by Fraser Hibbitt
It is not difficult to imagine that somewhere in our galaxy there is a planet like ours. A planet suited for life. The unimaginably vast, mind-boggling space outside our solar system always makes one wonder about other life out there. And extra-terrestrial life likely begins on a planet. This is what Dr. Borucki was entertaining when he imagined the Kepler spacecraft; it would take a further two decades to convince NASA to follow through with his dream.
In March 2009, the Kepler spacecraft was launched on a mission to gather data from 150,000 stars in a section of the Milky Way. It was observing, and looking for, changes in starlight which would have been caused by an exoplanet (An exoplanet is a planet lying outside our solar system). The Kepler mission was set for a three- and half-year course but ended up lasting nine years and seven months; the fuel in the Kepler’s telescope’s reaction control system ran out in 2018.
What the Kepler spacecraft was formally measuring was something called Eta-earth. Eta-earth is defined as the mean number per star (like our sun) of rocky planets residing in that star’s habitable zone. The habitable zone being a position in which an orbiting rocky planet can retain surface liquid water; whether the planet is warm enough to. The Eta-earth measurement gives us a frequency of earth-like planets, candidate worlds of habitation, in the galaxy.
It has taken two years to evaluate the data from the Kepler telescope and the results have now been published by the NASA Ames team in the Astronomical Journal. The team working on the data of the Kepler spacecraft calculated that at least thirty percent of stars similar in mass to our sun, have planets like earth in their habitable zones. ( See post on exoplanets).
NASA has estimated that there are at least a hundred billion stars in the Milky Way with four billion of them being like our sun. If we extrapolate from the Kepler data, then we can imagine just how many rocky planets there may be in habitable zones.
The closest earth-like planet has been calculated to be around twenty light-years away, with a further four ‘earths’ within thirty light-years. This new analysis has debunked earlier formulations on the Kepler data about the number of earth-like planets and is now suggesting that the Milky Way is at least as twice abundant in ‘earths’ as was previously believed.
Jon Jenkins was the “Analysis Lead” for NASA’s Kepler Mission. He headed the group of scientists and programmers that designed and built the software that made the search for other worlds possible. Standing next to him is Carl Kruse, holding a magnum of wine with a SETI Institute label titled “Prepare For Success. Open Only Upon Detection Of Extraterrestrial Signal.” Signed by Jill Tarter and Frank Drake.
This new and improved analysis of the Kepler data was helped by information from the European GAIA satellite, which has measured the position, and brightness, of one billion stars in the Milky Way. This enabled the astronomers to accurately cross reference the Kepler results to produce a more reliable statistic. The key is to approach greater reliability given that statistics preside on generalizations.
It is this statistical framework that revealed a limitation in the Kepler mission. When the telescope failed, The Kepler spacecraft had not reached its goal of detecting planets that had an orbital duration of less than 700 days. This means that the data from the Kepler spacecraft has detected no planets which are true analogues of the earth; there were no planets detected with the same radius or orbital duration meaning that none of these detected planets received the same amount of light as our earth.
Doug Vakoch, whose title at the SETI Institute is “Director of Interstellar Message Composition,” is the only social scientist employed in SETI where he investigates how you might craft messages that could be transmitted across interstellar space that could be understood by extraterrestrials even without face-to-face contact. He is particularly interested in how we might compose messages expressing what it’s like to be human. With Carl Kruse at SETIcon.
These Kepler planets are extrapolated from data; they may be rocky with a radius of a half to one and a half of the earth, but we have no idea what they are like. They have been detected using precise equipment looking for a precise pattern, and they are too far away to study in detail. The numbers are promising, but so far, we have no signs of life.
There has been, however, an adjustment to improve the parameters of Eta-earth. Kepler’s measurement of Eta-earth was only directed towards stars that were like our sun. A new focus in the search for exoplanets has implemented the use of red dwarfs. Red dwarfs, in comparison to our sun, are dimmer and smaller, but recent investigations have found that one-quarter to one-half of red dwarfs can produce habitable zone planets. Although, there is some controversy over whether the radiation flares from a red dwarf would preempt any kind of life.
The use of red dwarfs to scout for exoplanets within habitable zones has now been integrated into a new project. The Kepler mission has been transferred to a new spacecraft, TESS, which stands for Transiting Exoplanet Survey Satellite. TESS’s aim is to search in the vicinity of a few hundred light years from earth for exoplanets. TESS, launched in 2018, has discovered 66 new exoplanets and more than 2000 candidates.
It remains to be seen whether the value of Eta-earth can be improved by altering the parameters of the definition. The inclusion of red dwarfs has expanded the scope of Eta-earth and so there may be more ways to measure Eta-earth in the future. For now, we can only look for what we currently know of how life begins, and how life thrives.
The value of Eta-earth is an integral part of the mathematical expression known as the Drake equation. The Drake equation is used to calculate how many technologically advanced civilizations there may be in our galaxy, and that could make contact via technology such as radio transmissions, which travel at the speed of light.
Above is the Drake Equation where N is the total number of intelligent civilizations, which is arrived at by multiplying the following factors –
R is the rate at which new stars form in the galaxy,
F(p) are those stars that have planets,
n are the planets that could have life,
F(L) are those planets that go on to develop life,
F(i) are the percentage of those planets whose life becomes intelligent,
F(c) are those that develop radio communication and
L is the longevity of that civilization’s ability to transmit.
Eta-earth, up until now, has rested as an unknown factor, but now, as a part of a real investigation it adds to the excitement of the Drake equation; it is helping us to estimate what fraction of these exoplanets nurture life, and towards ending our bout of solitude among the stars. Exciting times.
Homepage for the Carl Kruse Blog
Contact: carl AT carlkruse DOT com
Further posts regarding SETI talk about the end of SETI@HOME, fundraising for the SETI Institute, the role SETI@HOME played in searching for alien intelligence, and a brief overview of the SETI Institute.
The blog’s last post was on a more earthly type of discovery, the excavation of the City of Troy.