Five starts into his 13th season in the big leagues, Ervin Santana has allowed a mere three earned runs and is carrying a 0.77 ERA, the lowest mark in the Major Leagues. Given that Santana has made a career out of being more of a solid mid-rotation starter than an ace, and that he's got by far the lowest Batting Average on Balls In Play mark in the Major Leagues at .129, the obvious question is: "Has he really been good, or has he just been lucky?" 
This early in the season, the correct answer to that is really "both." Five starts don't carry more weight than more than a decade of pitching, and as we investigated using Statcast™ earlier this year, the Twins' outfield was positioned to add far more defensive support than it did last year. Santana obviously won't carry a 0.77 ERA for too much longer. But what we can say is that at least so far, he's earned a lot more of that mark than you might think. Santana has legitimately been an elite pitcher for the season's first month.
How do we know it's something he's doing, and not just batted balls luckily finding gloves or stellar defense propping him up? Because we can use our new Statcast™ tools to look at the quality of contact Santana is allowing, independent of defense, and it's absolutely stellar.

There's two easy ways to show this. First, let's look at the starting pitchers who are collecting the largest amount of poor contact, based both on exit velocity and launch angle. We've identified six types of batted-ball contact, three of which are good for the pitcher (weakly hit, popups or topped into the ground), and three of which are good for the hitter, including barrels. The three "poor contact" types have a Major League average of .136 this year, while the the three "hard contact" types have result in an average of .562.
Clearly, the more poor contact you can get, the better. Santana's getting all of it.
Highest percentage of poor contact on batted balls in 2017 for starting pitchers
75.0 percent -- Miguel Gonzalez, White Sox
74.7 percent -- Andrew Triggs, A's
73.6 percent -- Santana, Twins
72.9 percent -- Kyle Freeland, Rockies
71.2 percent -- Joe Musgrove, Astros
71.0 percent -- Dallas Keuchel, Astros
70.1 percent -- Lance Lynn, Cardinals
70.6 percent -- Michael Wacha, Cardinals
70.5 percent -- Trevor Cahill, Padres
70.2 percent -- Shelby Miller, D-Backs
Major League average for all pitchers: 63.6 percent
Last year, Santana got poor contact on 62.5 percent of batted balls; the year before, it was 60 percent. So while this number is very likely to come down, it doesn't change what's already happened. So far, Santana is forcing hitters to make contact that's unlikely to turn into hits.
But pitching isn't just about contact, of course. It's also about missing bats. What we can do is look at the exit velocity and launch angle of each batted ball, and define the Hit Probability for each one -- that is, how likely that type of batted ball is to turn into a hit. When you combine that with real-world strikeouts, you can output Estimated Batting Average, or xBA. That's a number that's important, because we want to credit a pitcher for inducing terrible contact even if a poor defense can't get to it, and not credit a pitcher for allowing a screaming line drive that's hauled in by an elite outfielder.
If we look at the 138 starting pitchers who have faced at least 50 batters so far, we can see that based on quality of contact and strikeouts, Santana's not just in the top 10, he's again surrounded by some truly elite pitchers.
Lowest estimated batting average in 2017 for starting pitchers
.172 -- Scherzer, Nationals
.173 -- Chris Sale, Red Sox
.174 -- Dan Straily, Marlins
.178 -- Paxton, Mariners
.183 -- Jacob deGrom, Mets
.191 -- Marco Estrada, Blue Jays
.192 -- Lynn, Cardinals
.193 -- Santana, Twins
.194 -- Clayton Kershaw, Dodgers
.195 -- Jerad Eickhoff, Phillies
Major League average for all pitchers: .240
Remember, that's all independent of defense, so the fact that Byron Buxton, Max Kepler, Jorge Polanco and friends are adding solid fielding isn't even reflected here. It's not like he's had no help from his defense at all, of course, like when Danny Santana helped turn a sinking liner with an expected average of .743 into an out.

But mostly, it's been a lot of this (.051 expected average groundout):

And this (.100 expected average groundout):

Simply put, Santana has been combining elite contact prevention along with increased strikeout numbers to perform at an ace-like level. (Yes, increased strikeout numbers. Despite a K/9 mark that's down from 7.4 to 6.7, that's skewed by how quickly he's getting through innings. The far-superior K%, which simply looks at strikeouts per hitter faced, shows he's increased his whiffs from 19.9 percent to 21.1 percent.)
So what's changed to lead to this hot start? It's so early that it's hard to say, really. Santana isn't throwing harder, not really. The addition of catcher Jason Castro, signed this offseason in large part due to his elite framing skills, has likely benefited the entire staff. Backup catcher Chris Gimenez noted earlier this month that "the recipe for Santana's success has been pounding hitters with inside fastballs," which echoes what pitching coach Neil Allen preached this spring.
To pick just one example, fastballs against lefty hitters, there seems to be something to that. Looking at it from the catcher's viewpoint, the change is clear -- fewer strikes down the pipe, more pitches in on the hands.
Gif: Ervin Santana fastballs to lefties
So will Santana keep this up? No, of course not. That's not a slight against him so much as the simple acknowledgement that no pitcher, no matter how great, is going to maintain this pace all year. Regression is coming. At some point, Santana is going to leave a game with a runner on base and need to rely on his bullpen, which he's yet to do in 2017. But what he is doing right now is giving a young team a reliably solid starting pitcher every five days. He's perhaps giving the Twins an appealing trade piece later this summer if teams like the Astros or Tigers decide they need additional depth.
But mostly, Santana has been really good. This isn't good luck or good defense. It's a lot of poor contact that doesn't lead to damage. So long as Santana can keep that up, he'll be more than the Twins could have possibly hoped for.