Two papers were published today in Nature, each independently revealing that a planet discovered by the Kepler mission is the closest thing we’ve found yet to another Earth. But don’t pack your bags just yet — this new “Earth” is certainly not the habitable world we’ve all been waiting for.
A brief history of exoplanets
The search for planets around other stars is one of the most exciting fields of modern research, and cuts to the core of one of the oldest questions known to humanity — “are we alone?”.
The first planets discovered orbiting sun-like stars, in the mid 1990s, led to an immediate revolution in our understanding of planetary science and planetary formation. Before the discovery of 51 Pegasi b, in 1995, models of planet formation predicted that planets we found around other stars would be much like our own.
It was a shock, then, when the first exoplanets, such as 51 Pegasi b, turned out to be “Hot Jupiters”, giant planets orbiting their stars at a tiny fraction of the distance from Earth to the sun.
Those first discoveries revolutionised our understanding of planet formation, and of the variety of planetary systems we expect to find elsewhere. The surprises have kept coming ever since — from planets with two suns to diamond planets, and many, many more.
Ever smaller, ever colder
As time has passed, we have become ever more proficient at finding exoplanets. We have found planets at ever greater distances from their host stars. And we have found ever-smaller planets, which are significantly harder to detect.
The methods we use to look for planets are biased toward finding larger ones. The radial velocity method measures the wobble of a given star along our line of sight as it rocks back and forth in response to the gravitational pull of its unseen planetary companion. This tells us the planet’s mass. The closer the planet to its host, or the more massive the planet compared to its host, the larger the wobble will be.
The bigger the planet, the more of its host star’s light it will block as it passes between us and that star. The shorter the planet’s orbital period, the more frequently it will pass between us and the star, and so the more data we will be able to obtain to show that the planet is there.
The Kepler mission has helped to push us remarkably close to the goal of finding truly Earth-like planets, and it is almost certain that such planets lurk amongst the 3602 candidate planets awaiting confirmation.
Today’s discovery takes us one step further down the road towards the first true exo-Earth.
Kepler 78b - delving into the nature of a tiny transiting planet
The discovery of an Earth-sized planet on a very short period orbit around the star Kepler 78 was announced a couple of months ago. The size of the planet made it a fascinating target for further study, and two groups of astronomers began independent programs of radial velocity observations of the planet’s host star. The results of those studies are published today.
The two disparate measurement methods — transit and radial velocity — can each tell us something about the planet in question, but they are insufficient to give us the full picture. But if it possible to observe a given planet using both techniques, then suddenly things become far more exciting.
Once you have both the size of the planet and its mass, you can calculate its density — and at that point, you can tell whether it is gaseous, rocky, metallic.
This is the game that the astronomers have played with the planet orbiting the star Kepler 78. We now know that Kepler 78b is slightly bigger than Earth — one of the two papers gives its radius as 1.20 times that of Earth (something like 8080 km); the other gives a radius of 1.173 times that of Earth (approximately 7470 km).
The uncertainties on these measurements are relatively small, but they are mutually consistent (the uncertainties overlap) — so we can say with some certainty that Kepler 78b is a planet that is only slightly larger than our Earth.
A remarkably Earth-like world
The two groups whose work is published today carried out intensive radial velocity observations of the star Kepler 78 using the HARPS-N instrument (on La Palma, in the Canary Islands) and the HIRES spectrometer on the 10 metre Keck-I Telescope (which sits on top of the Hawaiian island of Mauna Kaea). The use of the world’s best facilities have allowed the authors to measure the tiny radial velocity variations in the light from Kepler 78, allowing them to precisely determine the mass of the Kepler 78b.
Again, the two papers offer slightly different values for the planet’s mass — 1.69 or 1.86 times that of the Earth. Again, given the size of the uncertainties in these measurements, they are entirely consistent with one another — and they reveal that Kepler 78b is not significantly more massive than our Earth. A super-Earth it is, but one not too dissimilar to our own planet — at least in terms of its mass and its size.
The similarities between the two planets go deeper. Based on the masses and radii determined in these works, they both calculate the density of the planet. Again, their final values differ slightly — with values of 5.3 and 5.57 grams per cubic centimetre. Remarkably, these values are essentially the same as the density of the Earth — 5.52 grams per cubic centimetre.
Because Kepler 78b is more massive than the Earth, you might expect the gravitational pull at its surface to be stronger than that we experience here on Earth. However, because the planet is also larger than our Earth, it turns out that the acceleration due to gravity at its surface is startlingly similar to that we experience here. One of the two papers calculates that, on the planet’s surface, you would experience an acceleration of around 11.4 metres per second squared (just 1.16 times greater than that at Earth’s surface).
Earth-like, if Earth was on fire
Physically, then, Kepler 78b seems remarkably similar to our Earth — it is slightly more massive, slightly larger, and the gravitational pull on its surface is only slightly stronger than that we experience here. There, unfortunately, the similarities end.
The Earth takes just over 365 days to orbit the Sun, at a distance of roughly 150 million kilometres. By contrast, Kepler 78b takes just eight and a half hours to orbit its host, at a separation of less than one and a half million kilometers.
Kepler 78b orbits less than a million kilometers above the photosphere of its star. As a result, the surface temperature of our Earth-like planet is estimated in one of the two new papers to lie in the range 1500 - 3000 K (~1230 - 2730 degree Celsius).
To put this intense heat in perspective, one only has to look at the temperatures at which given elements melt and boil.
Astronomers often discuss how the Venutian surface would be hot enough to melt lead. On Kepler 78b, even the “coldest” estimate of the temperature would be sufficient to melt gold, silver, and copper. At the high end of the temperature range, copper and silver would boil, and gold wouldn’t be far off.
Kepler 78b orbits so close to its host star that it is almost certainly locked in what astronomers call a “1:1 spin-orbit resonance”. In other words, the planet almost certainly keeps one face pointed towards the star, the other facing away. From one side of Kepler 78b (if one exists), its parent star will sit stationary, spanning a 40 degree swathe of the sky. From the other, the star will never rise.
A watershed moment
So Kepler 78b isn’t the exo-Earth we’ve been looking for, the twin of our own planet so dear to science fiction. But it is a vital step on the way. It is by far the most Earth-like planet found to date — almost the right size, and with almost the right composition, but located on an orbit which ensures it is about as hellish as possible.
In fact, aside from its orbit, the similarities with the Earth are striking — and to find a planet like this so soon as astronomers plough through the reams of data provided by Kepler is a hugely promising sign.
Generations have dreamt of discovering other Earths around distant stars — and these observations of Kepler 78b show us that dream is tantalisingly close to being realised.