Kepler Discovers New Planetary System and Announces Over 1200 Planet Candidates

The Kepler spacecraft started doing its work in May 2009, continuously watching the stars contained in a patch of sky with the hope of being able to discover exoplanets. Just recently, the Kepler team had a very exciting set of announcements resulting from this diligent work.

An artist’s conception of the Kepler-11 system.
Image: NASA/Tim Pyle

Comparison of Kepler-11 orbit sizes
and our Solar System’s orbit sizes.
Image: NASA/Tim Pyle

One of these was the discovery and confirmation of a newly found planetary system with six confirmed planets. Their star, Kepler-11, is a sun-like star, located about 2000 light years away from Earth. This star is only the second known so far to host multiple planets outside of our solar system, and now holds the record for hosting the most known planets. Plus, all of its planets have extremely tight orbits. The largest orbit is smaller than that of Venus in our solar system, while the other five orbits are even smaller than that of Mercury. This results in the most compact planetary system yet discovered in addition to the largest planetary system by number. Furthermore, the planets are bigger than the Earth, with the largest planet comparable to the size of Uranus and Neptune in our solar system. Still five of the six newfound planets are among the eight smallest extrasolar planets found so far. These interesting facts and characteristics of the system also lead to some clues about the formation and the dynamics of the system.

Kepler detects planets by measuring for drops in the light of their stars as they pass across, called the transit method (This method is detailed further in this previous post about Kepler). Measuring the changes in how much the light drops, at what times, and in what patterns can allow astronomers to calculate characteristics of the planet like size, distance from the star, and number of planets. Since the planets are located extremely close to each other in this particular system, five of the six planets produce significant perturbations on each others orbits that are measurable by Kepler. As they are detected, they will let the researchers calculate estimates for the masses of these planets. Further transits in the future will allow the estimates to be further refined. Adding this information with the sizes of the planets leads to the density of the planets that in turn could allow other researchers to hypothesize about the makeup of the planet. The densities of the planets in the Kepler-11 system appear low, suggesting that they are gaseous planets composed of light elements. These would be similar to planets like Neptune in our solar system rather than terrestrial planets like Earth. The conclusions also give suggestions about the formation of the planetary system. Presence of a large amount of light gas likely means that these planets were formed early in the history of their system. Finding out more about the formation of other planetary systems, such as this one, could, in turn, lead to valuable realizations about our own system.

Locations of Planet Candidates in Kepler FOV
Image: NASA/Wendy Stenzel

Yet, probably the more important announcement could be that in addition to these confirmed planets, Kepler has found over 1200 other planet candidates. These are discoveries that result in data that may be similar to that produced by the presence of an exoplanet, but needs to be verified and confirmed. The amount of actual planets from the list will almost definitely be less, but, more importantly, by how much? Some analyses are suggesting that about 80 to 90 percent of the objects on this preliminary list could be planets. This means that the amount of exoplanets discovered (currently about 530) could go up very significantly, by around 1000 (or about 200% of the current number) if the early analyses end up being accurate. Plus, in that preliminary list of planets, 54 are located in the habitable zone of their stars. This zone includes the orbits where a planet could potentially hold liquid water. The radius of one of these is about 0.9 times the size of the Earth’s, while the radii of four others are less than two Earth radii. A planet more Earth-like than any discovered so far may be among these data.

Results like this one are hugely important. They suggest the presence of the hundreds of other star systems, some unique systems like Kepler-11, or perhaps others also like our own solar system.

An Intriguing Planet From Outside the Galaxy

An artist's impression of HIP 13044 and the planet HIP 13044 b.
Image: ESO/L. Calçada

Recently, exoplanets¹ have been discovered at an extremely rapid pace. In just about two decades, astronomers have found and confirmed over 500 planets, with many more waiting to be confirmed. Despite the large number, we're still finding a great amount of new things to be excited about in many of these discoveries.

The telescope used to make the discovery.
Image: ESO/H. H. Heyer

A newly announced exoplanet can claim to be the first one discovered to have originated outside our galaxy. Discovered by the Max Planck Institute in Germany using a 2.2m telescope at the European Southern Observatory in Chile, the planet, HIP 13044 b, orbits around a star, HIP 13044, that is located in the Helmi stream about 2,000 light years from the Sun. This particular group of stars originated from a small satellite galaxy of the Milky Way. The galaxy was later absorbed by the Milky Way about six to nine billion years ago, and gravitational tidal forces subsequently tore it apart and stretched it into a stream of stars. Both the star and planet orbiting around it were likely swept along for the ride.

This particular planet was discovered by the “wobble” method, just as many other exoplanets have been discovered in the past. The host star is studied for a long period of time, and a wobble, found for this star by a doppler shift, indicates a planet gravitationally tugging back on the star as it orbits around. The wobble in this particular case suggests a giant massive planet (similar to Jupiter) orbiting very closely to the star. This is very surprising, since the host star has also already passed through its red giant phase. When a Sun-like star enters the red giant phase, it grows extremely large, increasing its radius by ten to even hundreds of times larger that its original radius. Our Sun is expected to have a radius extending beyond the Earth's orbit when it becomes a red giant in around five billion years. However HIP 13044 b lies very close to its star, inside the area that was likely taken up by the star's red giant size before. This is likely due to the planet migrating inwards from a larger original orbit after the star shrunk, which is very intriguing. Later, the host star is expected to become an asymptotic giant branch² star, and the discoverers of the planet believe it will be devoured by the star at that time.

Furthermore, this exoplanet's host star is very metal-poor, meaning that it does not have many elements heavier than helium. Other exoplanets discovered so far, on the other hand, have had host stars that are at least as metallic as the Sun. HIP 13044, like the other stars in the Helmi stream, has metal content of about 1% of that of the Sun, in a mass that is about equal to the Sun's. This is just simply not a curiosity, but a potential reconsideration of how planets are formed. In the widely accepted core-accretion planet formation model, the matter around a star gradually coalesces to form planets. However, this model requires heavier elements to begin the process of planet formation, by forming the rocky core first. Without a rocky core, a gas giant, like this particular exoplanet, could not be formed since there is not enough mass to retain the gas. There is an alternative model for the formation of giant planets, called the disk instability , where a giant disk of gas around a star breaks off into planet-sized self-gravitating pieces. These pieces eventually each result in a giant planet. The model may be relevant in this case and this exoplanet discovery may provide substantial evidence for the disk instability model, or perhaps may lead to another future model.

HIP 13044 b's discovery continues to show that although exoplanet discoveries may no longer by novel in and of themselves, they still bring forth fresh considerations and interesting ideas.

Footnotes:

1: Short for extrasolar planets, which are planets located outside our solar system. ↵
2: In order to explain the asymptotic giant branch, I should explain the Hertzsprung-Russell (H-R) diagram first. The H-R diagram is essentially a scatter graph of stars plotted by their temperatures and luminosities (some use other related classifications like absolute magnitude, spectral types, etc.). Most stars in the H-R diagram lie on an area that looks like a curved line called the main sequence. There are also some branches that come out of the main sequence. The asymptotic giant branch is one of these branches, and consists of low to intermediate mass stars (about 0.6 to 10 times the mass of the Sun) in the late part of their stellar evolution. ↵

New Exoplanet Could Be First Hospitable to Life

An artist’s rendition of Gliese 581g.
Image: Copyright Lynette Cook

The first Earth-like exoplanet (a planet outside our Solar System has been discovered orbiting in a star’s habitable zone, a range of distances around a planet in which an Earth-like planet can keep and maintain liquid water. Essentially, this could mean that the planet may be hospitable to Earth-like life. The range is often referred to as the “Goldilocks zone” in that it is neither too hot nor too cold for life.

The existence of the planet, Gliese 581g, was recently announced by a team of astronomers from the University of California, Santa Cruz and the Carnegie Institute of Washington. The discovery used date from observations having been collected for over a decade at the Keck Observatory in Hawaii.

The Planet and Consequences for Life

Gliese 581g is one of two planets that was recently discovered orbiting Gliese 58, bringing the the total known planets orbiting the star to six. The star itself is located about 20 light years away from the Earth, and some of its other planets, lying on the edge of the habitable zone, have also been debated to be capable of harboring life. However, life on these planets would likely only be possible if these other planets had certain specific conditions, such as a very thick atmosphere in one case to result in a greenhouse effect that would sufficiently warm up the planet.

On the other hand, Gliese 581g lies very comfortably in the habitable zone. The planet itself is about three to four times the mass of the Earth. This mass likely means that the planet has a definite and rocky surface, and with enough gravity to hold on to an atmosphere. If its density is close to that of the Earth’s, the planet’s radius would be 1.2 to 1.4 times the size of the Earth’s. The gravity on the surface, therefore, would be similar to or slightly higher than on Earth.

Astronomers are estimating that the distance of Gliese 581g from its star is about 0.15 AU (1 AU is the distance from the Earth to the Sun), meaning that it can orbit around its star in a little less than 37 days. In our Solar System, this orbit would be even smaller than Mercury’s, and would make the planet severely hot. However, its star, Gliese 581, is classified as a red dwarf star, making it much cooler than our star, and at this radius, the average surface temperature on the planet is estimated to be between -31 to -12 °C.

This may be surprising in that this temperature is well below freezing, and likely not conducive to life as we know it. However, astronomers have been able to deduce that the planet is also tidally locked to the star. This means that there is one side continually facing the star while the opposing side is continually facing away from the star, just like how the Moon orbits the Earth (we only see one side of the Moon from the Earth). The consequence of this is that one side of Gliese 581g is in perpetual daylight and likely with a very high surface temperature. Meanwhile, the opposing side experiences the opposite treatment, receiving permanent nighttime and a low surface temperature. Life, could like likely only exist in a band between the light and the dark sides of the planet. This has some interesting consequences.

It is possible that lifeforms that prefer warmer temperatures could develop and exist on the brighter, warmer side of the planet, while those preferring a more colder and darker environment could live more towards the darker side. Plus differing longitudes on the planet could result in a wide range of hospitable temperatures. Therefore, despite the potential existence of just a narrow band of life, there could be very diverse ecosystems existing on the planet.

Method of Discovery

The discovery of Gliese 581g comes from 11 years of observations of the star Gliese 581. The team of astronomers working on this project used the HIRES spectrometer installed on the Keck I Telescope at the Keck Observatory in Hawaii. Using this tool, the astronomers were able to make very precise measurements of Gliese 581’s radial velocity. The radial velocity is the velocity of any object, including a star like Gliese 581, in the line of sight from Earth. As a planet orbits around its star, the star pulls on the planet gravitationally to keep it in orbit. The planet also pulls on the star, and this result in the star “wobbling” a little bit. The wobble can be detected from Earth by measuring the radial velocity of a star. Astronomers on Earth usually cannot actually see an exoplanet, but can detect changes in the radial velocity of the star. Studying this wobble allows astronomers to calculate the mass and distance from the star of the planet. Looking at the wobble over time allows astronomers to actually calculate the orbit of the planet.

If there are multiple planets, like in the case of Gliese 581, this technique gets a little more complicated. There is no longer just one pull from a planet, but multiple pulls. These gravitational pulls result in a combined complex wobble of the star. Performing analyses on this complex wobble, allows astronomers to detect the planets, and also calculate their orbits and masses.

This work however, requires many observations and measurements of radial velocity, spaced out over time. Detecting this particular planet took 238 observations, each of which lasted about an entire evening on the telescope.

Additional steps are helpful to verify that the wobble is in fact caused by orbiting planets. In some cases, stars may wobble due to processes within the star itself. In this study, a separate group of astronomers working with a robotic telescope at Tennessee State University made careful and precise measurements of the brightness of the star. This step gives important evidence that the radial velocity changes are likely caused by this orbiting planet, and not something else.

What does it mean?

The planet’s position in the habitable zone itself is a great discovery. Although it does not confirm the existence of life, this planet is the most likely to harbor life out of all exoplanets that have yet been discovered. Plus, the circumstances surrounding its discovery also yield stunning realizations.

The discovery has been made relatively quickly. There are only a small number of stars that have been studied by astronomers for the existence of exoplanets. This particular exoplanet has been discovered at a time when only a few Earth-like exoplanets have been discovered. Plus this discovery is very nearby, only about 20 light years away. Both these suggest that the existence of habitable Earth-like planets is in no way rare, as some believe. In our galaxy alone, the astronomers working on the project hypothesize, there could be tens of billions of star systems containing habitable Earth-like exoplanets, which is definitely a mind-blowing realization.

A Nearby Watery Planet

A new rocky planet, GJ 1214b, has been found very close to the solar system, one of a few hundred exoplanets currently known. Making this discovery special, though, is that the planet contains water, being almost completely covered by it, and that its mass is just a little bit greater than the Earth's.