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The last 25 years has brought a veritable flood of exoplanets. Some of those finds include Jupiter-class planets that take just a few days to complete an orbit. Others showcase Earth-sized planets orbiting stars emitting less than 5 percent of the Sun’s light. Still others are stars with multiple planets orbiting 2-3 times more distant than Jupiter and having masses almost large enough to ignite nuclear fusion. The catalog of more than 3,000 exoplanets testifies to the remarkable technological achievements over the past two decades. However, as scientists continue to find potentially habitable planets, I find it worthwhile to remember what we actually know about planets outside the solar system.

Scientists use four main techniques to find exoplanets: radial velocity measurements, transit searches, gravitational lensing, and direct detection. Each has strengths and limitations, but all currently fall short of finding the necessary data to evaluate the true habitability of an exoplanet. Here is a brief rundown of each technique.

Radial Velocity

The light emitted by a star dwarfs the tiny amount reflected (or generated) from an exoplanet. Rather than trying to measure the minuscule exoplanet glow, the radial velocity technique searches for the gravitational tug from exoplanet on its host star. Compared to this host, planets typically have masses anywhere from 100 times smaller (Jupiter-sized) to 10,000 times smaller (Earth-sized).


Currently, radial velocity searches have detected almost 700 planets with more than 100 of those residing in multi-planet systems. The sensitivity of the technique grows as the planet mass increases or as the orbit size decreases. Both conditions cause larger motions in the host star. Although this technique ushered in the age of exoplanet discoveries, it yields little information about the host planet, namely the minimum mass, the orbital parameters (distance to star, eccentricity, etc.), and the age (assumed to be the same as the host star age). Because radial velocity searches look at the shift in wavelengths of light, ground-based telescopes work just as well as space-borne ones. However, current technology limits the sensitivity to planets larger than a few times the mass of Earth.

Transit Searches

In 1999, astronomers began exploiting another way to find planets outside our solar system. If the plane of a distant planet system is aligned with Earth, then any planets might cross the disk of the star as they orbit. If so, astronomers would see a dimming of the light while the planet transits across the star. The sensitivity of this technique depends on the physical size of the star and planet. Jupiter would block out 1 percent of the light from the Sun since it has a diameter one-tenth as large.


Just using the transit technique, astronomers know the physical size of an exoplanet as well as its orbital parameters. Current technology allows astronomers to find planets as small or smaller than Earth in Earth-like orbits. Earth would block one ten-thousandth the light from the Sun, so only telescopes in space achieve this sensitivity. The transit technique relies on the proper alignment of Earth, the exoplanet, and star, so astronomers can only find a small fraction of the exoplanets that might exist. Often radial velocity searches add the mass of the planet. Occasionally, astronomers get information from the exoplanet’s atmosphere as the transit begins and ends. So far, only Jupiter-like planets have yielded this atmospheric information.

Gravitational Microlensing

Occasionally, a star and an associated planet will pass in front of a background star. For specific alignments, the star and planet gravitationally lens (enhance the view of) the background star, causing a brief but dramatic increase in detected light.


Of the roughly 50 planets detected by microlensing, a handful have masses and orbits similar to Earth. In fact, microlensing is one of the few techniques with the sensitivity to detect such planets. However, the chance nature of the method means that astronomers get only one shot at a detection. Additionally, the only information obtained from the planets is the mass and orbital parameters.

Direct Detection

The direct detection method seeks to directly detect the light coming from an extrasolar planet. Because the light from the star dwarfs the planetary light, this technique currently detects only Jupiter-class planets orbiting relatively far (more than 10 times the Earth-Sun distance) from their host stars. However, the light from the planet carries much information about the planet size, temperature, orbit, and atmosphere. The planned Terrestrial Planet Finder mission seeks to directly image an Earth-like planet orbiting in the water habitable zone around a Sun-like star.


In my assessment, this method is the only one capable of actually answering whether or not a planet could truly host life. None of the other techniques give adequate information about the existence of liquid water on the surface or gases in the atmosphere indicative of microbial life. However, the technology to detect Earth-sized planets in Earth-like orbits requires another couple decades of development.

The Bottom Line

The last few decades have seen remarkable advances in our ability to find planets outside our solar system. Scientists have found thousands of planets, with some residing in the region where liquid water could exist. Searches for exoplanets reveal abundant information about the mass, size, and orbital characteristics of the planets, but we still have a long way to go to determine whether a planet could truly host life, and more importantly, if life exists beyond the solar system.


Subjects: Extrasolar Planets, Life on Other Planets

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About The Author

Jeff Zweerink

Since my earliest memories, science and the Christian faith have featured prominently in my life - but I struggled when my scientific studies seemed to collide with my early biblical training. My first contact with RTB came when I heard Hugh Ross speak at Iowa State University. It was the first time I realized it was possible to do professional work incorporating both my love of science and my desire to serve God. I knew RTB's ministry was something I was called to be a part of. While many Christians and non-Christians see the two as in perpetual conflict, I find they integrate well. They operate by the same principles and are committed to discovering foundational truths. My passion at RTB is helping Christians see how powerful a tool science is to declare God's glory and helping scientists understand how the established scientific discoveries demonstrate the legitimacy and rationality of the Christian faith. While many Christians and non-Christians see the two as in perpetual conflict, I find they integrate well. • Biography • Resources • Upcoming Events • Promotional Items Jeff Zweerink thought he would follow in his father's footsteps as a chemistry professor until a high school teacher piqued his interest in physics. Jeff pursued a BS in physics and a PhD in astrophysics at Iowa State University (ISU), where he focused his study on gamma rays - messengers from distant black holes and neutron stars. Upon completing his education, Jeff taught at Loras College in Dubuque, Iowa. Postdoctoral research took him to the West Coast, to the University of California, Riverside, and eventually to a research faculty position at UCLA. He has conducted research using STACEE and VERITAS gamma-ray telescopes, and currently works on GAPS, a balloon experiment seeking to detect dark matter. A Christian from childhood, Jeff desired to understand how the worlds of science and Scripture integrate. He struggled when his scientific studies seemed to collide with his early biblical training. While an undergrad at ISU, Jeff heard Hugh Ross speak and learned of Reasons to Believe (RTB) and its ministry of reconciliation - tearing down the presumed barriers between science and faith and introducing people to their personal Creator. Jeff knew this was something he was called to be a part of. Today, as a research scholar at RTB, Jeff speaks at churches, youth groups, universities, and professional groups around the country, encouraging people to consider the truth of Scripture and how it connects with the evidence of science. His involvement with RTB grows from an enthusiasm for helping others bridge the perceived science-faith gap. He seeks to assist others in avoiding the difficulties he experienced. Jeff is author of Who's Afraid of the Multiverse? and coauthor of more than 30 journal articles, as well as numerous conference proceedings. He still serves part-time on the physics and astronomy research faculty at UCLA. He directs RTB's online learning programs, Reasons Institute and Reasons Academy, and also contributes to the ministry's podcasts and daily blog, Today's New Reason to Believe. When he isn’t participating in science-faith apologetics Jeff enjoys fishing, camping, and working on home improvement projects. An enthusiastic sports fan, he coaches his children's teams and challenges his RTB colleagues in fantasy football. He roots for the Kansas City Chiefs and for NASCAR's Ryan Newman and Jeff Gordon. Jeff and his wife, Lisa, live in Southern California with their five children.

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