close-in exoplanets

All posts tagged close-in exoplanets

Illustration of the hot-Jupiter exoplanet WASP-12b. From https://www.nasa.gov/mission_pages/hubble/science/planet-eater.html.

WASP-12 b is in trouble. A giant ball of mostly hydrogen, the planet circles its star once every 25 hours. The resulting intense stellar irradiation drives super-sonic storms of plasma around the world, and the atmosphere has so much thermal energy, in fact, that some of it is escaping into space. But it gets worse. WASP-12 b is steadily tumbling toward its host star, and astronomers expect that, within a few million years, the star will eat the planet.

WASP-12 b is one of a few hundred hot Jupiters, gas giants very close to their stars, and so far, it’s the only one we have confirmed in a death spiral. Many other hot Jupiters probably are probably also condemned, but how many more can we find perched on the edge of destruction? And, come to think of it, how did the planets find themselves in such precarious positions in the first place? To answer these questions, astronomers need to understand how many hot Jupiters there are out there and how many more are left to be found.

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Jacob Peter Gowy’s The Flight of Icarus. From https://en.wikipedia.org/wiki/Icarus.

Daedalus’ situation was desperate: to keep the secret of the labyrinth, King Minos had trapped its creator, Daedalus, and his son Icarus in the maze with the man-eating Minotaur. In a fit of inspiration, Daedalus crafted two pairs of wings of feathers and wax. As they flew out over the Aegean Sea, Daedalus admonished Icarus not to fly too close to the Sun or else the wax holding the feathers would melt.

Famously full of flawed physics, this fabulous fable foreshadows the forbidding future for a planet newly discovered using data from NASA’s TESS Mission. Dubbed a “warm Jupiter”, TOI-3362 b swings around its host star every 18 days. Like Jupiter, it is a gas giant planet (with a mass five times greater than Jupiter’s), but, unlike Jupiter, It sweeps past its host star every 18 days on a wildly elongated orbit that flash-heats the planet. And like Icarus, TOI-3362 b will one day plunge disastrously into the sea (although in this case, it’s not the warm Aegean but a sea of molten plasma).

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The TESS Spacecraft.

At our journal club last week, we discussed the discovery of a new warm Jupiter from the TESS Mission.

TESS is the successor to the wildly successful Kepler/K2 Mission and is designed to find exoplanets using the same technique as Kepler – looking for their shadows as planets pass in front of their host stars, i.e. the transit technique.

Sadly, the Kepler spacecraft was officially shut down two weeks ag0 because it ran out of fuel, but TESS, launched last March, is off and running, having already discovered about half a dozen new planets.

One of those planets, we discussed in journal club on Friday – a planet orbiting the star HD1397. The gas giant planet is about the same size as Jupiter but half the mass, making it significantly less dense than Saturn.

The planet also has an unusually eccentric or stretched-out orbit that swings very near its host star, passing to within 8 stellar radii from its star at its closest point. By contrast, the Earth is 200 stellar radii away from the Sun.

If this planet had been discovered 20 years ago, it would have completely stumped astrophysicists, and many would likely have doubted its existence. Nowadays, though, such strange planets are practically the norm in exoplanet astronomy.

So with HD1397 b’s discovery, the exoplanet train rumbles on, and we should expect thousands upon thousands more bizzarities from TESS that will, like Kepler’s discoveries, again re-write the planetary rulebook.


At our research group meeting, we also discussed the art of scientific presentations. I’ve pasted the example presentation I gave below.

Artist’s impression of planet alignment in 2016. From here.

Anyone who’s done some stargazing has probably noticed that the Sun and the Moon appear along nearly the same arc in the sky. This Sun’s arc, called the ecliptic, corresponds to the plane of the Earth’s orbit. Since all planets in the solar system share nearly the same orbital plane, they likewise hew close to this arc. It turns out that the ecliptic also coincides closely with the Sun’s equator.

The near alignment of all planetary orbits in the solar system is one of the most important clues to their formation – the solar system originated billions of years ago from a thin disk of gas and dust girding the young Sun’s belly like a hula hoop, an idea going back at least to Immanuel Kant in the 1700s called the Nebular Hypothesis.

Once it was accepted, this idea was so successful at explaining and predicting features of the solar system, astronomers believed all planetary systems in our galaxy would resemble our own – with small, rocky planets close to their stars and large, gassy planets farther away, but all sharing the same orbital plane.

The discoveries of thousands of exoplanets have turned all that on its head – planets around other stars have orbits oriented every which way. For example, the Upsilon Andromeda system has three Jupiter-like planets, all on orbits that are widely misaligned.

Although these topsy-turvy planetary orbits were initially puzzling, astronomers are starting to tease out the explanations for these systems. Planets probably do start out in well-aligned orbits, but, like kids in the backseat on a long car trip, jostling between the planets (due to mutual gravitational tugs) soon upsets this delicate arrangement and upends the orbits. In the case of Upsilon Andromeda, planets may even have been ejected from the system.

A recent study from Fei Dai and colleagues explored connections between orbital misalignment and the origins of one puzzling class of exoplanet – small, short-period planets. These planets range in size (and probably composition) from Neptune-like to smaller than Earth but inhabit orbits very close to their host stars, some taking only hours to circle the star. Many of these short-period planets also have sibling planets farther out, and the arrangement of these orbits might tell us how the planets got so close to their stars.

As for the Upsilon Andromeda system, the mutual inclination between the orbits, if its big, may point to a history of violence in the system. Such violence may explain how the short-period planets got so close to their stars – they could have started out far away and been thrown by their siblings toward the star. By contrast, a small mutual inclination could mean the system has always been relatively quiescent, and the short-period planets may have gently migrated inward from farther out.

By analyzing the transit light curves of the planets as observed by the Kepler spacecraft, Dai and colleagues found a pattern in the mutual inclinations for these systems. From their paper, the figure below shows that when the distance of the shortest-period planet in a system a/R* is larger, the mutual inclination ΔI between orbits tends to be less widely distributed.

Figure 3 from Dai et al. (2018).

What does this result mean? Since the short-period planets closest to their stars (small a/R*) also seem to have a very wide range of mutual inclinations, maybe they experience the same kind of gravitational jostling that took place in Upsilon Andromeda, while planets farther out, they were moved in more gracefully.

Taking a wider perspective, evidence is mounting that, while planetary systems are common in the galaxy, our own solar system is unique in many ways – there’s really no place like home.