Hotter Stars, Habitable Planets?

The search for life on other planets could soon extend to solar systems that are very different from our own, according to a new study published in the Astrophysical Journal Letters. In fact, finding a terrestrial planet in such a solar system would offer unique scientific opportunities to test evolution, said Andrew Gould, professor of astronomy at Ohio State. The research describes how one might try to detect a hot, massive star and a far-away orbiting planet. Owing to the distance of its orbit, that planet might still be able to harbor liquid water–and thus life.

SIM reference design
SIM, scheduled for launch in 2009, will determine the positions and distances of stars several hundred times more accurately than any previous program.
Credit: NASA / JPL

Gould and his coauthors calculated that NASA’s upcoming Space Interferometry Mission (SIM) would be able to detect habitable planets near stars significantly more massive than the sun. Scientists have typically thought that the search for life "should focus on finding planets like Earth that orbit stars like the sun, but this new finding shows that the field is wide open," Gould said.

"Here’s a type of solar system that we never thought to look at," he added, "but now we’ll have the tools to do it."

Gould is on the science team that is helping to plan the SIM mission, and he is working to define the capabilities of the satellite.

SIM would help astronomers find habitable planets, Gould said. The key is detecting planets that circle a star at just the right distance to maintain a supply of liquid water. The range of most promising orbits depends on the type of the star, and is called the "habitable zone."

Time Enough to Evolve?

The earth resides directly in the habitable zone for our solar system, some 93 million miles from the sun. The nearest planets, Venus and Mars, barely lie within the edges of the habitable zone.

Hotter, more massive stars have always been considered less likely to harbor life, though not because they would be too hot. Planets could still enjoy temperate climates, just at orbits farther away from the star.

"The problem is one of time, not temperature, Gould said. "Hotter stars tend to burn out" faster — perhaps too fast for life to develop there.

Our sun is approximately 4.5 billion years old; in contrast, one of the stars examined in the study is 1.5 times more massive than the sun, and would probably only generate life-sustaining energy for about two billion years.

The gravitational pull of an unseen planet causes a star to wobble. As the star moves toward an observer, the wavelength of the star’s light is squeezed and becomes more blue. As the star moves away from the observer, the wavelength is stretched and the light becomes more red.

Given the billions of years required for evolution of life on earth, scientists could question whether life would stand a chance in a shorter-lived solar system.

"We have no idea how evolution would proceed on any planet other than our own," Gould said. "If we find a planet around a shorter-lived star, we may be able to test what would happen to evolution under those circumstances."

Triangulating On A Star

SIM will use Interferometry — a technique that involves the interference of light waves — to very accurately measure the position of stars in the sky. The satellite would notice, for instance, if a point of light on the surface of the moon moved the width of a dime.

In the case of distant stars, SIM will pick up on the tiny wobble in the position of a star caused by the gravity of its orbiting planets.

That’s what will make SIM ideal for studying hotter, massive stars, Gould said. Planets that orbit far from a star — as the habitable planets around a hot star would have to do — create a larger wobble. SIM, in solar orbit, won’t actually see Earth-sized planets. Instead, it will use astrometry, measuring the angle between two stars (as viewed from the spacecraft, which will form the third point of a triangle). By repeatedly measuring the angle between a target star and each of several more distant background stars, SIM will be able to determine whether the target star wobbles periodically because of gravity from orbiting planets, including planets as small as Earth.

The Problem to Solve

Gould and study coauthors Eric B. Ford of Princeton University and Debra A. Fischer of the University of California, Berkeley, determined that SIM is sensitive enough for the task. Their recent article is entitled "Resolving the Microlens Mass Degeneracy for Earth-Mass Planets".

The authors describe the problem they are trying to solve for planet-finders: "Of all planet-finding techniques, microlensing is potentially the most sensitive to Earth-mass planets. However, microlensing light curves generically yield only the planet-star mass ratio: the mass itself is uncertain to a factor of a few…Here we present a new method to measure microlens masses for terrestrial planets. We show that, with only a modest adjustment to the proposed orbit of the dedicated satellite that finds the events, and combined with observations from a ground-based observing program, the planet mass can be measured routinely."

One shortcoming of the detection method is that an alignment or micro-lensing event, by its random nature, is a one-shot affair. It happens; you watch it; it’s over. There is no second chance to verify the results, because the particular alignment of the distant star, the lensing star and Earth, which created the event, is never repeated. But because collaborations involves astronomers in different locations observing the same event, they can correlate their results.

The other problem is that even if astronomers do detect planets, they won’t be able to tell the precise masses of the stars or the planets they observe. Rather, they can determine only the ratio of the mass of the planet to that of the star. The majority of stars in the galaxy (about 80%) fall within a narrow range of masses, with M dwarfs on one end of the range and Sun-type stars, about 3 times as massive as the M dwarfs, on the other. So it follows that most detectable events are caused by these abundant types of stars. It’s just not possible – yet – to say for certain what type of star is responsible for any particular event.

This second problem is what Gould, Ford and Fischer address, by describing a clever orbital arrangement that can more accurately determine the detected planet’s mass.

What’s Next

Previously, Gould and Ohio State professor Darren DePoy and graduate student Joshua Pepper determined that another future NASA mission could be used to find habitable planets around very small stars, which are much more plentiful in the galaxy than stars like our sun.

Kepler image
This image shows a design of the spacecraft for the Kepler mission.
Credit: NASA

That mission, the Kepler Mission, will detect planetary transits — events where planets pass in front of a star and block the star’s light from reaching earth. Transits of planets orbiting close to a star are easier to detect, and because these small stars are very dim, the habitable zone would also be very close to the star.

Continuous ground-based observations are also always on the alert. OGLE (Optical Gravitational Lensing Experiment) is a collaboration of Polish and American astronomers observing in Chile; the other, MOA (Microlensing Observations in Astrophysics), is a collaboration between astronomers in New Zealand and Japan. Together OGLE and MOA monitor some 10 million stars, night after night. As soon as they spot a microlensing event, indicated by a star that appears suddenly to brighten, they flash an alert to PLANET (the Probing Lensing Anomalies NETwork). The astronomers in the PLANET collaboration, who are spread out among four telescopes throughout the southern hemisphere, then begin monitoring the events intensively. A typical microlensing event lasts for many weeks, but the part that could reveal a planet lasts only a few hour to a few days – and there’s no way to predict when the interesting part will occur.

The point is that the various methods for planet detection complement each other, and can be used to find habitable planets around a wide variety of stars," Gould said.

Scientists who use the different planet detection techniques are looking forward to the day – although it may be some years away – when their research overlaps and they can compare notes. Their overarching goal is to learn how planets form around different types of stars, and how they migrate inward – if, indeed, they do – after they form. This, in turn, should help to understand the likelihood of finding inhabited planets in the galaxy.

The Space Interferometry Mission is managed by JPL as part of NASA’s Origins program.

Related Web Pages

Star chart
Kepler mission
Space Interferometry Mission
Extrasolar Planets Encyclopedia
Info on the PLANET collaboration
Extrasolar Encyclopedia site in France
Planet Search Results Suggest Our Solar System May Be Uncommon