Kepler-22b: What you need to know

10 December 2011 by Phil Bull

The media is abuzz with news of the discovery of Kepler-22b, an exoplanet that might possibly bear some resemblance to the Earth. This is an exciting result, but have we really found Earth’s twin? Along with two of our resident exoplanet experts, Suzanne Aigrain (who was present at NASA’s announcement of the discovery) and Neale Gibson, we’ve compiled an “essential guide” to our newfound sibling.

Why are people excited about this?

Kepler-22b is the first roughly Earth-sized extrasolar planet we’ve found that definitely orbits inside its host star’s habitable zone. It’s possible that conditions on such a planet might be right for life and, if there are many other such Earth-like planets out there, the odds of life having evolved on at least one of them seem quite high. Discovering life elsewhere would be big news! Speculation on whether this planet could support life is extremely premature (see below), but the discovery of planets that might be relatively hospitable is a step in the right direction.

Kepler-22b resides inside its host star's habitable zone, just as Venus, Earth, and Mars are within the Sun's habitable zone. Credit: NASA

How did they find it?

Kepler is a satellite that stares intently at thousands of stars, measuring minute changes in their brightness over time. If a planet passes in front of a star (“transits”), we see a small dip in its brightness as the planet blots out some light. There are a number of other phenomena that can cause similar dips, so the Kepler team do follow-up observations to make sure that they’ve really seen a planet. This normally involves waiting for the planet to go around its star three times. If they see three dips at regularly-spaced intervals, it’s more likely to be an orbiting planet than some fluke brightness fluctuation. Kepler-22b has an orbital period of 290 Earth days, so it’s been about two and a half years from 22b first being detected to its existence being confirmed.

That’s not by any means all there is to it. As Suzanne explains, “There are literally thousands of targets where they have seen more than three transits, yet they don't publish them as validated planets but as candidates, because there are still many astrophysical false positives (real signals that look like planetary transits but are in fact due to various combinations of 3 or more stars, or even two stars and a bigger planet) that they have not yet excluded. In the case of Kepler-22b and similar systems, they have done a whole series of other ground-based observations, plus observations of one of the transits with the Spitzer space telescope in the infrared, plus some very in-depth modelling that excludes the vast majority of the astrophysical false positive scenarios. That modelling is so complex that it requires the use of one of the worlds fastest supercomputers, that's operated by NASA.”

What’s in a name?

When astronomers discover new objects, they typically give them rather precise, but rather boring names. Kepler-22b is no exception, but Suzanne explains the jargon. “First of all, “b” means it's the first planet candidate found around that target (not the second, as you might guess). By convention, when we find a group of two or more stars and/or planets orbiting each other, we label the stars using capital letters (A, B, ...), and the planets using small letters. The letter "A" always refers to the most massive star in the system, and is often omitted, particularly in cases when there are no other stars known in the system, as with Kepler 22. The letters for planets always start at "b", as we never have a case where a planet is the most massive object in the system.”

“Unlike stars, planets are named sequentially in the order of discovery, not by mass. The number 22 doesn't mean it's the 22nd in the list of targets Kepler is monitoring (that list is over 150,000 entries long), and neither is it the 22nd in the list of targets, in the light curves of which they found transit-like events (that list is over 3,000 entries long). It's the 22nd target for which they have enough information to assert, beyond reasonable doubt, that it really is a planet (beyond reasonable doubt here meaning roughly 99% probability).”

What is its parent star like?

Kepler-22b orbits a star that is broadly similar to our own Sun. It’s a G-class yellow dwarf star, like the Sun, but it’s a little smaller, fainter and older. The star is about 600 lightyears away, in the direction of the constellation of Cygnus, and it’s not very bright as seen from the Earth (11th magnitude). You’d need a fairly decent telescope to find it.

What is it made of?

Broadly speaking, planets fall into two main categories: those made of rock, and those made of gas. Gaseous planets, like Jupiter, tend to be larger, and lots of them have been found to date. Rocky ones, like the Earth or Mars, are trickier to find because they’re smaller and so block out less light when they transit.

It’s hard to directly measure the composition of a planet, but we can make educated guesses based on a number of factors. For example, we know that the radius of the planet is about 2.4 times the radius of Earth, so we can work out its volume. If we knew its mass, we could work out its (average) density: mass divided by volume. If the average density is 1000 kilograms per cubic metre, maybe it’s made of water? If it’s 5000 kilograms per cubic metre, it’s more likely to be rock, and so on. Unfortunately, we don’t know its mass yet! As Neale explains, “We are unable to measure the mass of the planet at the moment, since planets with long orbital periods and low masses like 22b produce the smallest radial velocity signal”.

Can the planet support life?

We don’t know yet! At present, we don’t have a good idea of what conditions are really necessary for life, but it’s reasonable to suggest that having liquid water on the surface of the planet would make a good start. (At the very least, this requires the planet to be rocky rather than gaseous, as it wouldn’t have a surface otherwise.) The range of distances from a star where an Earth-like planet would be able to have liquid water on its surface is called the “Habitable zone”, the size of which depends on the star’s temperature and brightness. Essentially, it’s how far away from the star you can go without conditions being too hot or too cold.

Venus would be much colder if it weren't for its thick atmosphere causing a gigantic greenhouse effect. Credit: NASA.

22b is inside its star’s habitable zone. The Kepler team have calculated that it should have a temperature of 22 degrees C, just right for liquid water - or so you might think. But this estimate depends on what sort of atmosphere the planet has. If the Earth had no atmosphere it would be about -20 degrees C on average, but the greenhouse effect warms it up by trapping some heat. At the moment, we know precious little about 22b’s atmosphere, so the actual surface temperature is unknown; 22 degrees is just an estimate based on the assumption of a mild greenhouse effect, like Earth’s. For some flavour of how much the atmosphere could affect the temperature, consider that Venus would be about -40 degrees C without an atmosphere*, but its thick atmosphere keeps it at a scorching 470 degrees C! So, until we know more about 22b’s atmosphere, we can’t really draw any conclusions about its suitability for life. According to Neale, that might take a while. “Kepler searches for planets around relatively faint stars, and current techniques are far from being able to measure the atmosphere. Future planned space missions such as PLATO and TESS are designed to find similar planets around nearby stars and enable these types of measurements.”

* You might be wondering why a Venus with no atmosphere would be colder than an Earth with no atmosphere - Venus is closer to the Sun than us, so shouldn’t it be warmer anyway? The discrepancy is to do with albedo, or how reflective the planet is. You can see a simple calculation of planetary equilibrium temperatures here.

Is it tidally locked?

Tidal locking is when a body rotates “in step” with another body that it’s orbiting, so that it rotates once on its axis for every orbit. This locking is caused by tidal forces, which you get when one part of a body is subjected to a stronger gravitational pull than another part. The Moon is tidally locked to the Earth, for example, so we always see the same face of the Moon (more or less). If 22b were tidally locked to its host star, one side of the planet would always be in daylight, and the other in the dark. This would make the planet very hot on one side but cold on the other, which wouldn’t make for too pleasant a climate.

Says Neale: “We can't (yet!) measure the rotation of planets themselves, so we can only ever make a guess if they are tidally locked based on their orbital and physical properties. With “Hot Jupiters” (large planets very close to their host stars), the tidal forces are strong and we expect many of them to be tidally locked. For planets that are further away from their host, the tidal forces drop off very sharply, and there is no reason to expect any planet on long period orbits to be tidally locked. In fact it is highly unlikely.” So 22b probably isn’t tidally locked, then.

Image credit: NASA

Categories: exoplanets | aigrain | earth | Kepler | gibson