A weird and wonderful galaxy of exoplanets

The planet-finding space-based Kepler telescope discovered thousands of planets. We are just now getting a glimpse of the scale and variety and, surprisingly, most are quite unlike anything in our own solar system. Phil Dooley takes us on a tour. 

The newly discovered Kepler-10c, a giant, rocky planet 17 times heavier than Earth in an artist's impression. – David A. Aguilar (CfA)

A gargantuan rocky giant version of Earth, a planetary pair being gobbled by a red giant, worlds lit by shimmering auroras – they sound like the creations of a science fiction writer. In fact, they were highlights on a tour of the galaxy’s exoplanets – planets outside our solar system – given by researchers at the American Astrophysical Society meeting in Boston this month.

The rash of new results comes courtesy of Kepler, our dedicated planet-finding satellite. Perhaps most surprising is that the majority of the planets Kepler scientists have discovered are quite unlike anything in our solar system. “We’re finding a much richer diversity of exoplanets than we ever imagined,” says David Latham, of the Harvard-Smithsonian Center for Astrophysics, which made many of the exoplanet discoveries.

It turns out our beloved home planet is a little under-sized. Most exoplanets have volumes somewhere between those of Earth and Neptune, whose radius is around four times greater than Earth’s. And most of these mid-sized planets consist of a thick atmosphere of hydrogen and helium wrapped around a rocky core, earning them the name “gas dwarves”. Despite being the most common kind of planet in the galaxy, our solar system doesn’t have one. “Mother Nature knows how to make ’em, but they didn’t get made around here,” says Latham.

Besides identifying thousands of these gas dwarves, researchers have also found strange and unique worlds that defy our current understanding of how planets form.

Kepler-10c, a giant, rocky planet 17 times heavier than Earth, is one such anomaly. It lies in the constellation Draco, about 564 light-years away, and circles its host star once every 45 days. Kepler deduced the planet’s size based on the degree to which it dimmed the light from its host star as the giant passed in front of it. The next step was to measure its mass, a task that fell to the ground-based telescope, Harps North, which measured tiny wobbles of the host star caused by the orbiting giant’s gravity.

The team thought Kepler-10c would be just another gas dwarf. “We were expecting low density, about two to three times the mass of the Earth.” However the planet turned out to be very dense – like Earth it is made mostly of rock. But the gravity of a rocky planet that size should be able to pull in a large gaseous atmosphere, like Jupiter in our own solar system, and Kepler-10c has none. “Nobody knows how to form a planet of 17 Earth masses and a rocky density without collecting a large gaseous envelope,” says Latham.

Exoplanet researcher Daniel Bayliss from the Australian National University is also perplexed by the finding. “There’ll be a flood of papers from people who study planet formation trying to explain this system – why it didn’t turn into a Jupiter,” he says. “The diversity of planetary systems surprises us every time.”

The solar system that hosts the rocky giant held an even bigger surprise. At 11 billion years old, it formed when the universe was just two billion years old. But the heavy elements from which rocky planets form, such as iron and silicon, were not thought to have been created until another billion years later. “It looks like that just isn’t the way it works,” says Latham. But if rocky planets first formed a billion years earlier than expected, “it pushes back the frontier of when life might have been able to start forming,” he says.

When the inevitable happens, where will Earthlings go? The number
of candidate homes could be a lot smaller than we thought.

As well as providing grounds for conjecture on life’s beginnings, results from the Harvard-Smithsonian Center also give a glimpse of how life on Earth will end. Gongjie Li presented the horrific tale of two planets bound for destruction as their host star, Kepler-56, mushrooms into a red giant.

Stars become red giants when they run out of hydrogen fuel. As the fire in the star’s centre goes out, the core collapses and the pressure and temperature increase until their normally stable helium content begins to burn. This releases an enormous amount of energy which causes the star to expand, swallowing any planets orbiting nearby.

Kepler-56 has already swelled and is now four times the size of our Sun. As it expands further toward the two hapless planets – both in orbits much closer than Mercury – their atmospheres will disappear, their surfaces will melt and they will be pulled into egg shapes by the gravitational forces of the expanding star’s edge. Li has estimated they will be engulfed 130 and 155 million years from now respectively – by astronomical standards imminent events.

The doomed world Kepler-56b is being shredded and consumed by its aging host star. An artist's impression. – David A. Aguilar (CfA)

For Mercury and Venus, it will take another five billion years before our Sun subjects them to the same treatment. Earth may escape – when the Sun goes red giant it may not quite reach our planet. However its surface will be close enough to subject us to the kind of conditions Kepler-56’s inner planets are about to experience.

When the inevitable happens, where will Earthlings go? The number of candidate homes could be a lot smaller than we thought, judging by the findings presented by Ofer Cohen, also from the Harvard-Smithsonian Center.

By far the most common kind of star in the galaxy is the red dwarf, so it makes sense to look to them as potential hosts for Earth-like planets. As red dwarves are cooler than our Sun, the goldilocks zone – the planetary orbital distance just right for liquid water – is closer to the star’s surface.

Cohen decided to check whether the proximity to the red dwarf would have any adverse consequences for an Earth-like planet. The planet he describes isn’t a place you’d want to move to. He found that, despite initially pleasant temperatures, a fearsome stellar wind would buffet any planet in the goldilocks zone, potentially stripping away its atmosphere.

Even if the planet did hold on to its atmosphere, conditions would deteriorate. Just as the Moon is “tidally locked” to Earth so that the same side always faces us, Cohen found the extreme gravitational gradients caused by being so close to the red dwarf would slow the planet’s rotation until one side was locked facing the star. This would leave that side boiling hot while the dark side would be freezing cold. Hurricane-force winds would circle the planet driven by the temperature difference. The only consolation for inhabitants of such a world would be a spectacular light-show in the sky as the powerful solar wind created an aurora 100,000 times stronger than Earth’s.

Despite the pessimistic results for red-dwarf planets, Latham is still entranced by the search for life on other planets – and thinks we are edging closer to being able to detect it. “It’s one of the great questions: are we alone, or is there life elsewhere?” he says. “If life has taken over a planet and has polluted its atmosphere with waste gases, like oxygen, we have actually got a chance of sensing those gases with spectroscopy. We’re making progress towards that goal, we may see it in the next 10 or 20 years. That’s the inspiration that I have.”

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Phil Dooley is a freelance science writer based in Canberra.