How Do You Measure A Planet Near A Tiny Star?

When you sit back and think about how far away exoplanets are — and how faint — it’s a scientific feat that we can find these distant worlds outside our Solar System at all. It’s even harder to learn about the world if the exoplanet is orbiting a dim star — say, about two-thirds the size of the Sun — that is faint through even the largest telescope.

In response to this problem, there’s one science team that thinks it’s found a way to solve it. Their research bumped a planet from the habitable zone to the not-so-friendly zone of a star. Here’s how it happened:

The usual way to measure a distant star is this: look at the light. A Sun-sized star, for example, would have its light waves measured at different wavelengths. Scientists then match what they see to spectra (light bands) that are created artificially.

This method doesn’t work so well for smaller stars, though. “The challenge is that small stars are incredibly difficult to characterize,” stated Sarah Ballard, a post-doctoral researcher at the University of Washington, in a press release. Worse, these small guys make up about two-thirds of the stars in the universe.

Red Dwarf star and planet. Artists impression (NASA)
Red Dwarf star and planet. Artists impression (NASA)

Ballard led a multi-university team describing a “characterization by proxy” method accepted for publication in The Astrophysical Journal and now available online.

The science team based their work on previous research performed by astronomer Tabetha Boyajian, who is currently at Yale University.

Boyajian combined the resources of several telescopes that measured wavelengths of light, wavelengths that are slightly longer than visible light. This technique allowed the interferometer (the combined telescopes) to figure out the size of stars that are close by.

With that data on hand, Ballard and her colleagues looked out into the universe. Their target was Kepler-61b (Kepler Object 1361.01), a “candidate” planet about double the size of Earth spotted by the planet-hunting Kepler space telescope. The candidate, if proven, is orbiting a low-mass star 900 light-years away that is hard to measure in a telescope.

Kepler space telescope's field of view. Credit: NASA
Kepler space telescope’s field of view. Credit: NASA

Next, the scientists picked four nearby stars that have similar light patterns, reasoning that they would be spectroscopially close enough to Kepler-61b’s parent star to make accurate measurements. The four stars are located in Ursa Major and Cygnus, ranging between 12 to 25 light years away from Earth.

When the scientists compared the measurements to Kepler 61’s star, a surprise emerged.

“Kepler-61 turned out to be bigger and hotter than expected,” the University of Washington stated. “This in turn recalibrated planet Kepler-61b’s relative size upward as well — meaning it, too, would be hotter than previously thought and no longer a resident of the star’s habitable zone.”

The newly refined planetary radius for Kepler-61b is 2.15 times the radius of Earth (plus or minus 0.13 radii). Astronomers estimate it orbits its star about once every 59.9 days and has a temperature of 273 Kelvin (plus or minus 13 Kelvin.)

Artist's impression of the habitable zone around variously sized stars. Credit: NASA
Artist’s impression of the habitable zone around variously sized stars. Credit: NASA

Just to wrap up, here’s a note about how likely it is that Kepler-61b is actually a planet — and not a planetary candidate.

The candidate was first described in this 2011 scientific paper. Kepler-61b is just one in a long list of 1,235 planetary candidates catalogued in that paper, all discovered in just four months — between May 2 and Sept. 16, 2009.

While the NASA Exoplanet Archive still lists Kepler-61b as a candidate planet — one that must be confirmed by independent observations — this 2011 paper says that most Kepler candidates have a strong possibility of being actual planets because the Kepler software is technologically apt.

In other words, Ballard and her co-authors write in the research, Kepler-61b is very likely to be a planet itself — with only 4.8 percent possibility of being a “false positive”, to be exact.

Source: University of Washington

4 Replies to “How Do You Measure A Planet Near A Tiny Star?”

  1. Ah, what life would be like on a planet with a star of class M. Your eyes would be color balanced to the ruddy hue would be essentially white. Stars like ours in the night sky would not be yellow, they would be blue.

  2. Hi, i have seen ligth curves of Kepler 62-e and Kepler 62-f and i remember reading a document wich says in theory how exo-moons can be detected by Kepler, now does anybody has study the ligth curves of these new planets from this point of view ?

    I see the curves from here (is in spanish but look for images of ligth curves for e and f):

    And the paper: “The detectability of habitable exomoons with Kepler” is here:

    What do you think ?

  3. I must say I find the article, the press release and the paper a bit confusing.

    The reported equilibrium temperature, which seems to be albedo-based only, doesn’t seem too bad. IIRC the greenhouse of Earth increases the albedo-temperature some 50 K, which would put Kepler-61b within the habitable zone.

    And a similar claim is seen early in the paper, given “an equilibrium temperature of 273±13 K”:

    “The most recent release of Kepler exoplanetary candidates (Batalha et al. 2013) contains 10 members <2 R_Earth and with equilibrium temperatures between 185 and 303 K. This temperature range is a generous de?nition of the habitable zone proposed by Kasting (2012)."

    More problematic is that the new radius pushes the planet out of the terrestrial range of <2 R_Earth into the mini-Neptune range of Kepler 11b. "We conclude that the planet is likely slightly too large to be terrestrial in composition, and likely possesses a signi?cant atmosphere."

    1. I haven’t read the press release or the paper, but I agree that this article doesn’t make sense when it talks about the habitable zone. It says that the new results indicate that the planet is warmer than originally thought, and so is no longer in the HZ. But the revised, warmer equilibrium temperature of 273 K is the freezing point of water, defining the outer edge of the so-called “narrow” or “conservative” HZ. So based on that revised warmer temperature, it is now (barely) in the HZ, and would not have been originally.

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