptune” gas planets, each about 6 times the size of Earth. This makes them remarkably similar in form, if not composition.  Their parent star is remarkably similar to our own, being a G0 main sequence star of a slightly greater mass (1.08) and luminescence (1.21). However, none of the planets is liable to be a particularly pleasant place, as they all occupy orbits exceptionally close to their parent, the outermost being just 0.38 times as distant as Mercury is from the Sun. Thus, all are probably tidally locked to the star, with one face constantly baked in searing heat and radiation.

But what makes the system extraordinary is that all six planets share the orbit resonance one to the next: a slightly off-perfect 3:2.The innermost, solid planet completes and orbit around the star every 2.17 terrestrial days, and the second in 3.4 days, and the rest 5.2, 7.9, 12 and 17.4 days respectively.

A 3.2 orbit resonance is quite common; it means that for every three orbits one planet completes around its star, the next one out from it will complete two. But here the resonance is slightly off: 2.17 x 3 and 3.4 x 2  (the first two planets) don’t quite match up as a perfect 3:2 resonance; nor does 3 x 3.4 and 2 x 5.2 (for the 2nd and 3rd planets), and so on. Thus the six planets of the system are like six musicians playing the same piece of music, but the arrangement each has is slightly off compared to the next so that they don’t all quite reach the next bar at the same time, resulting in the music played being slightly off.

This deviance from a “true” 3:2 resonance between succeeding pairs of the planets has been taken as a possible confirmation of the planetary migration hypothesis. This is an attempt to explain why many exoplanet systems have massive gas giants – as big as or bigger than Jupiter – orbiting so close to their primary. Stellar and planetary evolution as it has been understood means such massive worlds shouldn’t be able to form so close to their parent star, both the lack of material and the overwehelming gravity so close to the star should either prevent or destroy them before they can form to such a size.

Instead, the planetary migration hypothesis suggests that such planets actually formed much further from their parent stars, but then over time drifted closer and closer to their star. The HD 158259 system give weight to this hypothesis because the slightly off 3:2 resonance suggests that all six planets likely all formed further away from their parent star, but gravitational attraction drew them gradually closer and into synchronicity, trapping them in lockstep. However, continuing perturbations in their individual orbits has since pulled them out of an absolutely perfect 3:2 resonance, leaving them as we observe them today.

This idea still has yet to be verified in some way, but even without a perfect 3:2 resonance, such is the regularity of orbit they do share one to the next, it is likely astronomers will be able to use it to understand the composition and structure of each of the planets and offer insights into how this and other multi-planet systems containing planets of roughly equitable size / mass orbiting close to their parent stars may have formed.