Beyond Our Solar System
Twenty years ago, there would have been nothing to write about under this topic. There were no known planets circling stars beyond our own. But in 2009 a revolution happened with NASA’s launch of the Kepler telescope. Within a few years, Kepler had found so many planets that scientists realized that there were more planets than stars in our galaxy. Let that sink in for a moment: There are more planets than there are stars in the galaxy. Astronomy and planetary science will never be the same. The study of exoplanets, as they’re now called, is the new frontier.
Obviously, at least for now, we can’t see planets orbiting other stars directly because the light from the star overwhelms the dim glow that’s reflected by the planet. But there are three main ways we can detect planets orbiting other stars indirectly:
1) We can measure a tiny wobble of the star against the background of fixed stars caused by the tug of the planet going back and forth.
2) That tug also causes a wobble toward and away from us, producing a small Doppler shift in the light from the star.
3) If we have a sensitive enough telescope, we can detect slight dips in a star’s brightness when a planet passes in front of the star (called a transit).
The first two methods were used in the early days of planet-hunting to detect the first exoplanets. The last one is what the Kepler project used. Kepler’s sole purpose was to “stare” at a spot about the size of your fist at arm’s length near the constellation Cygnus. It had a 95 mega-pixel camera and it recorded the scene every 6.5 seconds. Astronomers on Earth then compared the brightness of about 150,000 stars in its field of view and plotted how their brightness varied. The first discoveries were Jupiter-sized planets orbiting close to their stars, “hot Jupiters,” but as the observations continued, smaller and further-out planets were detected, including a few in Earth-like orbits. After four years, Kepler was moved to a different mission, but its data are still being combed for more planets. The count is now over 4,000 with a few thousand waiting to be confirmed; some of the exoplanets are multi-planet systems like ours.
Once Kepler proved the usefulness of the transit method of exoplanet discovery, NASA initiated the Transiting Exoplanet Survey Satellite (TESS), which launched in 2018. Rather than focusing on one small patch of sky, TESS is switching around and surveying 200,000 bright stars over the entire sky. TESS has already confirmed over 100 exoplanets with a couple thousand awaiting confirmation. The European Space Agency has also gotten into the game with its CHaracterising ExOPlanet Satellite (these acronyms can get rather tortured) which launched at the end of 2019 and has already discovered a few exoplanets. NASA is also working on a new telescope called the Nancy Grace Roman Space Telescope to be launched in 2025 that will search for and characterize exoplanets in the infrared.
NASA’s Jet Propulsion Laboratory is working on advanced technology that could allow us to directly image a planet around another star. The only way to do that is to block the light of the star and to do that you need an occultation disk – a disk you can place between your telescope and the star which will block the starlight, but not the glow from the tiny planet right next to it. The challenge is that the telescope and disk need to be outside the atmosphere, and the disk needs to be large (over 100 feet) and far away (about 25,000 miles) from the telescope, while being aligned to a few feet. Project Starshade is working on that.
As we learn more about these other solar systems, JPL’s resident artists have set up an Exoplanet Travel Bureau complete with posters advertising the exotic locations we might someday be able to travel to, such as the lava world of 55 Cancri e, or Kepler 16b, “where your shadow always has company” due to its twin suns. You can download the posters at https://exoplanets.nasa.gov/alien-
Is There Anybody Out There?
Now that we know there are so many planets out there, interest has revived in an “equation” proposed by astronomer Frank Drake in 1961. He wanted a way to try and answer the question: “Are we alone?” So he wrote down the following, now known as the Drake Equation:
N x fp x f1 x f2 x f3 x f4 x L = X
Where N is the number of stars in our galaxy, fp is the fraction of stars that have planets, f1 is the fraction of planets that could support life, f2 is the fraction of those that do develop life, f3 is the fraction of those that go on to develop intelligent life (civilization), f4 is the fraction of civilizations that emit detectable signals into space, and L is the length of time they emit those signals. So you start with a big number and gradually whittle it down to a small number. The question is: Is X larger than 0? We know the approximate number of stars in our galaxy and, thanks to the Kepler mission, we now know that fp is about 100%. But the rest of the factors are unknown. If we move a little beyond science and into speculation, we can make estimates of the other factors and can come up with a range of guesses for X. The best guesses now range from 0 to about 15,000,000. In other words, we may be alone, or the galaxy may be teeming with intelligent life! Of the latter possibility, Enrico Fermi, another famous scientist, remarked: “well, where are they?” Because if there are that many civilizations, some must have invented interstellar travel and, given the age of the galaxy, must have colonized most of the other planets. Thus the Fermi Paradox casts some doubt on the idea of a teeming galaxy.
Meanwhile we’re still searching for ET: The Search for Extraterrestrial Intelligence (SETI), initiated by Drake, Carl Sagan, and others is using large radio telescopes to scan for artificial signals. They chose a radio wavelength near the resonance frequency of the hydrogen atom as hydrogen is the most common material in the galaxy. So far, no detections have been made. Other wider-ranging searches have also been tried, but one of the limitations is that factor L in the Drake Equation: civilizations may not emit strong signals for very long. Looking at our own history of radio communications shows that, while we had many strong TV transmitters from the ‘50s on up to early this millennium, those are mostly gone now that most TV is on cable or streamed on the internet. So our ‘L’ is only about 50 years or so. But maybe someone, somewhere will pick up an episode of I Love Lucy a few hundred years from now and wonder who those people are…