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Hidden Stars May Make Planets Appear Smaller

In the search for planets similar to our own, an important point of comparison is the planet’s density. A low density tells scientists a planet is more likely to be gaseous like Jupiter, and a high density is associated with rocky planets like Earth. But a new study suggests some are less dense than previously thought because of a second, hidden star in their systems.

As telescopes stare at particular patches of sky, they can’t always differentiate between one star and two. A system of two closely orbiting stars may appear in images as a single point of light, even from sophisticated observatories such as NASA’s Kepler space telescope.

This can have significant consequences for determining the sizes of planets that orbit just one of these stars, says a forthcoming study in the Astronomical Journal by Elise Furlan of Caltech/IPAC-NExScI in Pasadena, California, and Steve Howell at NASA’s Ames Research Center in California’s Silicon Valley.

“Our understanding of how many planets are small like Earth, and how many are big like Jupiter, may change as we gain more information about the stars they orbit,” Furlan said. “You really have to know the star well to get a good handle on the properties of its planets.”

Some of the most well-studied planets outside our solar system – or exoplanets – are known to orbit lone stars. We know Kepler-186f, an Earth-size planet in the habitable zone of its star, orbits a star that has no companion (the habitable zone is the distance at which a rocky planet could support liquid water on its surface). TRAPPIST-1, the ultra-cool dwarf star that is home to seven Earth-size planets, does not have a companion either. That means there is no second star complicating the estimation of the planets’ diameters, and therefore their densities.

But other stars have a nearby companion, high-resolution imaging has recently revealed. David Ciardi, chief scientist at the NASA Exoplanet Science Institute (NExScI) at Caltech, led a large-scale effort to follow up on stars that Kepler had studied using a variety of ground-based telescopes. This, combined with other research, has confirmed that many of the stars where Kepler found planets have binary companions. In some cases, the diameters of the planets orbiting these stars were calculated without taking the companion star into consideration.

That means estimates for their sizes should be smaller, and their densities higher, than their true values.

Previous studies determined that roughly half of all the sun-like stars in our sun’s neighborhood have a companion within 10,000 astronomical units (an astronomical unit is equal to the average distance between the sun and Earth, 93 million miles or 150 million kilometers). Based on this, about 15 percent of stars in the Kepler field could have a bright, close companion – meaning planets around these stars may be less dense than previously thought.

The Transit Problem for Binaries

When a telescope spots a planet crossing in front of its star – an event called a “transit” – astronomers measure the resulting apparent decrease in the star’s brightness. The amount of light blocked during a transit depends on the size of the planet – the bigger the planet, the more light it blocks, and the greater the dimming that is observed.

Scientists use this information to determine the radius – half the diameter – of the planet.

If there are two stars in the system, the telescope measures the combined light of both stars. But a planet orbiting one of these stars will cause just one of them to dim. So, if you don’t know that there is a second star, you will underestimate the size of the planet.

For example, if a telescope observes that a star dims by 5 percent, scientists would determine the transiting planet’s size relative to that one star. But if a second star adds its light, the planet must be larger to cause the same amount of dimming.

If the planet orbits the brighter star in a binary pair, most of the light in the system comes from that star anyway, so the second star won’t have a big effect on the planet’s calculated size. But if the planet orbits the fainter star, the larger, primary star contributes more light to the system, and the correction to the calculated planet radius can be large – it could double, triple or increase even more.

This will affect how the planet’s orbital distance is calculated, which could impact whether the planet is found to be in the habitable zone.

If the stars are roughly equal in brightness, the “new” radius of the planet is about 40 percent larger than if the light were assumed to come from a single star. Because density is calculated using the cube of the radius, this would mean a nearly three-fold decrease in density.

The impact of this correction is most significant for smaller planets because it means a planet that had once been considered rocky could, in fact, be gaseous.

The New Study

In the new study, Furlan and Howell focused on 50 planets in the Kepler observatory’s field of view whose masses and radii were previously estimated. These planets all orbit stars that have stellar companions within about 1,700 astronomical units. For 43 of the 50 planets, previous reports of their sizes did not take into account the contribution of light from a second star. That means a revision to their reported sizes is necessary.

In most cases, the change to the planets’ reported sizes would be small. Previous research showed that 24 of the 50 planets orbit the bigger, brighter star in a binary pair. Moreover, Furlan and Howell determined that 11 of these planets would be too large to be planets if they orbited the fainter companion star. So, for 35 of the 50 planets, the published sizes will not change substantially.

But for 15 of the planets, they could not determine whether they orbit the fainter or the brighter star in a binary pair. For five of the 15 planets, the stars in question are of roughly equal brightness, so their densities will decrease substantially regardless of which star they orbit.

This effect of companion stars is important for scientists characterizing planets discovered by Kepler, which has found thousands of exoplanets. It will also be significant for NASA’s upcoming Transiting Exoplanet Survey Satellite (TESS) mission, which will look for small planets around nearby, bright stars and small, cool stars.

“In further studies, we want to make sure we are observing the type and size of planet we believe we are,” Howell said. “Correct planet sizes and densities are critical for future observations of high-value planets by NASA’s James Webb Space Telescope. In the big picture, knowing which planets are small and rocky will help us understand how likely we are to find planets the size of our own elsewhere in the galaxy.”

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The Best Dive Watches Under $200

Whether you’re just diving into a huge workload at your desk, or actually spending some time deep underwater, one thing is clear — a dive watch would be great to have on your wrist. You don’t need to be a diver to appreciate these high-performing, beautifully designed, and undoubtedly utilitarian timepieces for everyday use (especially now that summer’s in full swing). In this Carry Smarter guide, you’ll get familiar with the basics of dive watches, what features to look for when buying a diver, and our picks for the best and most affordable options to help you take the plunge into the world of dive watches.

A Crash Course on Dive Watches

The purpose of a dive watch is to monitor how long you’ve been underwater, and more importantly - how much air you have left in your tank. They’ve been around since the turn of the 20th century and continue to be both fashionable and useful today. The quintessential dive watch has an immediately recognizable look. They’re larger in size (around 42mm), feature a rotating bezel, and rest on a metal bracelet or rubber strap. Dive watches are ideal for EDC use because they’re built like tanks, they’re easy to read, and they look just plain cool.

4 Hallmarks of Dive Watches

Water Resistance: If you’re buying a dive watch, it should have proper water resistance. While most watches claim 50m of water resistance, that really means that it will survive hand washes and maybe a shower. When looking at dive watches, 200m (660 feet!) of water resistance is common ground. If you plan on having a watch that will stand up to swimming, showering, and of course, diving - be sure to choose something with a high level of water resistance.

Build Quality: Divers entrust their watches with their lives to be able to know precisely how much time they have underwater. For dive watches, reliable durability and construction are critical. Look for a dive watch with a well-built case, a strong crystal (mineral and sapphire are best), and a good strap or bracelet. A solid dive watch will last for decades if maintained, and you can easily buy an heirloom piece in the $200 range.

Movement: The slight bump in price from our Military Watch Guide opens up more options for the type of movement that powers the watch. Automatic movements are popular in the diver market as they don’t require a battery. Automatic watches “wind” from the motion of your arm, so they’ll keep ticking as long as you keep them on your wrist. Also seen in this class of watches are day/date features, adding to the utility of the timepiece.

Legibility: When underwater, it’s crucial to know exactly how long you’ve been diving. The bezel, a key component of the dive watch, tells you exactly that. The bezel’s “12 o’clock” dot can be rotated to match up with the minute hand to keep track of time. As the minute hand moves, you can see how many minutes have elapsed by reading the bezel number as opposed to the watch face. Dive watches feature large, illuminated indices (the hour and minute markings on the face) that are easy to read. This illumination (or “lume” in the watch world) not only looks awesome, but it helps you quickly tell time when the lights are out.

With the features to look for in a dive watch in mind, here are some of our favorite examples — all coming in at under $200:

The 8 Best Affordable Dive Watches for EDC

Casio MDV106-1A

The Casio MDV106-1A is the most inexpensive watch on this list at well under $200, but Casio didn’t get to where they are today producing cheap, low-quality watches. This watch is a great entry point into the dive watch look without having to commit to the full mechanical experience (and price). Its 45mm case diameter is as big as they come, and its 200m water resistance, screw-down crown, and screw-lock back preserve its Japanese quartz movement from the water. Excellent features for a dive watch at a very affordable price point.

BUY ($40)

Seiko SKX007

The SKX007 is an excellent example of a classic dive watch. This model from Seiko has been around in one form or another for decades. Featuring a mechanical movement and tank-like construction, this capable diver will serve you well for years to come. The large, circular indices are easy to read and the bezel clicks securely in place. The day/date wheel, sweeping seconds hand, and bright lume add up to a stylish watch ideal for everyday wear.

BUY ($174)

Orient CEM65001B “Black Mako”

Orient’s Submariner homage gets everything right. It pays its respects to the quintessential dive watch design, but makes some very attractive tweaks to make it their own. The Arabic numerals, date window, sword hands, and striking red accent on the second hand are all welcome aesthetic choices, enhancing its look without overdoing it. The rest of the watch is solid: stainless steel bracelet, in-house automatic movement, 200m water resistance and mineral crystal window all give great value to the watch as well as the wearer, given how inexpensive it is. The Orient Black Mako is a great starting point to jump into the deep end of dive watches.

BUY ($133)

Timex Expedition T49799

The Timex Expedition series of watches go the extra mile in providing quality timepieces packed with features but not weighed down by price. The T49799 takes the brand under the waves, giving you everything you need for your next dive. The watch itself is beefy, with 44 millimeters of shock-resistant stainless steel sealed, chunky rivets and a mineral crystal window rated for 200m. The signature Timex Indiglo provides ample illumination for dark and murky environments, and its chronograph dials handle all your timing needs. An outer bezel Tachymeter and date window round out the watch’s data features.

BUY ($119)

Seiko SNZH53

This diver is from Seiko’s popular “5 Series” of watches. Each watch in the 5 Series features automatic winding, a day/date display, water resistance, a recessed crown, and a durable case and bracelet. This particular watch features a more vintage look thanks to the wide bezel and thin indices on the face. The dark blue face nicely accents the stainless steel and the transparent casebook allows you to see the mechanical movement in motion. The SNZH53 also comes on a stainless steel bracelet, which adds to the value of this affordable diver.

BUY ($169)

Parnis GMT-Master

Understated excellence is the name of the game for Parnis pieces, and the GMT-Master is winning at it. Only simple and effective components grace the watch, from its scratch-resistant sapphire window to its automatic, hacking movement. Its design pays tribute to the classic dive design, and its stainless steel construction capped with a ceramic bezel ensures that design is preserved against wear and tear. If you want the dive watch quality but prefer not to make waves with aesthetics, this Parnis could be for you.

BUY ($120)

Luminox 3051 EVO Colormark

Developed together with the U.S. Navy Seals, the Luminox 3051 is as rugged as it is striking in appearance. Perfect for low-light environments, its tritium tubes stay visible long after other the strongest paint-on lumes have lost their brightness. Its thick, 44mm polyurethane case protects its Swiss-quartz movement, and its 200m water resistance ensures the 3051 doesn’t spring a leak while in service. Even its face styling is designed to make visibility the priority, with block Arabic numerals painted in bright white contrast to the black case. Eye-catching and tough, Luminox’s flagship 3051 leads the way in underwater timekeeping.

BUY ($197)

Seiko SRP307 “Black Monster”

You can’t have a list about dive watches (regardless of the price) and not mention the Seiko Monster. This timepiece sets the bar for the value you get from an automatic watch, regardless of price or brand. From its mammoth 45mm case design to its reliable 4r36 movement to the most aggressive lume applied on a production watch, the list of its features just goes on and on. This second-generation SRP307 takes all the respectable features of its predecessor and improves on all its former weaknesses. Its second hand can now be stopped (hacked) during adjustment, its crown is easier to grip, it has a more thematic and less complicated face, and they’ve somehow made its lume even brighter. Make no mistake, its nickname is “Monster” for a reason. (Editor’s Note: At the time of writing, the Monster was $200 on the nose. Its price has since fluctuated higher, but it’s still a worthwhile mention for this list.)

BUY ($200)

Words by Ed Jelley, Mikey Bautista, and Bernard Capulong. Photos by Ed Jelley.