Star Wars planets migrate into position around stellar pairs

One of Kepler’s most exciting discoveries was proving the existence of circumbinary planets: planets that orbit two stars, which are themselves bound together by gravity in an often-tight orbital dance (just like Luke Skywalker’s home planet of Tatooine i

The giant planet Kepler-34b orbits round two stars. Now that’s just greedy. Image: David A. Aguilar

Planetary science is beginning to catch up with science fiction. Since the launch of the Kepler space telescope in 2009, a deluge of planets outside of our solar system has been found, with many oddball, exotic worlds among them. One of Kepler’s most exciting discoveries was proving the existence of circumbinary planets: planets that orbit two stars, which are themselves bound together by gravity in an often-tight orbital dance.

Luke Skywalker’s home planet of Tatooine – invented by George Lucas' for the Star Wars series – was envisioned to exist in this kind of binary system. Now, using computer models, a team from Bristol University has shed more light on how this kind of planet was formed.

In the beginning

Planetary scientists are in general agreement that planets form inside a thin, gaseous disk surrounding nascent stars. Within this disk, solid particles (evocatively named “dust”) collide and progressively grow to asteroid-sized bodies. These bodies, called planetesimals, are the essential bricks of planet formation. Further collisions among them build protoplanets – rocky, Earth-sized bodies.



Giant planet accumulating its atmosphere. Image: Stefano Meschiari




Further out from the central star, water and other compounds “freeze out” and become part of the solid component. Past this so-called “ice line”, protoplanets can grow even larger and amass thick, massive atmospheres. This sharp divide between small, Earth-sized planets close to the central star (Mercury to Mars) and giant planets further out (Jupiter to Neptune) is easily recognised in the Solar System.

For this theory to work, it demands an incredible feat: growth from microscopic dust particles more than a hundred times smaller than a grain of sand, all the way to Jupiter-sized objects. It is a very delicate process, involving many physical mechanisms, some of which are still poorly understood today.

Double trouble

One sticking point is the stage in which planetesimals collide. Planetesimals need to collide surprisingly gingerly in order to come together; smash them too fast, and they will break into smaller rocks. Regions with high-speed collisions are sterile for planet formation, as no further growth can occur. This is why the recent discovery of circumbinary planets had astronomy theorists raise an eyebrow (or two).

There are few environments more violent than a binary star system. In the early stages of planet formation, the powerful gravitational perturbations around two stars should lead to destructive collisions that grind down the material.

And yet, all circumbinary planets discovered so far orbit very close to their parent binary stars. So close, in fact, that if they were any closer, their orbit would be destabilised to the point of ejection from the system or collision with one of the two stars. This is because the stars, moving along on their orbit, tug and perturb the planet with their gravity from different directions. Inside this unstable region, then, no planet could survive for long.

The Bristol University team devised sophisticated computer simulations of the early formation stages of the giant circumbinary planet Kepler-34(AB)b in order to better understand its birth environment. Their models found that at the current location of the planet, impacts between planetesimals would always be catastrophically destructive. Only far away from the gravitational pull of the two stars can we expect collision speeds to be low enough for planet building to occur.

Explaining giant planets in a location where they should never have been able to form requires invoking an old idea: that of planetary migration.

The first giant planet discovered outside of our solar system, 51 Peg b, orbits its parent star closer than Mercury does our sun. It is impossible for such a planet to form so close to its star, as the high temperatures would eliminate the rocks and ices before they could come together. Theorists quickly understood what had occurred early on in this system’s history: the planet probably formed further away from its star and subsequently migrated closer to it.

The picture, then, becomes clearer: Kepler-34(AB)b must have formed far from the two stars, in a more tranquil environment, and later migrated to its current location. Several computer models have shown this idea to be feasible, and compatible with our observations. The same models also help us understand why all the circumbinary planets found have been relatively small.

This had puzzled planetary scientists because normally the bigger a planet is, the easier it is to detect. But now we know that this isn’t the case: we would not observe such planets simply because they did not survive their turbulent beginnings. Simulations of planetary migration show that Jupiter-sized circumbinary planets end up strongly interacting with the gravitational field of the stars and are subsequently flung out from the system.

Although there are still many details to be worked out, this theoretical framework appears to be in step with Kepler’s discoveries so far. But yet-to-be-made planetary discoveries are bound to surprise us in the near future.

Stefano Meschiari does not work for, consult to, own shares in or receive funding from any company or organisation that would benefit from this article, and has no relevant affiliations.

The Conversation

This article was originally published at The Conversation. Read the original article.

Stefano Meschiari is a W J McDonald Postdoctoral Fellow at the University of Texas.

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The Tories are the zombie party: with an ageing, falling membership, still they stagger on to victory

One Labour MP in Brighton spotted a baby in a red Babygro and said to me: “There’s our next [Labour] prime minister.”

All football clubs have “ultras” – and, increasingly, political parties do, too: although, in the case of political parties, their loudest and angriest supporters are mostly found on the internet. The SNP got there first: in the early days of email, journalists at the Scotsman used to receive bilious missives complaining about its coverage – or, on occasion, lack of coverage – of what the Scottish National Party was up to. The rest soon followed, with Ukip, the Labour Party and even the crushed Liberal Democrats now boasting a furious electronic horde.

The exception is the Conservative Party. Britain’s table-topping team might have its first majority in 18 years and is widely expected in Westminster to remain in power for another decade. But it doesn’t have any fans. The party’s conference in Manchester, like Labour’s in Brighton, will be full to bursting. But where the Labour shindig is chock-full of members, trade unionists and hangers-on from the charitable sector, the Conservative gathering is a more corporate affair: at the fringes I attended last year, lobbyists outnumbered members by four to one. At one, the journalist Peter Oborne demanded to know how many people in the room were party members. It was standing room only – but just four people put their hands up.

During Grant Shapps’s stint at Conservative headquarters, serious attempts were made to revive membership. Shapps, a figure who is underrated because of his online blunders, and his co-chair Andrew Feldman were able to reverse some of the decline, but they were running just to stand still. Some of the biggest increases in membership came in urban centres where the Tories are not in contention to win a seat.

All this made the 2015 election win the triumph of a husk. A party with a membership in long-term and perhaps irreversible decline, which in many seats had no activists at all, delivered crushing defeats to its opponents across England and Wales.

Like José Mourinho’s sides, which, he once boasted, won “without the ball”, the Conservatives won without members. In Cumbria the party had no ground campaign and two paper candidates. But letters written by the Defence Secretary, Michael Fallon, were posted to every household where someone was employed making Trident submarines, warning that their jobs would be under threat under a Labour government. This helped the Tories come close to taking out both Labour MPs, John Woodcock in Barrow and Furness and Jamie Reed in Copeland. It was no small feat: Labour has held Barrow since 1992 and has won Copeland at every election it has fought.

The Tories have become the zombies of British politics: still moving though dead from the neck down. And not only moving, but thriving. One Labour MP in Brighton spotted a baby in a red Babygro and said to me: “There’s our next [Labour] prime minister.” His Conservative counterparts also believe that their rivals are out of power for at least a decade.

Yet there are more threats to the zombie Tories than commonly believed. The European referendum will cause endless trouble for their whips over the coming years. And for all there’s a spring in the Conservative step at the moment, the party has a majority of only 12 in the Commons. Parliamentary defeats could easily become commonplace. But now that Labour has elected Jeremy Corbyn – either a more consensual or a more chaotic leader than his predecessors, depending on your perspective – division within parties will become a feature, rather than a quirk, at Westminster. There will be “splits” aplenty on both sides of the House.

The bigger threat to Tory hegemony is the spending cuts to come, and the still vulnerable state of the British economy. In the last parliament, George Osborne’s cuts fell predominantly on the poorest and those working in the public sector. They were accompanied by an extravagant outlay to affluent retirees. As my colleague Helen Lewis wrote last week, over the next five years, cuts will fall on the sharp-elbowed middle classes, not just the vulnerable. Reductions in tax credits, so popular among voters in the abstract, may prove just as toxic as the poll tax and the abolition of the 10p bottom income-tax rate – both of which were popular until they were actually implemented.

Added to that, the British economy has what the economist Stephen King calls “the Titanic problem”: a surplus of icebergs, a deficit of lifeboats. Many of the levers used by Gordon Brown and Mervyn King in the last recession are not available to David Cameron and the chief of the Bank of England, Mark Carney: debt-funded fiscal stimulus is off the table because the public finances are already in the red. Interest rates are already at rock bottom.

Yet against that grim backdrop, the Conservatives retain the two trump cards that allowed them to win in May: questions about Labour’s economic competence, and the personal allure of David Cameron. The public is still convinced that the cuts are the result of “the mess” left by Labour, however unfair that charge may be. If a second crisis strikes, it could still be the Tories who feel the benefit, if they can convince voters that the poor state of the finances is still the result of New Labour excess rather than Cameroon failure.

As for Cameron, in 2015 it was his lead over Ed Miliband as Britons’ preferred prime minister that helped the Conservatives over the line. This time, it is his withdrawal from politics which could hand the Tories a victory even if the economy tanks or cuts become widely unpopular. He could absorb the hatred for the failures and the U-turns, and then hand over to a fresher face. Nicky Morgan or a Sajid Javid, say, could yet repeat John Major’s trick in 1992, breathing life into a seemingly doomed Conservative project. For Labour, the Tory zombie remains frustratingly lively. 

Stephen Bush is editor of the Staggers, the New Statesman’s political blog.

This article first appeared in the 01 October 2015 issue of the New Statesman, The Tory tide